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Object literals

An object literal is a list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}). You should not use an object literal at the beginning of a statement. This will lead to an error or not behave as you expect, because the { will be interpreted as the beginning of a block. The following is an example of an object literal. The first element of the car object defines a property, myCar, and assigns to it a new string, "Saturn"; the second element, the getCar property, is immediately assigned the result of invoking the function (carTypes("Honda")); the third element, the special property, uses an existing variable (sales). var sales = 'Toyota'; function carTypes(name) { if (name === 'Honda') { return name; } else { return "Sorry, we don't sell " + name + "."; } } var car = { myCar: 'Saturn', getCar: carTypes('Honda'), special: sales }; console.log(car.myCar); // Saturn console.log(car.getCar); // Honda console.log(car.special); // Toyota Additionally, you can use a numeric or string literal for the name of a property or nest an object inside another. The following example uses these options. var car = { manyCars: {a: 'Saab', 'b': 'Jeep'}, 7: 'Mazda' }; console.log(car.manyCars.b); // Jeep console.log(car[7]); // Mazda Object property names can be any string, including the empty string. If the property name would not be a valid JavaScript identifier or number, it must be enclosed in quotes. Property names that are not valid identifiers also cannot be accessed as a dot (.) property, but can be accessed and set with the array-like notation("[]"). var unusualPropertyNames = { '': 'An empty string', '!': 'Bang!' } console.log(unusualPropertyNames.''); // SyntaxError: Unexpected string console.log(unusualPropertyNames['']); // An empty string console.log(unusualPropertyNames.!); // SyntaxError: Unexpected token ! console.log(unusualPropertyNames['!']); // Bang!

Boolean literals

The Boolean type has two literal values: true and false. Do not confuse the primitive Boolean values true and false with the true and false values of the Boolean object. The Boolean object is a wrapper around the primitive Boolean data type. See Boolean for more information.

console.log()

command is used to print, or log, text to the console. Consider the following example:

declarations

here are three kinds of declarations in JavaScript. var Declares a variable, optionally initializing it to a value. let Declares a block-scoped, local variable, optionally initializing it to a value. const Declares a block-scoped, read-only named constan

nstance

is an individual case (or object) of a data type.

Floating-point literals

A floating-point literal can have the following parts: A decimal integer which can be signed (preceded by "+" or "-"), A decimal point ("."), A fraction (another decimal number), An exponent. The exponent part is an "e" or "E" followed by an integer, which can be signed (preceded by "+" or "-"). A floating-point literal must have at least one digit and either a decimal point or "e" (or "E"). More succinctly, the syntax is: [(+|-)][digits][.digits][(E|e)[(+|-)]digits] For example: 3.1415926 -.123456789 -3.1E+12 .1e-23

evaluating variables

A variable declared using the var or let statement with no assigned value specified has the value of undefined. An attempt to access an undeclared variable will result in a ReferenceError exception being thrown: var a; console.log('The value of a is ' + a); // The value of a is undefined console.log('The value of b is ' + b); // The value of b is undefined var b; console.log('The value of c is ' + c); // Uncaught ReferenceError: c is not defined let x; console.log('The value of x is ' + x); // The value of x is undefined console.log('The value of y is ' + y); // Uncaught ReferenceError: y is not defined let y; You can use undefined to determine whether a variable has a value. In the following code, the variable input is not assigned a value, and the if statement evaluates to true. var input; if (input === undefined) { doThis(); } else { doThat(); } The undefined value behaves as false when used in a boolean context. For example, the following code executes the function myFunction because the myArray element is undefined: var myArray = []; if (!myArray[0]) myFunction(); The undefined value converts to NaN when used in numeric context. var a; a + 2; // Evaluates to NaN When you evaluate a null variable, the null value behaves as 0 in numeric contexts and as false in boolean contexts. For example: var n = null; console.log(n * 32); // Will log 0 to the console

Array literal

An array literal is a list of zero or more expressions, each of which represents an array element, enclosed in square brackets ([]). When you create an array using an array literal, it is initialized with the specified values as its elements, and its length is set to the number of arguments specified. The following example creates the coffees array with three elements and a length of three: var coffees = ['French Roast', 'Colombian', 'Kona']; Note : An array literal is a type of object initializer. See Using Object Initializers. If an array is created using a literal in a top-level script, JavaScript interprets the array each time it evaluates the expression containing the array literal. In addition, a literal used in a function is created each time the function is called. Array literals are also Array objects. See Array and Indexed collections for details on Array objects.

variable hoisting

Another unusual thing about variables in JavaScript is that you can refer to a variable declared later, without getting an exception. This concept is known as hoisting; variables in JavaScript are in a sense "hoisted" or lifted to the top of the function or statement. However, variables that are hoisted will return a value of undefined. So even if you declare and initialize after you use or refer to this variable, it will still return undefined. /** * Example 1 */ console.log(x === undefined); // true var x = 3; /** * Example 2 */ // will return a value of undefined var myvar = 'my value'; (function() { console.log(myvar); // undefined var myvar = 'local value'; })(); The above examples will be interpreted the same as: /** * Example 1 */ var x; console.log(x === undefined); // true x = 3; /** * Example 2 */ var myvar = 'my value'; (function() { var myvar; console.log(myvar); // undefined myvar = 'local value'; })(); Because of hoisting, all var statements in a function should be placed as near to the top of the function as possible. This best practice increases the clarity of the code. In ECMAScript 2015, let (const) will not hoist the variable to the top of the block. However, referencing the variable in the block before the variable declaration results in a ReferenceError. The variable is in a "temporal dead zone" from the start of the block until the declaration is processed. console.log(x); // ReferenceError let x = 3;

Function hoisting

For functions, only the function declaration gets hoisted to the top and not the function expression. /* Function declaration */ foo(); // "bar" function foo() { console.log('bar'); } /* Function expression */ baz(); // TypeError: baz is not a function var baz = function() { console.log('bar2'); };

Global variables

Global variables are in fact properties of the global object. In web pages the global object is window, so you can set and access global variables using the window.variable syntax. Consequently, you can access global variables declared in one window or frame from another window or frame by specifying the window or frame name. For example, if a variable called phoneNumber is declared in a document, you can refer to this variable from an iframe as parent.phoneNumber.

Enhanced Object literals

In ES2015, object literals are extended to support setting the prototype at construction, shorthand for foo: foo assignments, defining methods, making super calls, and computing property names with expressions. Together, these also bring object literals and class declarations closer together, and let object-based design benefit from some of the same conveniences. var obj = { // __proto__ __proto__: theProtoObj, // Shorthand for 'handler: handler' handler, // Methods toString() { // Super calls return 'd ' + super.toString(); }, // Computed (dynamic) property names [ 'prop_' + (() => 42)() ]: 42 }; Please note: var foo = {a: 'alpha', 2: 'two'}; console.log(foo.a); // alpha console.log(foo[2]); // two //console.log(foo.2); // Error: missing ) after argument list //console.log(foo[a]); // Error: a is not defined console.log(foo['a']); // alpha console.log(foo['2']); // two

Converting strings to numbers

In the case that a value representing a number is in memory as a string, there are methods for conversion. parseInt() parseFloat() parseInt will only return whole numbers, so its use is diminished for decimals. Additionally, a best practice for parseInt is to always include the radix parameter. The radix parameter is used to specify which numerical system is to be used. An alternative method of retrieving a number from a string is with the + (unary plus) operator: '1.1' + '1.1' = '1.11.1' (+'1.1') + (+'1.1') = 2.2 // Note: the parentheses are added for clarity, not required.

Integers

Integers can be expressed in decimal (base 10), hexadecimal (base 16), octal (base 8) and binary (base 2). Decimal integer literal consists of a sequence of digits without a leading 0 (zero). Leading 0 (zero) on an integer literal, or leading 0o (or 0O) indicates it is in octal. Octal integers can include only the digits 0-7. Leading 0x (or 0X) indicates hexadecimal. Hexadecimal integers can include digits (0-9) and the letters a-f and A-F. Leading 0b (or 0B) indicates binary. Binary integers can include digits only 0 and 1. Some examples of integer literals are: 0, 117 and -345 (decimal, base 10) 015, 0001 and -0o77 (octal, base 8) 0x1123, 0x00111 and -0xF1A7 (hexadecimal, "hex" or base 16) 0b11, 0b0011 and -0b11 (binary, base 2) For more information, see Numeric literals in the Lexical grammar reference.

Data type conversion

JavaScript is a dynamically typed language. That means you don't have to specify the data type of a variable when you declare it, and data types are converted automatically as needed during script execution. So, for example, you could define a variable as follows: var answer = 42; And later, you could assign the same variable a string value, for example: answer = 'Thanks for all the fish...'; Because JavaScript is dynamically typed, this assignment does not cause an error message. In expressions involving numeric and string values with the + operator, JavaScript converts numeric values to strings. For example, consider the following statements: x = 'The answer is ' + 42 // "The answer is 42" y = 42 + ' is the answer' // "42 is the answer" In statements involving other operators, JavaScript does not convert numeric values to strings. For example: '37' - 7 // 30 '37' + 7 // "377"

Data type

The latest ECMAScript standard defines seven data types: Six data types that are primitives: Boolean. true and false. null. A special keyword denoting a null value. Because JavaScript is case-sensitive, null is not the same as Null, NULL, or any other variant. undefined. A top-level property whose value is undefined. Number. 42 or 3.14159. String. "Howdy" Symbol (new in ECMAScript 2015). A data type whose instances are unique and immutable. and Object Although these data types are a relatively small amount, they enable you to perform useful functions with your applications. Objects and functions are the other fundamental elements in the language. You can think of objects as named containers for values, and functions as procedures that your application can perform.

comments

The syntax of comments is the same as in C++ and in many other languages: // a one line comment /* this is a longer, multi-line comment */ /* You can't, however, /* nest comments */ SyntaxError */

variable scope

When you declare a variable outside of any function, it is called a global variable, because it is available to any other code in the current document. When you declare a variable within a function, it is called a local variable, because it is available only within that function. JavaScript before ECMAScript 2015 does not have block statement scope; rather, a variable declared within a block is local to the function (or global scope) that the block resides within. For example the following code will log 5, because the scope of x is the function (or global context) within which x is declared, not the block, which in this case is an if statement. if (true) { var x = 5; } console.log(x); // x is 5 This behavior changes, when using the let declaration introduced in ECMAScript 2015. if (true) { let y = 5; } console.log(y); // ReferenceError: y is not defined

Constants

You can create a read-only, named constant with the const keyword. The syntax of a constant identifier is the same as for a variable identifier: it must start with a letter, underscore or dollar sign ($) and can contain alphabetic, numeric, or underscore characters. const PI = 3.14; A constant cannot change value through assignment or be re-declared while the script is running. It has to be initialized to a value. The scope rules for constants are the same as those for let block-scope variables. If the const keyword is omitted, the identifier is assumed to represent a variable. You cannot declare a constant with the same name as a function or variable in the same scope. For example: // THIS WILL CAUSE AN ERROR function f() {}; const f = 5; // THIS WILL CAUSE AN ERROR ALSO function f() { const g = 5; var g; //statements } However, the properties of objects assigned to constants are not protected, so the following statement is executed without problems. const MY_OBJECT = {'key': 'value'}; MY_OBJECT.key = 'otherValue';

declaration variables

You can declare a variable in three ways: With the keyword var. For example, var x = 42. This syntax can be used to declare both local and global variables. By simply assigning it a value. For example, x = 42. This always declares a global variable, if it is declared outside of any function. It generates a strict JavaScript warning. You shouldn't use this variant. With the keyword let. For example, let y = 13. This syntax can be used to declare a block-scope local variable. See Variable scope below.

Extra commas in array literals

You do not have to specify all elements in an array literal. If you put two commas in a row, the array is created with undefined for the unspecified elements. The following example creates the fish array: var fish = ['Lion', , 'Angel']; This array has two elements with values and one empty element (fish[0] is "Lion", fish[1] is undefined, and fish[2] is "Angel"). If you include a trailing comma at the end of the list of elements, the comma is ignored. In the following example, the length of the array is three. There is no myList[3]. All other commas in the list indicate a new element. Note : Trailing commas can create errors in older browser versions and it is a best practice to remove them. var myList = ['home', , 'school', ]; In the following example, the length of the array is four, and myList[0] and myList[2] are missing. var myList = [ ,'home', , 'school']; In the following example, the length of the array is four, and myList[1] and myList[3] are missing. Only the last comma is ignored. var myList = ['home', , 'school', , ]; Understanding the behavior of extra commas is important to understanding JavaScript as a language, however when writing your own code: explicitly declaring the missing elements as undefined will increase your code's clarity and maintainability.

Literals

You use literals to represent values in JavaScript. These are fixed values, not variables, that you literally provide in your script. This section describes the following types of literals:

variables

You use variables as symbolic names for values in your application. The names of variables, called identifiers, conform to certain rules. A JavaScript identifier must start with a letter, underscore (_), or dollar sign ($); subsequent characters can also be digits (0-9). Because JavaScript is case sensitive, letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase). You can use most of ISO 8859-1 or Unicode letters such as å and ü in identifiers (for more details see this blog post). You can also use the Unicode escape sequences as characters in identifiers. Some examples of legal names are Number_hits, temp99, $credit, and _name