JavaScript Questions

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###### 71. How can we log the values that are commented out after the console.log statement? ```javascript function* startGame() { const answer = yield 'Do you love JavaScript?'; if (answer !== 'Yes') { return "Oh wow... Guess we're gone here"; } return 'JavaScript loves you back ❤️'; } const game = startGame(); console.log(/* 1 */); // Do you love JavaScript? console.log(/* 2 */); // JavaScript loves you back ❤️ ``` - A: `game.next("Yes").value` and `game.next().value` - B: `game.next.value("Yes")` and `game.next.value()` - C: `game.next().value` and `game.next("Yes").value` - D: `game.next.value()` and `game.next.value("Yes")` <details><summary>Answer</summary>

C A generator function "pauses" its execution when it sees the `yield` keyword. First, we have to let the function yield the string "Do you love JavaScript?", which can be done by calling `game.next().value`. Every line is executed, until it finds the first `yield` keyword. There is a `yield` keyword on the first line within the function: the execution stops with the first yield! _This means that the variable `answer` is not defined yet!_ When we call `game.next("Yes").value`, the previous `yield` is replaced with the value of the parameters passed to the `next()` function, `"Yes"` in this case. The value of the variable `answer` is now equal to `"Yes"`. The condition of the if-statement returns `false`, and `JavaScript loves you back ❤️` gets logged. </details>

###### 77. Is this a pure function? ```javascript function sum(a, b) { return a + b; } ``` - A: Yes - B: No <details><summary>Answer</summary>

A A pure function is a function that _always_ returns the same result, if the same arguments are passed. The `sum` function always returns the same result. If we pass `1` and `2`, it will _always_ return `3` without side effects. If we pass `5` and `10`, it will _always_ return `15`, and so on. This is the definition of a pure function. </details>

###### 43. What does this return? ```javascript [...'Lydia']; ``` - A: `["L", "y", "d", "i", "a"]` - B: `["Lydia"]` - C: `[[], "Lydia"]` - D: `[["L", "y", "d", "i", "a"]]` <details><summary>Answer</summary>

A A string is an iterable. The spread operator maps every character of an iterable to one element. </details>

###### 51. What's the output? ```javascript function getInfo(member, year) { member.name = 'Lydia'; year = '1998'; } const person = { name: 'Sarah' }; const birthYear = '1997'; getInfo(person, birthYear); console.log(person, birthYear); ``` - A: `{ name: "Lydia" }, "1997"` - B: `{ name: "Sarah" }, "1998"` - C: `{ name: "Lydia" }, "1998"` - D: `{ name: "Sarah" }, "1997"` <details><summary>Answer</summary>

A Arguments are passed by _value_, unless their value is an object, then they're passed by _reference_. `birthYear` is passed by value, since it's a string, not an object. When we pass arguments by value, a _copy_ of that value is created (see question 46). The variable `birthYear` has a reference to the value `"1997"`. The argument `year` also has a reference to the value `"1997"`, but it's not the same value as `birthYear` has a reference to. When we update the value of `year` by setting `year` equal to `"1998"`, we are only updating the value of `year`. `birthYear` is still equal to `"1997"`. The value of `person` is an object. The argument `member` has a (copied) reference to the _same_ object. When we modify a property of the object `member` has a reference to, the value of `person` will also be modified, since they both have a reference to the same object. `person`'s `name` property is now equal to the value `"Lydia"` </details>

###### 68. What's the output? ```javascript console.log(Number(2) === Number(2)); console.log(Boolean(false) === Boolean(false)); console.log(Symbol('foo') === Symbol('foo')); ``` - A: `true`, `true`, `false` - B: `false`, `true`, `false` - C: `true`, `false`, `true` - D: `true`, `true`, `true` <details><summary>Answer</summary>

A Every Symbol is entirely unique. The purpose of the argument passed to the Symbol is to give the Symbol a description. The value of the Symbol is not dependent on the passed argument. As we test equality, we are creating two entirely new symbols: the first `Symbol('foo')`, and the second `Symbol('foo')`. These two values are unique and not equal to each other, `Symbol('foo') === Symbol('foo')` returns `false`. </details>

###### 12. What's the output? ```javascript function Person(firstName, lastName) { this.firstName = firstName; this.lastName = lastName; } const lydia = new Person('Lydia', 'Hallie'); const sarah = Person('Sarah', 'Smith'); console.log(lydia); console.log(sarah); ``` - A: `Person {firstName: "Lydia", lastName: "Hallie"}` and `undefined` - B: `Person {firstName: "Lydia", lastName: "Hallie"}` and `Person {firstName: "Sarah", lastName: "Smith"}` - C: `Person {firstName: "Lydia", lastName: "Hallie"}` and `{}` - D: `Person {firstName: "Lydia", lastName: "Hallie"}` and `ReferenceError` <details><summary>Answer</summary>

A For `sarah`, we didn't use the `new` keyword. When using `new`, `this` refers to the new empty object we create. However, if you don't add `new`, `this` refers to the **global object**! We said that `this.firstName` equals `"Sarah"` and `this.lastName` equals `"Smith"`. What we actually did, is defining `global.firstName = 'Sarah'` and `global.lastName = 'Smith'`. `sarah` itself is left `undefined`, since we don't return a value from the `Person` function. </details>

###### 32. When you click the paragraph, what's the logged output? ```html <div onclick="console.log('div')"> Click here! </div> ``` - A: `p` `div` - B: `div` `p` - C: `p` - D: `div` <details><summary>Answer</summary>

A If we click `p`, we see two logs: `p` and `div`. During event propagation, there are 3 phases: capturing, target, and bubbling. By default, event handlers are executed in the bubbling phase (unless you set `useCapture` to `true`). It goes from the deepest nested element outwards. </details>

###### 5. Which one is true? ```javascript const bird = { size: 'small', }; const mouse = { name: 'Mickey', small: true, }; ``` - A: `mouse.bird.size` is not valid - B: `mouse[bird.size]` is not valid - C: `mouse[bird["size"]]` is not valid - D: All of them are valid <details><summary>Answer</summary>

A In JavaScript, all object keys are strings (unless it's a Symbol). Even though we might not _type_ them as strings, they are always converted into strings under the hood. JavaScript interprets (or unboxes) statements. When we use bracket notation, it sees the first opening bracket `[` and keeps going until it finds the closing bracket `]`. Only then, it will evaluate the statement. `mouse[bird.size]`: First it evaluates `bird.size`, which is `"small"`. `mouse["small"]` returns `true` However, with dot notation, this doesn't happen. `mouse` does not have a key called `bird`, which means that `mouse.bird` is `undefined`. Then, we ask for the `size` using dot notation: `mouse.bird.size`. Since `mouse.bird` is `undefined`, we're actually asking `undefined.size`. This isn't valid, and will throw an error similar to `Cannot read property "size" of undefined`. </details>

###### 6. What's the output? ```javascript let c = { greeting: 'Hey!' }; let d; d = c; c.greeting = 'Hello'; console.log(d.greeting); ``` - A: `Hello` - B: `Hey!` - C: `undefined` - D: `ReferenceError` - E: `TypeError` <details><summary>Answer</summary>

A In JavaScript, all objects interact by _reference_ when setting them equal to each other. First, variable `c` holds a value to an object. Later, we assign `d` with the same reference that `c` has to the object. <img src="https://i.imgur.com/ko5k0fs.png" width="200"> When you change one object, you change all of them. </details>

###### 11. What's the output? ```javascript function Person(firstName, lastName) { this.firstName = firstName; this.lastName = lastName; } const member = new Person('Lydia', 'Hallie'); Person.getFullName = function() { return `${this.firstName} ${this.lastName}`; }; console.log(member.getFullName()); ``` - A: `TypeError` - B: `SyntaxError` - C: `Lydia Hallie` - D: `undefined` `undefined` <details><summary>Answer</summary>

A In JavaScript, functions are objects, and therefore, the method `getFullName` gets added to the constructor function object itself. For that reason, we can call `Person.getFullName()`, but `member.getFullName` throws a `TypeError`. If you want a method to be available to all object instances, you have to add it to the prototype property: ```js Person.prototype.getFullName = function() { return `${this.firstName} ${this.lastName}`; }; ``` </details>

###### 9. What's the output? ```javascript let greeting; greetign = {}; // Typo! console.log(greetign); ``` - A: `{}` - B: `ReferenceError: greetign is not defined` - C: `undefined` <details><summary>Answer</summary>

A It logs the object, because we just created an empty object on the global object! When we mistyped `greeting` as `greetign`, the JS interpreter actually saw this as `global.greetign = {}` (or `window.greetign = {}` in a browser). In order to avoid this, we can use `"use strict"`. This makes sure that you have declared a variable before setting it equal to anything. </details>

###### 42. What does the `setInterval` method return in the browser? ```javascript setInterval(() => console.log('Hi'), 1000); ``` - A: a unique id - B: the amount of milliseconds specified - C: the passed function - D: `undefined` <details><summary>Answer</summary>

A It returns a unique id. This id can be used to clear that interval with the `clearInterval()` function. </details>

###### 39. Everything in JavaScript is either a... - A: primitive or object - B: function or object - C: trick question! only objects - D: number or object <details><summary>Answer</summary>

A JavaScript only has primitive types and objects. Primitive types are `boolean`, `null`, `undefined`, `bigint`, `number`, `string`, and `symbol`. What differentiates a primitive from an object is that primitives do not have any properties or methods; however, you'll note that `'foo'.toUpperCase()` evaluates to `'FOO'` and does not result in a `TypeError`. This is because when you try to access a property or method on a primitive like a string, JavaScript will implicitly wrap the primitive type using one of the wrapper classes, i.e. `String`, and then immediately discard the wrapper after the expression evaluates. All primitives except for `null` and `undefined` exhibit this behaviour. </details>

###### 38. What's the output? ```javascript (() => { let x, y; try { throw new Error(); } catch (x) { (x = 1), (y = 2); console.log(x); } console.log(x); console.log(y); })(); ``` - A: `1` `undefined` `2` - B: `undefined` `undefined` `undefined` - C: `1` `1` `2` - D: `1` `undefined` `undefined` <details><summary>Answer</summary>

A The `catch` block receives the argument `x`. This is not the same `x` as the variable when we pass arguments. This variable `x` is block-scoped. Later, we set this block-scoped variable equal to `1`, and set the value of the variable `y`. Now, we log the block-scoped variable `x`, which is equal to `1`. Outside of the `catch` block, `x` is still `undefined`, and `y` is `2`. When we want to `console.log(x)` outside of the `catch` block, it returns `undefined`, and `y` returns `2`. </details>

###### 58. What's the output? ```javascript const name = 'Lydia'; age = 21; console.log(delete name); console.log(delete age); ``` - A: `false`, `true` - B: `"Lydia"`, `21` - C: `true`, `true` - D: `undefined`, `undefined` <details><summary>Answer</summary>

A The `delete` operator returns a boolean value: `true` on a successful deletion, else it'll return `false`. However, variables declared with the `var`, `const` or `let` keyword cannot be deleted using the `delete` operator. The `name` variable was declared with a `const` keyword, so its deletion is not successful: `false` is returned. When we set `age` equal to `21`, we actually added a property called `age` to the global object. You can successfully delete properties from objects this way, also the global object, so `delete age` returns `true`. </details>

###### 98. What's the output? ```javascript const getList = ([x, ...y]) => [x, y] const getUser = user => { name: user.name, age: user.age } const list = [1, 2, 3, 4] const user = { name: "Lydia", age: 21 } console.log(getList(list)) console.log(getUser(user)) ``` - A: `[1, [2, 3, 4]]` and `SyntaxError` - B: `[1, [2, 3, 4]]` and `{ name: "Lydia", age: 21 }` - C: `[1, 2, 3, 4]` and `{ name: "Lydia", age: 21 }` - D: `Error` and `{ name: "Lydia", age: 21 }` <details><summary>Answer</summary>

A The `getList` function receives an array as its argument. Between the parentheses of the `getList` function, we destructure this array right away. You could see this as: `[x, ...y] = [1, 2, 3, 4]` With the rest parameter `...y`, we put all "remaining" arguments in an array. The remaining arguments are `2`, `3` and `4` in this case. The value of `y` is an array, containing all the rest parameters. The value of `x` is equal to `1` in this case, so when we log `[x, y]`, `[1, [2, 3, 4]]` gets logged. The `getUser` function receives an object. With arrow functions, we don't _have_ to write curly brackets if we just return one value. However, if you want to instantly return an _object_ from an arrow function, you have to write it between parentheses, otherwise everything between the two braces will be interpreted as a block statement. In this case the code between the braces is not a valid JavaScript code, so a `SyntaxError` gets thrown. The following function would have returned an object: `const getUser = user => ({ name: user.name, age: user.age })` </details>

###### 26. The JavaScript global execution context creates two things for you: the global object, and the "this" keyword. - A: true - B: false - C: it depends <details><summary>Answer</summary>

A The base execution context is the global execution context: it's what's accessible everywhere in your code. </details>

###### 62. What's the output? ```javascript const settings = { username: 'lydiahallie', level: 19, health: 90, }; const data = JSON.stringify(settings, ['level', 'health']); console.log(data); ``` - A: `"{"level":19, "health":90}"` - B: `"{"username": "lydiahallie"}"` - C: `"["level", "health"]"` - D: `"{"username": "lydiahallie", "level":19, "health":90}"` <details><summary>Answer</summary>

A The second argument of `JSON.stringify` is the _replacer_. The replacer can either be a function or an array, and lets you control what and how the values should be stringified. If the replacer is an _array_, only the property names included in the array will be added to the JSON string. In this case, only the properties with the names `"level"` and `"health"` are included, `"username"` is excluded. `data` is now equal to `"{"level":19, "health":90}"`. If the replacer is a _function_, this function gets called on every property in the object you're stringifying. The value returned from this function will be the value of the property when it's added to the JSON string. If the value is `undefined`, this property is excluded from the JSON string. </details>

###### 63. What's the output? ```javascript let num = 10; const increaseNumber = () => num++; const increasePassedNumber = number => number++; const num1 = increaseNumber(); const num2 = increasePassedNumber(num1); console.log(num1); console.log(num2); ``` - A: `10`, `10` - B: `10`, `11` - C: `11`, `11` - D: `11`, `12` <details><summary>Answer</summary>

A The unary operator `++` _first returns_ the value of the operand, _then increments_ the value of the operand. The value of `num1` is `10`, since the `increaseNumber` function first returns the value of `num`, which is `10`, and only increments the value of `num` afterwards. `num2` is `10`, since we passed `num1` to the `increasePassedNumber`. `number` is equal to `10`(the value of `num1`. Again, the unary operator `++` _first returns_ the value of the operand, _then increments_ the value of the operand. The value of `number` is `10`, so `num2` is equal to `10`. </details>

###### 4. What's the output? ```javascript +true; !'Lydia'; ``` - A: `1` and `false` - B: `false` and `NaN` - C: `false` and `false` <details><summary>Answer</summary>

A The unary plus tries to convert an operand to a number. `true` is `1`, and `false` is `0`. The string `'Lydia'` is a truthy value. What we're actually asking, is "is this truthy value falsy?". This returns `false`. </details>

###### 35. Which of these values are falsy? ```javascript 0; new Number(0); (''); (' '); new Boolean(false); undefined; ``` - A: `0`, `''`, `undefined` - B: `0`, `new Number(0)`, `''`, `new Boolean(false)`, `undefined` - C: `0`, `''`, `new Boolean(false)`, `undefined` - D: All of them are falsy <details><summary>Answer</summary>

A There are 8 falsy values: - `undefined` - `null` - `NaN` - `false` - `''` (empty string) - `0` - `-0` - `0n` (BigInt(0)) Function constructors, like `new Number` and `new Boolean` are truthy. </details>

###### 10. What happens when we do this? ```javascript function bark() { console.log('Woof!'); } bark.animal = 'dog'; ``` - A: Nothing, this is totally fine! - B: `SyntaxError`. You cannot add properties to a function this way. - C: `"Woof"` gets logged. - D: `ReferenceError` <details><summary>Answer</summary>

A This is possible in JavaScript, because functions are objects! (Everything besides primitive types are objects) A function is a special type of object. The code you write yourself isn't the actual function. The function is an object with properties. This property is invocable. </details>

###### 55. What's the output? ```javascript class Dog { constructor(name) { this.name = name; } } Dog.prototype.bark = function() { console.log(`Woof I am ${this.name}`); }; const pet = new Dog('Mara'); pet.bark(); delete Dog.prototype.bark; pet.bark(); ``` - A: `"Woof I am Mara"`, `TypeError` - B: `"Woof I am Mara"`, `"Woof I am Mara"` - C: `"Woof I am Mara"`, `undefined` - D: `TypeError`, `TypeError` <details><summary>Answer</summary>

A We can delete properties from objects using the `delete` keyword, also on the prototype. By deleting a property on the prototype, it is not available anymore in the prototype chain. In this case, the `bark` function is not available anymore on the prototype after `delete Dog.prototype.bark`, yet we still try to access it. When we try to invoke something that is not a function, a `TypeError` is thrown. In this case `TypeError: pet.bark is not a function`, since `pet.bark` is `undefined`. </details>

###### 83. What is the output? ```javascript const person = { name: 'Lydia', age: 21, }; let city = person.city; city = 'Amsterdam'; console.log(person); ``` - A: `{ name: "Lydia", age: 21 }` - B: `{ name: "Lydia", age: 21, city: "Amsterdam" }` - C: `{ name: "Lydia", age: 21, city: undefined }` - D: `"Amsterdam"` <details><summary>Answer</summary>

A We set the variable `city` equal to the value of the property called `city` on the `person` object. There is no property on this object called `city`, so the variable `city` has the value of `undefined`. Note that we are _not_ referencing the `person` object itself! We simply set the variable `city` equal to the current value of the `city` property on the `person` object. Then, we set `city` equal to the string `"Amsterdam"`. This doesn't change the person object: there is no reference to that object. When logging the `person` object, the unmodified object gets returned. </details>

###### 86. Which option is a way to set `hasName` equal to `true`, provided you cannot pass `true` as an argument? ```javascript function getName(name) { const hasName = // } ``` - A: `!!name` - B: `name` - C: `new Boolean(name)` - D: `name.length` <details><summary>Answer</summary>

A With `!!name`, we determine whether the value of `name` is truthy or falsy. If name is truthy, which we want to test for, `!name` returns `false`. `!false` (which is what `!!name` practically is) returns `true`. By setting `hasName` equal to `name`, you set `hasName` equal to whatever value you passed to the `getName` function, not the boolean value `true`. `new Boolean(true)` returns an object wrapper, not the boolean value itself. `name.length` returns the length of the passed argument, not whether it's `true`. </details>

###### 79. What is the output? ```javascript const myLifeSummedUp = ['☕', '💻', '🍷', '🍫']; for (let item in myLifeSummedUp) { console.log(item); } for (let item of myLifeSummedUp) { console.log(item); } ``` - A: `0` `1` `2` `3` and `"☕"` `"💻"` `"🍷"` `"🍫"` - B: `"☕"` `"💻"` `"🍷"` `"🍫"` and `"☕"` `"💻"` `"🍷"` `"🍫"` - C: `"☕"` `"💻"` `"🍷"` `"🍫"` and `0` `1` `2` `3` - D: `0` `1` `2` `3` and `{0: "☕", 1: "💻", 2: "🍷", 3: "🍫"}` <details><summary>Answer</summary>

A With a _for-in_ loop, we can iterate over **enumerable** properties. In an array, the enumerable properties are the "keys" of array elements, which are actually their indexes. You could see an array as: `{0: "☕", 1: "💻", 2: "🍷", 3: "🍫"}` Where the keys are the enumerable properties. `0` `1` `2` `3` get logged. With a _for-of_ loop, we can iterate over **iterables**. An array is an iterable. When we iterate over the array, the variable "item" is equal to the element it's currently iterating over, `"☕"` `"💻"` `"🍷"` `"🍫"` get logged. </details>

###### 70. What's the output? ```javascript console.log('' + '💻'); ``` - A: `"💻"` - B: `257548` - C: A string containing their code points - D: Error <details><summary>Answer</summary>

A With the `+` operator, you can concatenate strings. In this case, we are concatenating the string `""` with the string `"💻"`, resulting in `"💻"`. </details>

###### 89. What's the output? ```javascript // module.js export default () => 'Hello world'; export const name = 'Lydia'; // index.js import * as data from './module'; console.log(data); ``` - A: `{ default: function default(), name: "Lydia" }` - B: `{ default: function default() }` - C: `{ default: "Hello world", name: "Lydia" }` - D: Global object of `module.js` <details><summary>Answer</summary>

A With the `import * as name` syntax, we import _all exports_ from the `module.js` file into the `index.js` file as a new object called `data` is created. In the `module.js` file, there are two exports: the default export, and a named export. The default export is a function which returns the string `"Hello World"`, and the named export is a variable called `name` which has the value of the string `"Lydia"`. The `data` object has a `default` property for the default export, other properties have the names of the named exports and their corresponding values. </details>

###### 93. What's the output? ```javascript const person = { name: 'Lydia', age: 21, }; for (const [x, y] of Object.entries(person)) { console.log(x, y); } ``` - A: `name` `Lydia` and `age` `21` - B: `["name", "Lydia"]` and `["age", 21]` - C: `["name", "age"]` and `undefined` - D: `Error` <details><summary>Answer</summary>

A `Object.entries(person)` returns an array of nested arrays, containing the keys and objects: `[ [ 'name', 'Lydia' ], [ 'age', 21 ] ]` Using the `for-of` loop, we can iterate over each element in the array, the subarrays in this case. We can destructure the subarrays instantly in the for-of loop, using `const [x, y]`. `x` is equal to the first element in the subarray, `y` is equal to the second element in the subarray. The first subarray is `[ "name", "Lydia" ]`, with `x` equal to `"name"`, and `y` equal to `"Lydia"`, which get logged. The second subarray is `[ "age", 21 ]`, with `x` equal to `"age"`, and `y` equal to `21`, which get logged. </details>

###### 28. What's the output? ```javascript String.prototype.giveLydiaPizza = () => { return 'Just give Lydia pizza already!'; }; const name = 'Lydia'; name.giveLydiaPizza(); ``` - A: `"Just give Lydia pizza already!"` - B: `TypeError: not a function` - C: `SyntaxError` - D: `undefined` <details><summary>Answer</summary>

A `String` is a built-in constructor, which we can add properties to. I just added a method to its prototype. Primitive strings are automatically converted into a string object, generated by the string prototype function. So, all strings (string objects) have access to that method! </details>

###### 21. What's the value of `sum`? ```javascript const sum = eval('10*10+5'); ``` - A: `105` - B: `"105"` - C: `TypeError` - D: `"10*10+5"` <details><summary>Answer</summary>

A `eval` evaluates codes that's passed as a string. If it's an expression, like in this case, it evaluates the expression. The expression is `10 * 10 + 5`. This returns the number `105`. </details>

###### 54. What's the output? ```javascript (() => { let x = (y = 10); })(); console.log(typeof x); console.log(typeof y); ``` - A: `"undefined", "number"` - B: `"number", "number"` - C: `"object", "number"` - D: `"number", "undefined"` <details><summary>Answer</summary>

A `let x = (y = 10);` is actually shorthand for: ```javascript y = 10; let x = y; ``` When we set `y` equal to `10`, we actually add a property `y` to the global object (`window` in browser, `global` in Node). In a browser, `window.y` is now equal to `10`. Then, we declare a variable `x` with the value of `y`, which is `10`. Variables declared with the `let` keyword are _block scoped_, they are only defined within the block they're declared in; the immediately invoked function expression (IIFE) in this case. When we use the `typeof` operator, the operand `x` is not defined: we are trying to access `x` outside of the block it's declared in. This means that `x` is not defined. Values who haven't been assigned a value or declared are of type `"undefined"`. `console.log(typeof x)` returns `"undefined"`. However, we created a global variable `y` when setting `y` equal to `10`. This value is accessible anywhere in our code. `y` is defined, and holds a value of type `"number"`. `console.log(typeof y)` returns `"number"`. </details>

###### 14. All object have prototypes. - A: true - B: false <details><summary>Answer</summary>

B All objects have prototypes, except for the **base object**. The base object is the object created by the user, or an object that is created using the `new` keyword. The base object has access to some methods and properties, such as `.toString`. This is the reason why you can use built-in JavaScript methods! All of such methods are available on the prototype. Although JavaScript can't find it directly on your object, it goes down the prototype chain and finds it there, which makes it accessible for you. </details>

###### 81. What is the output? ```javascript function sayHi(name) { return `Hi there, ${name}`; } console.log(sayHi()); ``` - A: `Hi there,` - B: `Hi there, undefined` - C: `Hi there, null` - D: `ReferenceError` <details><summary>Answer</summary>

B By default, arguments have the value of `undefined`, unless a value has been passed to the function. In this case, we didn't pass a value for the `name` argument. `name` is equal to `undefined` which gets logged. In ES6, we can overwrite this default `undefined` value with default parameters. For example: `function sayHi(name = "Lydia") { ... }` In this case, if we didn't pass a value or if we passed `undefined`, `name` would always be equal to the string `Lydia` </details>

###### 17. What's the output? ```javascript function getPersonInfo(one, two, three) { console.log(one); console.log(two); console.log(three); } const person = 'Lydia'; const age = 21; getPersonInfo`${person} is ${age} years old`; ``` - A: `"Lydia"` `21` `["", " is ", " years old"]` - B: `["", " is ", " years old"]` `"Lydia"` `21` - C: `"Lydia"` `["", " is ", " years old"]` `21` <details><summary>Answer</summary>

B If you use tagged template literals, the value of the first argument is always an array of the string values. The remaining arguments get the values of the passed expressions! </details>

###### 95. What's the output? ```javascript function nums(a, b) { if (a > b) console.log('a is bigger'); else console.log('b is bigger'); return a + b; } console.log(nums(4, 2)); console.log(nums(1, 2)); ``` - A: `a is bigger`, `6` and `b is bigger`, `3` - B: `a is bigger`, `undefined` and `b is bigger`, `undefined` - C: `undefined` and `undefined` - D: `SyntaxError` <details><summary>Answer</summary>

B In JavaScript, we don't _have_ to write the semicolon (`;`) explicitly, however the JavaScript engine still adds them after statements. This is called **Automatic Semicolon Insertion**. A statement can for example be variables, or keywords like `throw`, `return`, `break`, etc. Here, we wrote a `return` statement, and another value `a + b` on a _new line_. However, since it's a new line, the engine doesn't know that it's actually the value that we wanted to return. Instead, it automatically added a semicolon after `return`. You could see this as: ```javascript return; a + b; ``` This means that `a + b` is never reached, since a function stops running after the `return` keyword. If no value gets returned, like here, the function returns `undefined`. Note that there is no automatic insertion after `if/else` statements! </details>

###### 66. With which constructor can we successfully extend the `Dog` class? ```javascript class Dog { constructor(name) { this.name = name; } }; class Labrador extends Dog { // 1 constructor(name, size) { this.size = size; } // 2 constructor(name, size) { super(name); this.size = size; } // 3 constructor(size) { super(name); this.size = size; } // 4 constructor(name, size) { this.name = name; this.size = size; } }; ``` - A: 1 - B: 2 - C: 3 - D: 4 <details><summary>Answer</summary>

B In a derived class, you cannot access the `this` keyword before calling `super`. If you try to do that, it will throw a ReferenceError: 1 and 4 would throw a reference error. With the `super` keyword, we call that parent class's constructor with the given arguments. The parent's constructor receives the `name` argument, so we need to pass `name` to `super`. The `Labrador` class receives two arguments, `name` since it extends `Dog`, and `size` as an extra property on the `Labrador` class. They both need to be passed to the constructor function on `Labrador`, which is done correctly using constructor 2. </details>

###### 87. What's the output? ```javascript console.log('I want pizza'[0]); ``` - A: `"""` - B: `"I"` - C: `SyntaxError` - D: `undefined` <details><summary>Answer</summary>

B In order to get a character at a specific index of a string, you can use bracket notation. The first character in the string has index 0, and so on. In this case, we want to get the element with index 0, the character `"I'`, which gets logged. Note that this method is not supported in IE7 and below. In that case, use `.charAt()`. </details>

###### 60. What's the output? ```javascript const user = { name: 'Lydia', age: 21 }; const admin = { admin: true, ...user }; console.log(admin); ``` - A: `{ admin: true, user: { name: "Lydia", age: 21 } }` - B: `{ admin: true, name: "Lydia", age: 21 }` - C: `{ admin: true, user: ["Lydia", 21] }` - D: `{ admin: true }` <details><summary>Answer</summary>

B It's possible to combine objects using the spread operator `...`. It lets you create copies of the key/value pairs of one object, and add them to another object. In this case, we create copies of the `user` object, and add them to the `admin` object. The `admin` object now contains the copied key/value pairs, which results in `{ admin: true, name: "Lydia", age: 21 }`. </details>

###### 3. What's the output? ```javascript const shape = { radius: 10, diameter() { return this.radius * 2; }, perimeter: () => 2 * Math.PI * this.radius, }; console.log(shape.diameter()); console.log(shape.perimeter()); ``` - A: `20` and `62.83185307179586` - B: `20` and `NaN` - C: `20` and `63` - D: `NaN` and `63` <details><summary>Answer</summary>

B Note that the value of `diameter` is a regular function, whereas the value of `perimeter` is an arrow function. With arrow functions, the `this` keyword refers to its current surrounding scope, unlike regular functions! This means that when we call `perimeter`, it doesn't refer to the shape object, but to its surrounding scope (window for example). There is no value `radius` on that object, which returns `NaN`. </details>

###### 29. What's the output? ```javascript const a = {}; const b = { key: 'b' }; const c = { key: 'c' }; a[b] = 123; a[c] = 456; console.log(a[b]); ``` - A: `123` - B: `456` - C: `undefined` - D: `ReferenceError` <details><summary>Answer</summary>

B Object keys are automatically converted into strings. We are trying to set an object as a key to object `a`, with the value of `123`. However, when we stringify an object, it becomes `"[object Object]"`. So what we are saying here, is that `a["[object Object]"] = 123`. Then, we can try to do the same again. `c` is another object that we are implicitly stringifying. So then, `a["[object Object]"] = 456`. Then, we log `a[b]`, which is actually `a["[object Object]"]`. We just set that to `456`, so it returns `456`. </details>

###### 48. What's the output? ```javascript console.log(3 + 4 + '5'); ``` - A: `"345"` - B: `"75"` - C: `12` - D: `"12"` <details><summary>Answer</summary>

B Operator associativity is the order in which the compiler evaluates the expressions, either left-to-right or right-to-left. This only happens if all operators have the _same_ precedence. We only have one type of operator: `+`. For addition, the associativity is left-to-right. `3 + 4` gets evaluated first. This results in the number `7`. `7 + '5'` results in `"75"` because of coercion. JavaScript converts the number `7` into a string, see question 15. We can concatenate two strings using the `+`operator. `"7" + "5"` results in `"75"`. </details>

###### 74. What's the output? ```javascript function addToList(item, list) { return list.push(item); } const result = addToList('apple', ['banana']); console.log(result); ``` - A: `['apple', 'banana']` - B: `2` - C: `true` - D: `undefined` <details><summary>Answer</summary>

B The `.push()` method returns the _length_ of the new array! Previously, the array contained one element (the string `"banana"`) and had a length of `1`. After adding the string `"apple"` to the array, the array contains two elements, and has a length of `2`. This gets returned from the `addToList` function. The `push` method modifies the original array. If you wanted to return the _array_ from the function rather than the _length of the array_, you should have returned `list` after pushing `item` to it. </details>

###### 34. What's the output? ```javascript function sayHi() { return (() => 0)(); } console.log(typeof sayHi()); ``` - A: `"object"` - B: `"number"` - C: `"function"` - D: `"undefined"` <details><summary>Answer</summary>

B The `sayHi` function returns the returned value of the immediately invoked function expression (IIFE). This function returned `0`, which is type `"number"`. FYI: there are only 7 built-in types: `null`, `undefined`, `boolean`, `number`, `string`, `object`, and `symbol`. `"function"` is not a type, since functions are objects, it's of type `"object"`. </details>

###### 22. How long is cool_secret accessible? ```javascript sessionStorage.setItem('cool_secret', 123); ``` - A: Forever, the data doesn't get lost. - B: When the user closes the tab. - C: When the user closes the entire browser, not only the tab. - D: When the user shuts off their computer. <details><summary>Answer</summary>

B The data stored in `sessionStorage` is removed after closing the _tab_. If you used `localStorage`, the data would've been there forever, unless for example `localStorage.clear()` is invoked. </details>

###### 82. What is the output? ```javascript var status = '😎'; setTimeout(() => { const status = '😍'; const data = { status: '', getStatus() { return this.status; }, }; console.log(data.getStatus()); console.log(data.getStatus.call(this)); }, 0); ``` - A: `""` and `"😍"` - B: `""` and `"😎"` - C: `"😍"` and `"😎"` - D: `"😎"` and `"😎"` <details><summary>Answer</summary>

B The value of the `this` keyword is dependent on where you use it. In a **method**, like the `getStatus` method, the `this` keyword refers to _the object that the method belongs to_. The method belongs to the `data` object, so `this` refers to the `data` object. When we log `this.status`, the `status` property on the `data` object gets logged, which is `""`. With the `call` method, we can change the object to which the `this` keyword refers. In **functions**, the `this` keyword refers to the _the object that the function belongs to_. We declared the `setTimeout` function on the _global object_, so within the `setTimeout` function, the `this` keyword refers to the _global object_. On the global object, there is a variable called _status_ with the value of `"😎"`. When logging `this.status`, `"😎"` gets logged. </details>

###### 96. What's the output? ```javascript class Person { constructor() { this.name = 'Lydia'; } } Person = class AnotherPerson { constructor() { this.name = 'Sarah'; } }; const member = new Person(); console.log(member.name); ``` - A: `"Lydia"` - B: `"Sarah"` - C: `Error: cannot redeclare Person` - D: `SyntaxError` <details><summary>Answer</summary>

B We can set classes equal to other classes/function constructors. In this case, we set `Person` equal to `AnotherPerson`. The name on this constructor is `Sarah`, so the name property on the new `Person` instance `member` is `"Sarah"`. </details>

###### 30. What's the output? ```javascript const foo = () => console.log('First'); const bar = () => setTimeout(() => console.log('Second')); const baz = () => console.log('Third'); bar(); foo(); baz(); ``` - A: `First` `Second` `Third` - B: `First` `Third` `Second` - C: `Second` `First` `Third` - D: `Second` `Third` `First` <details><summary>Answer</summary>

B We have a `setTimeout` function and invoked it first. Yet, it was logged last. This is because in browsers, we don't just have the runtime engine, we also have something called a `WebAPI`. The `WebAPI` gives us the `setTimeout` function to start with, and for example the DOM. After the _callback_ is pushed to the WebAPI, the `setTimeout` function itself (but not the callback!) is popped off the stack. <img src="https://i.imgur.com/X5wsHOg.png" width="200"> Now, `foo` gets invoked, and `"First"` is being logged. <img src="https://i.imgur.com/Pvc0dGq.png" width="200"> `foo` is popped off the stack, and `baz` gets invoked. `"Third"` gets logged. <img src="https://i.imgur.com/WhA2bCP.png" width="200"> The WebAPI can't just add stuff to the stack whenever it's ready. Instead, it pushes the callback function to something called the _queue_. <img src="https://i.imgur.com/NSnDZmU.png" width="200"> This is where an event loop starts to work. An **event loop** looks at the stack and task queue. If the stack is empty, it takes the first thing on the queue and pushes it onto the stack. <img src="https://i.imgur.com/uyiScAI.png" width="200"> `bar` gets invoked, `"Second"` gets logged, and it's popped off the stack. </details>

###### 45. What does this return? ```javascript const firstPromise = new Promise((res, rej) => { setTimeout(res, 500, 'one'); }); const secondPromise = new Promise((res, rej) => { setTimeout(res, 100, 'two'); }); Promise.race([firstPromise, secondPromise]).then(res => console.log(res)); ``` - A: `"one"` - B: `"two"` - C: `"two" "one"` - D: `"one" "two"` <details><summary>Answer</summary>

B When we pass multiple promises to the `Promise.race` method, it resolves/rejects the _first_ promise that resolves/rejects. To the `setTimeout` method, we pass a timer: 500ms for the first promise (`firstPromise`), and 100ms for the second promise (`secondPromise`). This means that the `secondPromise` resolves first with the value of `'two'`. `res` now holds the value of `'two'`, which gets logged. </details>

###### 53. What's the output? ```javascript function Car() { this.make = 'Lamborghini'; return { make: 'Maserati' }; } const myCar = new Car(); console.log(myCar.make); ``` - A: `"Lamborghini"` - B: `"Maserati"` - C: `ReferenceError` - D: `TypeError` <details><summary>Answer</summary>

B When you return a property, the value of the property is equal to the _returned_ value, not the value set in the constructor function. We return the string `"Maserati"`, so `myCar.make` is equal to `"Maserati"`. </details>

###### 47. What's the output? ```javascript const person = { name: 'Lydia', age: 21, }; for (const item in person) { console.log(item); } ``` - A: `{ name: "Lydia" }, { age: 21 }` - B: `"name", "age"` - C: `"Lydia", 21` - D: `["name", "Lydia"], ["age", 21]` <details><summary>Answer</summary>

B With a `for-in` loop, we can iterate through object keys, in this case `name` and `age`. Under the hood, object keys are strings (if they're not a Symbol). On every loop, we set the value of `item` equal to the current key it's iterating over. First, `item` is equal to `name`, and gets logged. Then, `item` is equal to `age`, which gets logged. </details>

###### 61. What's the output? ```javascript const person = { name: 'Lydia' }; Object.defineProperty(person, 'age', { value: 21 }); console.log(person); console.log(Object.keys(person)); ``` - A: `{ name: "Lydia", age: 21 }`, `["name", "age"]` - B: `{ name: "Lydia", age: 21 }`, `["name"]` - C: `{ name: "Lydia"}`, `["name", "age"]` - D: `{ name: "Lydia"}`, `["age"]` <details><summary>Answer</summary>

B With the `defineProperty` method, we can add new properties to an object, or modify existing ones. When we add a property to an object using the `defineProperty` method, they are by default _not enumerable_. The `Object.keys` method returns all _enumerable_ property names from an object, in this case only `"name"`. Properties added using the `defineProperty` method are immutable by default. You can override this behavior using the `writable`, `configurable` and `enumerable` properties. This way, the `defineProperty` method gives you a lot more control over the properties you're adding to an object. </details>

###### 57. What's the output? ```javascript // counter.js let counter = 10; export default counter; ``` ```javascript // index.js import myCounter from './counter'; myCounter += 1; console.log(myCounter); ``` - A: `10` - B: `11` - C: `Error` - D: `NaN` <details><summary>Answer</summary>

C An imported module is _read-only_: you cannot modify the imported module. Only the module that exports them can change its value. When we try to increment the value of `myCounter`, it throws an error: `myCounter` is read-only and cannot be modified. </details>

###### 67. What's the output? ```javascript // index.js console.log('running index.js'); import { sum } from './sum.js'; console.log(sum(1, 2)); // sum.js console.log('running sum.js'); export const sum = (a, b) => a + b; ``` - A: `running index.js`, `running sum.js`, `3` - B: `running sum.js`, `running index.js`, `3` - C: `running sum.js`, `3`, `running index.js` - D: `running index.js`, `undefined`, `running sum.js` <details><summary>Answer</summary>

B With the `import` keyword, all imported modules are _pre-parsed_. This means that the imported modules get run _first_, the code in the file which imports the module gets executed _after_. This is a difference between `require()` in CommonJS and `import`! With `require()`, you can load dependencies on demand while the code is being run. If we would have used `require` instead of `import`, `running index.js`, `running sum.js`, `3` would have been logged to the console. </details>

###### 23. What's the output? ```javascript var num = 8; var num = 10; console.log(num); ``` - A: `8` - B: `10` - C: `SyntaxError` - D: `ReferenceError` <details><summary>Answer</summary>

B With the `var` keyword, you can declare multiple variables with the same name. The variable will then hold the latest value. You cannot do this with `let` or `const` since they're block-scoped. </details>

###### 88. What's the output? ```javascript function sum(num1, num2 = num1) { console.log(num1 + num2); } sum(10); ``` - A: `NaN` - B: `20` - C: `ReferenceError` - D: `undefined` <details><summary>Answer</summary>

B You can set a default parameter's value equal to another parameter of the function, as long as they've been defined _before_ the default parameter. We pass the value `10` to the `sum` function. If the `sum` function only receives 1 argument, it means that the value for `num2` is not passed, and the value of `num1` is equal to the passed value `10` in this case. The default value of `num2` is the value of `num1`, which is `10`. `num1 + num2` returns `20`. If you're trying to set a default parameter's value equal to a parameter which is defined _after_ (to the right), the parameter's value hasn't been initialized yet, which will throw an error. </details>

###### 75. What's the output? ```javascript const box = { x: 10, y: 20 }; Object.freeze(box); const shape = box; shape.x = 100; console.log(shape); ``` - A: `{ x: 100, y: 20 }` - B: `{ x: 10, y: 20 }` - C: `{ x: 100 }` - D: `ReferenceError` <details><summary>Answer</summary>

B `Object.freeze` makes it impossible to add, remove, or modify properties of an object (unless the property's value is another object). When we create the variable `shape` and set it equal to the frozen object `box`, `shape` also refers to a frozen object. You can check whether an object is frozen by using `Object.isFrozen`. In this case, `Object.isFrozen(shape)` returns true, since the variable `shape` has a reference to a frozen object. Since `shape` is frozen, and since the value of `x` is not an object, we cannot modify the property `x`. `x` is still equal to `10`, and `{ x: 10, y: 20 }` gets logged. </details>

###### 100. What's the value of output? ```javascript // 🎉✨ This is my 100th question! ✨🎉 const output = `${[] && 'Im'}possible! You should${'' && `n't`} see a therapist after so much JavaScript lol`; ``` - A: `possible! You should see a therapist after so much JavaScript lol` - B: `Impossible! You should see a therapist after so much JavaScript lol` - C: `possible! You shouldn't see a therapist after so much JavaScript lol` - D: `Impossible! You shouldn't see a therapist after so much JavaScript lol` <details><summary>Answer</summary>

B `[]` is a truthy value. With the `&&` operator, the right-hand value will be returned if the left-hand value is a truthy value. In this case, the left-hand value `[]` is a truthy value, so `"Im'` gets returned. `""` is a falsy value. If the left-hand value is falsy, nothing gets returned. `n't` doesn't get returned. </details>

###### 41. What's the output? ```javascript !!null; !!''; !!1; ``` - A: `false` `true` `false` - B: `false` `false` `true` - C: `false` `true` `true` - D: `true` `true` `false` <details><summary>Answer</summary>

B `null` is falsy. `!null` returns `true`. `!true` returns `false`. `""` is falsy. `!""` returns `true`. `!true` returns `false`. `1` is truthy. `!1` returns `false`. `!false` returns `true`. </details>

###### 36. What's the output? ```javascript console.log(typeof typeof 1); ``` - A: `"number"` - B: `"string"` - C: `"object"` - D: `"undefined"` <details><summary>Answer</summary>

B `typeof 1` returns `"number"`. `typeof "number"` returns `"string"` </details>

###### 24. What's the output? ```javascript const obj = { 1: 'a', 2: 'b', 3: 'c' }; const set = new Set([1, 2, 3, 4, 5]); obj.hasOwnProperty('1'); obj.hasOwnProperty(1); set.has('1'); set.has(1); ``` - A: `false` `true` `false` `true` - B: `false` `true` `true` `true` - C: `true` `true` `false` `true` - D: `true` `true` `true` `true` <details><summary>Answer</summary>

C All object keys (excluding Symbols) are strings under the hood, even if you don't type it yourself as a string. This is why `obj.hasOwnProperty('1')` also returns true. It doesn't work that way for a set. There is no `'1'` in our set: `set.has('1')` returns `false`. It has the numeric type `1`, `set.has(1)` returns `true`. </details>

###### 73. What's the output? ```javascript async function getData() { return await Promise.resolve('I made it!'); } const data = getData(); console.log(data); ``` - A: `"I made it!"` - B: `Promise {<resolved>: "I made it!"}` - C: `Promise {<pending>}` - D: `undefined` <details><summary>Answer</summary>

C An async function always returns a promise. The `await` still has to wait for the promise to resolve: a pending promise gets returned when we call `getData()` in order to set `data` equal to it. If we wanted to get access to the resolved value `"I made it"`, we could have used the `.then()` method on `data`: `data.then(res => console.log(res))` This would've logged `"I made it!"` </details>

###### 80. What is the output? ```javascript const list = [1 + 2, 1 * 2, 1 / 2]; console.log(list); ``` - A: `["1 + 2", "1 * 2", "1 / 2"]` - B: `["12", 2, 0.5]` - C: `[3, 2, 0.5]` - D: `[1, 1, 1]` <details><summary>Answer</summary>

C Array elements can hold any value. Numbers, strings, objects, other arrays, null, boolean values, undefined, and other expressions such as dates, functions, and calculations. The element will be equal to the returned value. `1 + 2` returns `3`, `1 * 2` returns `2`, and `1 / 2` returns `0.5`. </details>

###### 2. What's the output? ```javascript for (var i = 0; i < 3; i++) { setTimeout(() => console.log(i), 1); } for (let i = 0; i < 3; i++) { setTimeout(() => console.log(i), 1); } ``` - A: `0 1 2` and `0 1 2` - B: `0 1 2` and `3 3 3` - C: `3 3 3` and `0 1 2` <details><summary>Answer</summary>

C Because of the event queue in JavaScript, the `setTimeout` callback function is called _after_ the loop has been executed. Since the variable `i` in the first loop was declared using the `var` keyword, this value was global. During the loop, we incremented the value of `i` by `1` each time, using the unary operator `++`. By the time the `setTimeout` callback function was invoked, `i` was equal to `3` in the first example. In the second loop, the variable `i` was declared using the `let` keyword: variables declared with the `let` (and `const`) keyword are block-scoped (a block is anything between `{ }`). During each iteration, `i` will have a new value, and each value is scoped inside the loop. </details>

###### 90. What's the output? ```javascript class Person { constructor(name) { this.name = name; } } const member = new Person('John'); console.log(typeof member); ``` - A: `"class"` - B: `"function"` - C: `"object"` - D: `"string"` <details><summary>Answer</summary>

C Classes are syntactical sugar for function constructors. The equivalent of the `Person` class as a function constructor would be: ```javascript function Person() { this.name = name; } ``` Calling a function constructor with `new` results in the creation of an instance of `Person`, `typeof` keyword returns `"object"` for an instance. `typeof member` returns `"object"`. </details>

###### 25. What's the output? ```javascript const obj = { a: 'one', b: 'two', a: 'three' }; console.log(obj); ``` - A: `{ a: "one", b: "two" }` - B: `{ b: "two", a: "three" }` - C: `{ a: "three", b: "two" }` - D: `SyntaxError` <details><summary>Answer</summary>

C If you have two keys with the same name, the key will be replaced. It will still be in its first position, but with the last specified value. </details>

###### 64. What's the output? ```javascript const value = { number: 10 }; const multiply = (x = { ...value }) => { console.log((x.number *= 2)); }; multiply(); multiply(); multiply(value); multiply(value); ``` - A: `20`, `40`, `80`, `160` - B: `20`, `40`, `20`, `40` - C: `20`, `20`, `20`, `40` - D: `NaN`, `NaN`, `20`, `40` <details><summary>Answer</summary>

C In ES6, we can initialize parameters with a default value. The value of the parameter will be the default value, if no other value has been passed to the function, or if the value of the parameter is `"undefined"`. In this case, we spread the properties of the `value` object into a new object, so `x` has the default value of `{ number: 10 }`. The default argument is evaluated at _call time_! Every time we call the function, a _new_ object is created. We invoke the `multiply` function the first two times without passing a value: `x` has the default value of `{ number: 10 }`. We then log the multiplied value of that number, which is `20`. The third time we invoke multiply, we do pass an argument: the object called `value`. The `*=` operator is actually shorthand for `x.number = x.number * 2`: we modify the value of `x.number`, and log the multiplied value `20`. The fourth time, we pass the `value` object again. `x.number` was previously modified to `20`, so `x.number *= 2` logs `40`. </details>

###### 15. What's the output? ```javascript function sum(a, b) { return a + b; } sum(1, '2'); ``` - A: `NaN` - B: `TypeError` - C: `"12"` - D: `3` <details><summary>Answer</summary>

C JavaScript is a **dynamically typed language**: we don't specify what types certain variables are. Values can automatically be converted into another type without you knowing, which is called _implicit type coercion_. **Coercion** is converting from one type into another. In this example, JavaScript converts the number `1` into a string, in order for the function to make sense and return a value. During the addition of a numeric type (`1`) and a string type (`'2'`), the number is treated as a string. We can concatenate strings like `"Hello" + "World"`, so what's happening here is `"1" + "2"` which returns `"12"`. </details>

###### 49. What's the value of `num`? ```javascript const num = parseInt('7*6', 10); ``` - A: `42` - B: `"42"` - C: `7` - D: `NaN` <details><summary>Answer</summary>

C Only the first numbers in the string is returned. Based on the _radix_ (the second argument in order to specify what type of number we want to parse it to: base 10, hexadecimal, octal, binary, etc.), the `parseInt` checks whether the characters in the string are valid. Once it encounters a character that isn't a valid number in the radix, it stops parsing and ignores the following characters. `*` is not a valid number. It only parses `"7"` into the decimal `7`. `num` now holds the value of `7`. </details>

###### 44. What's the output? ```javascript function* generator(i) { yield i; yield i * 2; } const gen = generator(10); console.log(gen.next().value); console.log(gen.next().value); ``` - A: `[0, 10], [10, 20]` - B: `20, 20` - C: `10, 20` - D: `0, 10 and 10, 20` <details><summary>Answer</summary>

C Regular functions cannot be stopped mid-way after invocation. However, a generator function can be "stopped" midway, and later continue from where it stopped. Every time a generator function encounters a `yield` keyword, the function yields the value specified after it. Note that the generator function in that case doesn't _return_ the value, it _yields_ the value. First, we initialize the generator function with `i` equal to `10`. We invoke the generator function using the `next()` method. The first time we invoke the generator function, `i` is equal to `10`. It encounters the first `yield` keyword: it yields the value of `i`. The generator is now "paused", and `10` gets logged. Then, we invoke the function again with the `next()` method. It starts to continue where it stopped previously, still with `i` equal to `10`. Now, it encounters the next `yield` keyword, and yields `i * 2`. `i` is equal to `10`, so it returns `10 * 2`, which is `20`. This results in `10, 20`. </details>

###### 16. What's the output? ```javascript let number = 0; console.log(number++); console.log(++number); console.log(number); ``` - A: `1` `1` `2` - B: `1` `2` `2` - C: `0` `2` `2` - D: `0` `1` `2` <details><summary>Answer</summary>

C The **postfix** unary operator `++`: 1. Returns the value (this returns `0`) 2. Increments the value (number is now `1`) The **prefix** unary operator `++`: 1. Increments the value (number is now `2`) 2. Returns the value (this returns `2`) This returns `0 2 2`. </details>

###### 78. What is the output? ```javascript const add = () => { const cache = {}; return num => { if (num in cache) { return `From cache! ${cache[num]}`; } else { const result = num + 10; cache[num] = result; return `Calculated! ${result}`; } }; }; const addFunction = add(); console.log(addFunction(10)); console.log(addFunction(10)); console.log(addFunction(5 * 2)); ``` - A: `Calculated! 20` `Calculated! 20` `Calculated! 20` - B: `Calculated! 20` `From cache! 20` `Calculated! 20` - C: `Calculated! 20` `From cache! 20` `From cache! 20` - D: `Calculated! 20` `From cache! 20` `Error` <details><summary>Answer</summary>

C The `add` function is a _memoized_ function. With memoization, we can cache the results of a function in order to speed up its execution. In this case, we create a `cache` object that stores the previously returned values. If we call the `addFunction` function again with the same argument, it first checks whether it has already gotten that value in its cache. If that's the case, the caches value will be returned, which saves on execution time. Else, if it's not cached, it will calculate the value and store it afterwards. We call the `addFunction` function three times with the same value: on the first invocation, the value of the function when `num` is equal to `10` isn't cached yet. The condition of the if-statement `num in cache` returns `false`, and the else block gets executed: `Calculated! 20` gets logged, and the value of the result gets added to the cache object. `cache` now looks like `{ 10: 20 }`. The second time, the `cache` object contains the value that gets returned for `10`. The condition of the if-statement `num in cache` returns `true`, and `'From cache! 20'` gets logged. The third time, we pass `5 * 2` to the function which gets evaluated to `10`. The `cache` object contains the value that gets returned for `10`. The condition of the if-statement `num in cache` returns `true`, and `'From cache! 20'` gets logged. </details>

###### 27. What's the output? ```javascript for (let i = 1; i < 5; i++) { if (i === 3) continue; console.log(i); } ``` - A: `1` `2` - B: `1` `2` `3` - C: `1` `2` `4` - D: `1` `3` `4` <details><summary>Answer</summary>

C The `continue` statement skips an iteration if a certain condition returns `true`. </details>

###### 31. What is the event.target when clicking the button? ```html <div onclick="console.log('first div')"> <div onclick="console.log('second div')"> <button onclick="console.log('button')"> Click! </button> </div> </div> ``` - A: Outer `div` - B: Inner `div` - C: `button` - D: An array of all nested elements. <details><summary>Answer</summary>

C The deepest nested element that caused the event is the target of the event. You can stop bubbling by `event.stopPropagation` </details>

###### 19. What's the output? ```javascript function getAge(...args) { console.log(typeof args); } getAge(21); ``` - A: `"number"` - B: `"array"` - C: `"object"` - D: `"NaN"` <details><summary>Answer</summary>

C The rest parameter (`...args`) lets us "collect" all remaining arguments into an array. An array is an object, so `typeof args` returns `"object"` </details>

###### 85. What kind of information would get logged? ```javascript fetch('https://www.website.com/api/user/1') .then(res => res.json()) .then(res => console.log(res)); ``` - A: The result of the `fetch` method. - B: The result of the second invocation of the `fetch` method. - C: The result of the callback in the previous `.then()`. - D: It would always be undefined. <details><summary>Answer</summary>

C The value of `res` in the second `.then` is equal to the returned value of the previous `.then`. You can keep chaining `.then`s like this, where the value is passed to the next handler. </details>

###### 99. What's the output? ```javascript const name = 'Lydia'; console.log(name()); ``` - A: `SyntaxError` - B: `ReferenceError` - C: `TypeError` - D: `undefined` <details><summary>Answer</summary>

C The variable `name` holds the value of a string, which is not a function, thus cannot invoke. TypeErrors get thrown when a value is not of the expected type. JavaScript expected `name` to be a function since we're trying to invoke it. It was a string however, so a TypeError gets thrown: name is not a function! SyntaxErrors get thrown when you've written something that isn't valid JavaScript, for example when you've written the word `return` as `retrun`. ReferenceErrors get thrown when JavaScript isn't able to find a reference to a value that you're trying to access. </details>

###### 84. What is the output? ```javascript function checkAge(age) { if (age < 18) { const message = "Sorry, you're too young."; } else { const message = "Yay! You're old enough!"; } return message; } console.log(checkAge(21)); ``` - A: `"Sorry, you're too young."` - B: `"Yay! You're old enough!"` - C: `ReferenceError` - D: `undefined` <details><summary>Answer</summary>

C Variables with the `const` and `let` keyword are _block-scoped_. A block is anything between curly brackets (`{ }`). In this case, the curly brackets of the if/else statements. You cannot reference a variable outside of the block it's declared in, a ReferenceError gets thrown. </details>

###### 59. What's the output? ```javascript const numbers = [1, 2, 3, 4, 5]; const [y] = numbers; console.log(y); ``` - A: `[[1, 2, 3, 4, 5]]` - B: `[1, 2, 3, 4, 5]` - C: `1` - D: `[1]` <details><summary>Answer</summary>

C We can unpack values from arrays or properties from objects through destructuring. For example: ```javascript [a, b] = [1, 2]; ``` <img src="https://i.imgur.com/ADFpVop.png" width="200"> The value of `a` is now `1`, and the value of `b` is now `2`. What we actually did in the question, is: ```javascript [y] = [1, 2, 3, 4, 5]; ``` <img src="https://i.imgur.com/NzGkMNk.png" width="200"> This means that the value of `y` is equal to the first value in the array, which is the number `1`. When we log `y`, `1` is returned. </details>

###### 50. What's the output? ```javascript [1, 2, 3].map(num => { if (typeof num === 'number') return; return num * 2; }); ``` - A: `[]` - B: `[null, null, null]` - C: `[undefined, undefined, undefined]` - D: `[ 3 x empty ]` <details><summary>Answer</summary>

C When mapping over the array, the value of `num` is equal to the element it's currently looping over. In this case, the elements are numbers, so the condition of the if statement `typeof num === "number"` returns `true`. The map function creates a new array and inserts the values returned from the function. However, we don't return a value. When we don't return a value from the function, the function returns `undefined`. For every element in the array, the function block gets called, so for each element we return `undefined`. </details>

###### 18. What's the output? ```javascript function checkAge(data) { if (data === { age: 18 }) { console.log('You are an adult!'); } else if (data == { age: 18 }) { console.log('You are still an adult.'); } else { console.log(`Hmm.. You don't have an age I guess`); } } checkAge({ age: 18 }); ``` - A: `You are an adult!` - B: `You are still an adult.` - C: `Hmm.. You don't have an age I guess` <details><summary>Answer</summary>

C When testing equality, primitives are compared by their _value_, while objects are compared by their _reference_. JavaScript checks if the objects have a reference to the same location in memory. The two objects that we are comparing don't have that: the object we passed as a parameter refers to a different location in memory than the object we used in order to check equality. This is why both `{ age: 18 } === { age: 18 }` and `{ age: 18 } == { age: 18 }` return `false`. </details>

###### 76. What's the output? ```javascript const { name: myName } = { name: 'Lydia' }; console.log(name); ``` - A: `"Lydia"` - B: `"myName"` - C: `undefined` - D: `ReferenceError` <details><summary>Answer</summary>

C When we unpack the property `name` from the object on the right-hand side, we assign its value `"Lydia"` to a variable with the name `myName`. With `{ name: myName }`, we tell JavaScript that we want to create a new variable called `myName` with the value of the `name` property on the right-hand side. Since we try to log `name`, a variable that is not defined, `undefined` is returned on the left side assignment. Later, the value of `Lydia` is stored through the destructuring assignment. </details>

###### 37. What's the output? ```javascript const numbers = [1, 2, 3]; numbers[10] = 11; console.log(numbers); ``` - A: `[1, 2, 3, 7 x null, 11]` - B: `[1, 2, 3, 11]` - C: `[1, 2, 3, 7 x empty, 11]` - D: `SyntaxError` <details><summary>Answer</summary>

C When you set a value to an element in an array that exceeds the length of the array, JavaScript creates something called "empty slots". These actually have the value of `undefined`, but you will see something like: `[1, 2, 3, 7 x empty, 11]` depending on where you run it (it's different for every browser, node, etc.) </details>

###### 20. What's the output? ```javascript function getAge() { 'use strict'; age = 21; console.log(age); } getAge(); ``` - A: `21` - B: `undefined` - C: `ReferenceError` - D: `TypeError` <details><summary>Answer</summary>

C With `"use strict"`, you can make sure that you don't accidentally declare global variables. We never declared the variable `age`, and since we use `"use strict"`, it will throw a reference error. If we didn't use `"use strict"`, it would have worked, since the property `age` would have gotten added to the global object. </details>

###### 69. What's the output? ```javascript const name = 'Lydia Hallie'; console.log(name.padStart(13)); console.log(name.padStart(2)); ``` - A: `"Lydia Hallie"`, `"Lydia Hallie"` - B: `" Lydia Hallie"`, `" Lydia Hallie"` (`"[13x whitespace]Lydia Hallie"`, `"[2x whitespace]Lydia Hallie"`) - C: `" Lydia Hallie"`, `"Lydia Hallie"` (`"[1x whitespace]Lydia Hallie"`, `"Lydia Hallie"`) - D: `"Lydia Hallie"`, `"Lyd"`, <details><summary>Answer</summary>

C With the `padStart` method, we can add padding to the beginning of a string. The value passed to this method is the _total_ length of the string together with the padding. The string `"Lydia Hallie"` has a length of `12`. `name.padStart(13)` inserts 1 space at the start of the string, because 12 + 1 is 13. If the argument passed to the `padStart` method is smaller than the length of the array, no padding will be added. </details>

###### 72. What's the output? ```javascript console.log(String.raw`Hello\nworld`); ``` - A: `Hello world!` - B: `Hello`      `world` - C: `Hello\nworld` - D: `Hello\n`      `world` <details><summary>Answer</summary>

C `String.raw` returns a string where the escapes (`\n`, `\v`, `\t` etc.) are ignored! Backslashes can be an issue since you could end up with something like: `` const path = `C:\Documents\Projects\table.html` `` Which would result in: `"C:DocumentsProjects able.html"` With `String.raw`, it would simply ignore the escape and print: `C:\Documents\Projects\table.html` In this case, the string is `Hello\nworld`, which gets logged. </details>

###### 40. What's the output? ```javascript [[0, 1], [2, 3]].reduce( (acc, cur) => { return acc.concat(cur); }, [1, 2], ); ``` - A: `[0, 1, 2, 3, 1, 2]` - B: `[6, 1, 2]` - C: `[1, 2, 0, 1, 2, 3]` - D: `[1, 2, 6]` <details><summary>Answer</summary>

C `[1, 2]` is our initial value. This is the value we start with, and the value of the very first `acc`. During the first round, `acc` is `[1,2]`, and `cur` is `[0, 1]`. We concatenate them, which results in `[1, 2, 0, 1]`. Then, `[1, 2, 0, 1]` is `acc` and `[2, 3]` is `cur`. We concatenate them, and get `[1, 2, 0, 1, 2, 3]` </details>

###### 7. What's the output? ```javascript let a = 3; let b = new Number(3); let c = 3; console.log(a == b); console.log(a === b); console.log(b === c); ``` - A: `true` `false` `true` - B: `false` `false` `true` - C: `true` `false` `false` - D: `false` `true` `true` <details><summary>Answer</summary>

C `new Number()` is a built-in function constructor. Although it looks like a number, it's not really a number: it has a bunch of extra features and is an object. When we use the `==` operator, it only checks whether it has the same _value_. They both have the value of `3`, so it returns `true`. However, when we use the `===` operator, both value _and_ type should be the same. It's not: `new Number()` is not a number, it's an **object**. Both return `false.` </details>

###### 97. What's the output? ```javascript const info = { [Symbol('a')]: 'b', }; console.log(info); console.log(Object.keys(info)); ``` - A: `{Symbol('a'): 'b'}` and `["{Symbol('a')"]` - B: `{}` and `[]` - C: `{ a: "b" }` and `["a"]` - D: `{Symbol('a'): 'b'}` and `[]` <details><summary>Answer</summary>

D A Symbol is not _enumerable_. The Object.keys method returns all _enumerable_ key properties on an object. The Symbol won't be visible, and an empty array is returned. When logging the entire object, all properties will be visible, even non-enumerable ones. This is one of the many qualities of a symbol: besides representing an entirely unique value (which prevents accidental name collision on objects, for example when working with 2 libraries that want to add properties to the same object), you can also "hide" properties on objects this way (although not entirely. You can still access symbols using the `Object.getOwnPropertySymbols()` method). </details>

###### 13. What are the three phases of event propagation? - A: Target > Capturing > Bubbling - B: Bubbling > Target > Capturing - C: Target > Bubbling > Capturing - D: Capturing > Target > Bubbling <details><summary>Answer</summary>

D During the **capturing** phase, the event goes through the ancestor elements down to the target element. It then reaches the **target** element, and **bubbling** begins. <img src="https://i.imgur.com/N18oRgd.png" width="200"> </details>

###### 46. What's the output? ```javascript let person = { name: 'Lydia' }; const members = [person]; person = null; console.log(members); ``` - A: `null` - B: `[null]` - C: `[{}]` - D: `[{ name: "Lydia" }]` <details><summary>Answer</summary>

D First, we declare a variable `person` with the value of an object that has a `name` property. <img src="https://i.imgur.com/TML1MbS.png" width="200"> Then, we declare a variable called `members`. We set the first element of that array equal to the value of the `person` variable. Objects interact by _reference_ when setting them equal to each other. When you assign a reference from one variable to another, you make a _copy_ of that reference. (note that they don't have the _same_ reference!) <img src="https://i.imgur.com/FSG5K3F.png" width="300"> Then, we set the variable `person` equal to `null`. <img src="https://i.imgur.com/sYjcsMT.png" width="300"> We are only modifying the value of the `person` variable, and not the first element in the array, since that element has a different (copied) reference to the object. The first element in `members` still holds its reference to the original object. When we log the `members` array, the first element still holds the value of the object, which gets logged. </details>

###### 92. What's the output? ```javascript function giveLydiaPizza() { return 'Here is pizza!'; } const giveLydiaChocolate = () => "Here's chocolate... now go hit the gym already."; console.log(giveLydiaPizza.prototype); console.log(giveLydiaChocolate.prototype); ``` - A: `{ constructor: ...}` `{ constructor: ...}` - B: `{}` `{ constructor: ...}` - C: `{ constructor: ...}` `{}` - D: `{ constructor: ...}` `undefined` <details><summary>Answer</summary>

D Regular functions, such as the `giveLydiaPizza` function, have a `prototype` property, which is an object (prototype object) with a `constructor` property. Arrow functions however, such as the `giveLydiaChocolate` function, do not have this `prototype` property. `undefined` gets returned when trying to access the `prototype` property using `giveLydiaChocolate.prototype`. </details>

###### 91. What's the output? ```javascript let newList = [1, 2, 3].push(4); console.log(newList.push(5)); ``` - A: `[1, 2, 3, 4, 5]` - B: `[1, 2, 3, 5]` - C: `[1, 2, 3, 4]` - D: `Error` <details><summary>Answer</summary>

D The `.push` method returns the _new length_ of the array, not the array itself! By setting `newList` equal to `[1, 2, 3].push(4)`, we set `newList` equal to the new length of the array: `4`. Then, we try to use the `.push` method on `newList`. Since `newList` is the numerical value `4`, we cannot use the `.push` method: a TypeError is thrown. </details>

###### 56. What's the output? ```javascript const set = new Set([1, 1, 2, 3, 4]); console.log(set); ``` - A: `[1, 1, 2, 3, 4]` - B: `[1, 2, 3, 4]` - C: `{1, 1, 2, 3, 4}` - D: `{1, 2, 3, 4}` <details><summary>Answer</summary>

D The `Set` object is a collection of _unique_ values: a value can only occur once in a set. We passed the iterable `[1, 1, 2, 3, 4]` with a duplicate value `1`. Since we cannot have two of the same values in a set, one of them is removed. This results in `{1, 2, 3, 4}`. </details>

###### 8. What's the output? ```javascript class Chameleon { static colorChange(newColor) { this.newColor = newColor; return this.newColor; } constructor({ newColor = 'green' } = {}) { this.newColor = newColor; } } const freddie = new Chameleon({ newColor: 'purple' }); console.log(freddie.colorChange('orange')); ``` - A: `orange` - B: `purple` - C: `green` - D: `TypeError` <details><summary>Answer</summary>

D The `colorChange` function is static. Static methods are designed to live only on the constructor in which they are created, and cannot be passed down to any children or called upon class instances. Since `freddie` is an instance of class Chameleon, the function cannot be called upon it. A `TypeError` is thrown. </details>

###### 65. What's the output? ```javascript [1, 2, 3, 4].reduce((x, y) => console.log(x, y)); ``` - A: `1` `2` and `3` `3` and `6` `4` - B: `1` `2` and `2` `3` and `3` `4` - C: `1` `undefined` and `2` `undefined` and `3` `undefined` and `4` `undefined` - D: `1` `2` and `undefined` `3` and `undefined` `4` <details><summary>Answer</summary>

D The first argument that the `reduce` method receives is the _accumulator_, `x` in this case. The second argument is the _current value_, `y`. With the reduce method, we execute a callback function on every element in the array, which could ultimately result in one single value. In this example, we are not returning any values, we are simply logging the values of the accumulator and the current value. The value of the accumulator is equal to the previously returned value of the callback function. If you don't pass the optional `initialValue` argument to the `reduce` method, the accumulator is equal to the first element on the first call. On the first call, the accumulator (`x`) is `1`, and the current value (`y`) is `2`. We don't return from the callback function, we log the accumulator and current value: `1` and `2` get logged. If you don't return a value from a function, it returns `undefined`. On the next call, the accumulator is `undefined`, and the current value is `3`. `undefined` and `3` get logged. On the fourth call, we again don't return from the callback function. The accumulator is again `undefined`, and the current value is `4`. `undefined` and `4` get logged. </details>

###### 33. What's the output? ```javascript const person = { name: 'Lydia' }; function sayHi(age) { return `${this.name} is ${age}`; } console.log(sayHi.call(person, 21)); console.log(sayHi.bind(person, 21)); ``` - A: `undefined is 21` `Lydia is 21` - B: `function` `function` - C: `Lydia is 21` `Lydia is 21` - D: `Lydia is 21` `function` <details><summary>Answer</summary>

D With both, we can pass the object to which we want the `this` keyword to refer to. However, `.call` is also _executed immediately_! `.bind.` returns a _copy_ of the function, but with a bound context! It is not executed immediately. </details>

###### 52. What's the output? ```javascript function greeting() { throw 'Hello world!'; } function sayHi() { try { const data = greeting(); console.log('It worked!', data); } catch (e) { console.log('Oh no an error:', e); } } sayHi(); ``` - A: `It worked! Hello world!` - B: `Oh no an error: undefined` - C: `SyntaxError: can only throw Error objects` - D: `Oh no an error: Hello world!` <details><summary>Answer</summary>

D With the `throw` statement, we can create custom errors. With this statement, you can throw exceptions. An exception can be a string, a number, a boolean or an object. In this case, our exception is the string `'Hello world!'`. With the `catch` statement, we can specify what to do if an exception is thrown in the `try` block. An exception is thrown: the string `'Hello world!'`. `e` is now equal to that string, which we log. This results in `'Oh an error: Hello world!'`. </details>

###### 1. What's the output? ```javascript function sayHi() { console.log(name); console.log(age); var name = 'Lydia'; let age = 21; } sayHi(); ``` - A: `Lydia` and `undefined` - B: `Lydia` and `ReferenceError` - C: `ReferenceError` and `21` - D: `undefined` and `ReferenceError` <details><summary>Answer</summary>

D Within the function, we first declare the `name` variable with the `var` keyword. This means that the variable gets hoisted (memory space is set up during the creation phase) with the default value of `undefined`, until we actually get to the line where we define the variable. We haven't defined the variable yet on the line where we try to log the `name` variable, so it still holds the value of `undefined`. Variables with the `let` keyword (and `const`) are hoisted, but unlike `var`, don't get initialized. They are not accessible before the line we declare (initialize) them. This is called the "temporal dead zone". When we try to access the variables before they are declared, JavaScript throws a `ReferenceError`. </details>

###### 94. What's the output? ```javascript function getItems(fruitList, ...args, favoriteFruit) { return [...fruitList, ...args, favoriteFruit] } getItems(["banana", "apple"], "pear", "orange") ``` - A: `["banana", "apple", "pear", "orange"]` - B: `[["banana", "apple"], "pear", "orange"]` - C: `["banana", "apple", ["pear"], "orange"]` - D: `SyntaxError` <details><summary>Answer</summary>

D `...args` is a rest parameter. The rest parameter's value is an array containing all remaining arguments, **and can only be the last parameter**! In this example, the rest parameter was the second parameter. This is not possible, and will throw a syntax error. ```javascript function getItems(fruitList, favoriteFruit, ...args) { return [...fruitList, ...args, favoriteFruit]; } getItems(['banana', 'apple'], 'pear', 'orange'); ``` The above example works. This returns the array `[ 'banana', 'apple', 'orange', 'pear' ]` </details>

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