JavaScript Interview Questions

What is Closure?

Closures are created whenever a variable that is defined outside the current scope is accessed from within some inner scope. It gives you access to an outer function's scope from an inner function. In JavaScript, closures are created every time a function is created. To use a closure, simply define a function inside another function and expose it. Example: function create () { var count = 0; return { increment: function () { count ++; }, print: function () { console.log(count); } } } var c = create(); c.increment(); c.print(); // ==> 1

What are the features of JavaScript?

Control over the browser. Designing network centric apps. Complemented and integrated w/Java & HTML. Creates new functions w/in scripts. Open and is a cross-platform language.

What is Callback?

A callback is a plain JavaScript function passed to some method as an argument or option. It is a function that is to be executed after another function has finished executing, hence the name 'call back'. In JavaScript, functions are objects. Because of this, functions can take functions as arguments, and can be returned by other functions.

What is a name function in JavaScript?

A function is a JS procedure and block of code composed by components (names). A set of statements that performs a task or calculates value. It has a name when it is defined. Example: function named () { // do some stuff here // }

What is a prompt box in JavaScript?

A prompt box is a box which allows the user to enter input by providing a text box. The prompt() method displays a dialog box that prompts the visitor for input. A prompt box is often used if you want the user to input a value before entering a page. When a prompt box pops up, the user will have to click either "OK" or "Cancel" to proceed after entering an input value.

What are the data types supported by JavaScript?

A: Objects, in general, do have methods and are non-primitive B: Primitives types are built-in and are not considered objects. Undefined, Null, Booleans, Strings, Symbols, Numbers

What are the 4 different ways an HTML element can be accessed in a JavaScript code?

(i) getElementById('idname'): Gets an element by its ID name (ii) getElementsByClass('classname'): Gets all the elements that have the given classname. (iii) getElementsByTagName('tagname'): Gets all the elements that have the given tag name. (iv) querySelector(): This function takes css style selector and returns the first selected element.

What is the difference between window & document in JavaScript?

*Window JavaScript window is a global object which holds variables, functions, history, location. *Document The document also comes under the window and can be considered as the property of the window. It's basically a subset of the window.

What is the difference between Attributes and Property?

1. Attributes provide more details on an element like id, type, value etc. 2. Property is the value assigned to the property like type="text", value='Name' etc.

Name some of the built-in methods and the values returned by them.

1. CharAt()It returns the character at the specified index. 2. Concat()It joins two or more strings. 3. forEach()It calls a function for each element in the array. 4. indexOf()It returns the index within the calling String object of the first occurrence of the specified value. 5. length()It returns the length of the string. 6. pop()It removes the last element from an array and returns that element. 7. push()It adds one or more elements to the end of an array and returns the new length of the array. 8. reverse()It reverses the order of the elements of an array.

What are the ways to define a variable in JavaScript?

1. Var - The JavaScript variables statement is used to declare a variable and, optionally, we can initialize the value of that variable. Example: var a =10; Variable declarations are processed before the execution of the code. 2. Const - The idea of const functions is not allow them to modify the object on which they are called. When a function is declared as const, it can be called on any type of object. 3. Let - It is a signal that the variable may be reassigned, such as a counter in a loop, or a value swap in an algorithm. It also signals that the variable will be used only in the block it's defined in.

What is an event bubbling in JavaScript?

Event bubbling is a way of event propagation in the HTML DOM API, when an event occurs in an element inside another element, and both elements have registered a handle for that event. With bubbling, the event is first captured and handled by the innermost element and then propagated to outer elements. The execution starts from that event and goes to its parent element. Then the execution passes to its parent element and so on till the body element.

How to delete a cookie using JavaScript?

If you want to delete a cookie so that subsequent attempts to read the cookie return nothing, you just need to set the expiration date to a time in the past. You should define the cookie path to ensure that you delete the right cookie. Some browsers will not let you delete a cookie if you don't specify the path.

What are Exports & Imports?

Imports and exports help us to write modular JavaScript code. Using Imports and exports we can split our code into multiple files. Example: //------ lib.js ------ export const sqrt = Math.sqrt; export function square(x) { return x * x; } export function diag(x, y) { return sqrt(square(x) + square(y)); } //------ main.js ------ { square, diag } from 'lib'; console.log(square(5)); // 25 console.log(diag(4, 3)); // 5

How can you create an array in JavaScript?

Initialization of the object with var and add function key words. Examples: var x = []; var y = [1,2,3,4,5]; let numArray = [1,2,3,4,5];

What is the difference between Java and JavaScript?

Java is a complete programming language whereas JavaScript is a coded program that can be introduced to HTML pages. Java is procedural. JavaScript is client-side scripting.

What are escape characters in JavaScript?

JavaScript escape characters enable you to write special characters without breaking your application. Escape characters (Backslash) is used when working with special characters like single quotes, double quotes, apostrophes and ampersands. Place backslash before the characters to make it display. For example- document.write "I am a "good" boy" document.write /I am a 'good" boy'/

JavaScript

JavaScript is an object-oriented computer programming language commonly used to create interactive effects within web browsers. It allows you to build into otherwise static HTML pages. It's language has been embedded in Netscape, Internet Explorer and other web browsers.

Who invented JavaScript and why?

JavaScript was created by Brendan Eich in 1995 to give web pages a little more pep than the <blink> tag could provide.

What is the purpose of 'This' operator in JavaScript?

The JavaScript this keyword refers to the object it belongs to. This has different values depending on where it is used. In a method, this refers to the owner object and in a function, this refers to the global object.

What are the advantages of JavaScript?

Less server interaction. Immediate feedback. Increased interactivity. Richer interfaces.

What is the difference between Local storage & Session storage?

Local Storage - The data is not sent back to the server for every HTTP request (HTML, images, JavaScript, CSS, etc) - reducing the amount of traffic between client and server. It will stay until it is manually cleared through settings or program. Session Storage - It is similar to local storage; the only difference is while data stored in local storage has no expiration time, data stored in session storage gets cleared when the page session ends. Session Storage will leave when the browser is closed.

What is NaN in JavaScript?

NaN is a short form of Not a Number. Since NaN always compares unequal to any number, including NaN, it is usually used to indicate an error condition for a function that should return a valid number. When a string or something else is being converted into a number and that cannot be done, then we get to see NaN.

How do JavaScript primitive/object types passed in functions?

One of the differences between the two is that Primitive Data Types are passed By Value and Objects are passed By Reference. *By Value means creating a COPY of the original. Picture it like twins: they are born exactly the same, but the first twin doesn't lose a leg when the second twin loses his in the war. *By Reference means creating an ALIAS to the original. When that World calls you "Brad Vizcaya" although your name is Brad, this doesn't suddenly give birth to a clone of yourself: you are still one, but you can be called by these two very different names.

How to read a cookie using JavaScript?

Reading a cookie is just as simple as writing one, because the value of the document.cookie object is the cookie. So you can use this string whenever you want to access the cookie. The document.cookie string will keep a list of name = value pairs separated by semicolons, where name is the name of a cookie and value is its string value. You can use strings' split() function to break the string into key and values.

What would be the result of 2+5+"4″?

Since 2 and 5 are integers, they will be added numerically. And since 4 is a string, its concatenation will be done. So the result would be 74. *The " " makes all the difference here and represents 4 as a string and not a number.

What is the 'Strict' mode in JavaScript and how can it be enabled?

Strict mode is a way to introduce better error-checking into your code. When you use strict mode, you cannot use implicitly declared variables, or assign a value to a read-only property, or add a property to an object that is not extensible. You can enable strict mode by adding "use strict" at the beginning of a file, a program, or a function.

What is argument object in JavaScript and how to get the type of arguments passed to the function?

The arguments object is a local variable available with in all non error functions. Example: function func (x) { console,log (typeof x, arguments.length); } func (); // ==> "undefined", 0 func (1); // ==> "number", 1 fun ("1", "2", "3",); // ==> "string", 3 This is using typeof operator.

What is the difference between Call & Apply?

The call() method calls a function with a given this value and arguments provided individually. Syntax- fun.call(thisArg[, arg1[, arg2[, ...]]]) The apply() method calls a function with a given this value, and arguments provided as an array. Syntax- fun.apply(thisArg, [argsArray])

What are the variable naming conventions in JavaScript?

The following rules are to be followed while naming variables in JavaScript: You should not use any of the JavaScript reserved keywords as a variable name. For example, break or boolean variable names are not valid. JavaScript variable names should not start with a numeral (0-9). They must begin with a letter or the underscore character. For example, 123name is an invalid variable name but _123name or name123 is a valid one. JavaScript variable names are case sensitive. For example, Test and test are two different variables.

What is the difference between the operators '==' & '==='?

The main difference between "==" and "===" operator is that formerly compares variable by making type correction e.g. if you compare a number with a string with numeric literal, == allows that, but === doesn't allow that, because it not only checks the value but also type of two variables, if two variables are not of the same type "===" return false, while "==" return true.

What will be the output of the following code? var Output = (function(x) { Delete X; return X; } )(0); console.log(output);

The output would be 0. The delete operator is used to delete properties from an object. Here x is not an object but a local variable. delete operators don't affect local variables.

What will be the output of the code below? //nfe (named function expression) var Foo = Function Bar() { return 7; }; typeof Bar();

The output would be Reference Error. A function definition can have only one reference variable as its function name.

What will be the output of the following code? var X = { Foo : 1}; var Output = (function() { delete X.foo; return X.foo; } )(); console.log(output);

The output would be undefined. The delete operator is used to delete the property of an object. Here, x is an object which has the property foo, and as it is a self-invoking function, we will delete the foo property from object x. After doing so, when we try to reference a deleted property foo, the result is undefined.

What will be the output of the following code? var Employee = { company: 'xyz' } var Emp1 = Object.create(employee); delete Emp1.company Console.log(emp1.company);

The output would be xyz. Here, emp1 object has company as its prototype property. The delete operator doesn't delete prototype property. emp1 object doesn't have company as its own property. However, we can delete the company property directly from the Employee object using delete Employee.company.

How can you convert the string of any base to integer in JavaScript?

The parseInt() function is used to convert numbers between different bases. It takes the string to be converted as its first parameter, and the second parameter is the base of the given string. Example: parseInt("4F", 16)

What are the scopes of a variable in JavaScript?

The scope of a variable is the region of your program in which it is defined. JavaScript variable will have only two scopes.• Global Variables − A global variable has global scope which means it is visible everywhere in your JavaScript code.• Local Variables − A local variable will be visible only within a function where it is defined. Function parameters are always local to that function.

How to create a cookie using JavaScript?

The simplest way to create a cookie is to assign a string value to the document.cookie object, which looks like this- Syntax : document.cookie = "key1 = value1; key2 = value2; expires = date";

How does TypeOf Operator work?

The typeof operator is used to get the data type of its operand. The operand can be either a literal or a data structure such as a variable, a function, or an object. It is a unary operator that is placed before its single operand, which can be of any type. Its value is a string indicating the data type of the operand.

In how many ways a JavaScript code can be involved in an HTML file?

There are 3 different ways in which a JavaScript code can be involved in an HTML file: 1. Inline 2. Internal 3. External An inline function is a JavaScript function, which is assigned to a variable created at runtime. You can differentiate between Inline Functions and Anonymous since an inline function is assigned to a variable and can be easily reused. When you need a JavaScript for a function, you can either have the script integrated in the page you are working on, or you can have it placed in a separate file that you call, when needed. This is the difference between an internal script and an external script.

How to empty an array in JavaScript ?

There are a number of methods you can use to empty an array: 1. Method 1 - arrayList = [] Above code will set the variable arrayList to a new empty array. This is recommended if you don't have references to the original array arrayList anywhere else, because it will actually create a new, empty array. You should be careful with this method of emptying the array, because if you have referenced this array from another variable, then the original reference array will remain unchanged. 2. Method 2 - arrayList.length = 0; The code above will clear the existing array by setting its length to 0. This way of emptying the array also updates all the reference variables that point to the original array. Therefore, this method is useful when you want to update all reference variables pointing to arrayList. 3. Method 3 - arrayList.splice(0, arrayList.length); The implementation above will also work perfectly. This way of emptying the array will also update all the references to the original array. 4. Method 4 - while(arrayList.length) { arrayList.pop(); } The implementation above can also empty arrays, but it is usually not recommended to use this method often.

Name some of the JavaScript Frameworks

There are many JavaScript Frameworks available but some of the most commonly used frameworks are: Angular React Vue

What is the reason for wrapping the entire content of a JavaScript source file in a function book?

This is an increasingly common practice, employed by many popular JavaScript libraries. This technique creates a closure around the entire contents of the file which, perhaps most importantly, creates a private namespace and thereby helps avoid potential name clashes between different JavaScript modules and libraries. Another feature of this technique is to allow for an easy alias for a global variable. This is often used in jQuery plugins.

What is a Typed language?

Typed Language is in which the values are associated with values and not with variables. * Dynamically: in this, the variable can hold multiple types; like in JS, a variable can take number, chars. *Statically: in this, the variable can hold only one type, like in Java a variable declared of string can take only set of characters and nothing else.

What is the difference between undeclared & undefined?

Undeclared variables are those that do not exist in a program and are not declared. *If the program tries to read the value of an undeclared variable, then a run-time error is encountered. Undefined variables are those that are declared in the program but have not been given any value. *If the program tries to read the value of an undefined variable, an undefined value is returned.

What is the difference between null & undefined?

Undefined means a variable has been declared but has not yet been assigned a value. *Null is an assignment value. It can be assigned to a variable as a representation of *no value. Also, undefined & null are two distinct types: undefined is a type itself (undefined) while null is an object.

How can you create an object in JavaScript?

Use constructors to make objects. That is constructor function syntax. Example: let person = { first_name: 'Brad', last_name: 'Vizcaya', //method getFunction: function() { return ('The name of the person is ${person.firts_name} ${person.last_name}') }, //object within object phone_number: { mobile: '12345', landline: '6789' } } console.log(person.getFunction()); console.log(person_number.landline);

Can you assign a anonymous function to a variable and pass it as a argument to another function?

Yes. An anonymous function can be assigned to a variable and pass it as an argument to another function.

Is JavaScript a case-sensitive language?

Yes. Capitalization must be consistent.

What is the difference between innerHTML & innerText?

innerHTML - It will process an HTML tag if found in a string innerText - It will not process an HTML tag if found in a string

What will be the output of the code below?

var Y = 1; if (function F(){}) { y += Typeof F; } console.log(y); The output would be 1undefined. The if condition statement evaluates using eval, so eval(function f(){}) returns function f(){} (which is true). Therefore, inside the if statement, executing typeof f returns undefined because the if statement code executes at run time, and the statement inside the if condition is evaluated during run time.