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JavaScript ES5

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ECMAScript 2009, also known as ES5, was the first major revision to JavaScript.

This chapter describes the most important features of ES5.

ES5 Features

Browser Support

ES5 is fully supported in all modern browsers:

Chrome IE Edge Firefox Safari Opera
Yes 9.0 Yes Yes Yes Yes

The "use strict" Directive

"use strict" defines that the JavaScript code should be executed in "strict mode".

With strict mode you can, for example, not use undeclared variables.

You can use strict mode in all your programs. It helps you to write cleaner code, like preventing you from using undeclared variables.

"use strict" is just a string expression. Old browsers will not throw an error if they don't understand it.

Read more in JS Strict Mode.

Property Access on Strings

The charAt() method returns the character at a specified index (position) in a string:

Example

var str = "HELLO WORLD";
str.charAt(0);            // returns H
Try it Yourself »

ES5 allows property access on strings:

Example

var str = "HELLO WORLD";
str[0];                   // returns H
Try it Yourself »

Property access on string might be a little unpredictable.

Read more in JS String Methods.

Strings Over Multiple Lines

Example

"Hello \
Dolly!";
Try it Yourself »

The \ method might not have universal support.
Older browsers might treat the spaces around the backslash differently.
Some older browsers do not allow spaces behind the \ character.

A safer way to break up a string literal, is to use string addition:

Example

"Hello " +
"Dolly!";
Try it Yourself »

Reserved Words as Property Names

ES5 allows reserved words as property names:

Object Example

var obj = {name: "John", new: "yes"}
Try it Yourself »

String trim()

The trim() method removes whitespace from both sides of a string.

Example

var str = "       Hello World!        ";
alert(str.trim());
Try it Yourself »

Read more in JS String Methods.

Array.isArray()

The isArray() method checks whether an object is an array.

Example

function myFunction() {
  var fruits = ["Banana", "Orange", "Apple", "Mango"];
  var x = document.getElementById("demo");
  x.innerHTML = Array.isArray(fruits);
}
Try it Yourself »

Read more in JS Arrays.

Array forEach()

The forEach() method calls a function once for each array element.

Example

var txt = "";
var numbers = [45, 4, 9, 16, 25];
numbers.forEach(myFunction);

function myFunction(value) {
  txt = txt + value + "<br>";
}
Try it Yourself »

Learn more in JS Array Iteration Methods.

Array map()

This example multiplies each array value by 2:

Example

var numbers1 = [45, 4, 9, 16, 25];
var numbers2 = numbers1.map(myFunction);

function myFunction(value) {
  return value * 2;
}
Try it Yourself »

Learn more in JS Array Iteration Methods.

Array filter()

This example creates a new array from elements with a value larger than 18:

Example

var numbers = [45, 4, 9, 16, 25];
var over18 = numbers.filter(myFunction);

function myFunction(value) {
  return value > 18;
}
Try it Yourself »

Learn more in JS Array Iteration Methods.

Array reduce()

This example finds the sum of all numbers in an array:

Example

var numbers1 = [45, 4, 9, 16, 25];
var sum = numbers1.reduce(myFunction);

function myFunction(total, value) {
  return total + value;
}
Try it Yourself »

Learn more in JS Array Iteration Methods.

Array reduceRight()

This example also finds the sum of all numbers in an array:

Example

var numbers1 = [45, 4, 9, 16, 25];
var sum = numbers1.reduceRight(myFunction);

function myFunction(total, value) {
  return total + value;
}
Try it Yourself »

Learn more in JS Array Iteration Methods.

Array every()

This example checks if all values are over 18:

Example

var numbers = [45, 4, 9, 16, 25];
var allOver18 = numbers.every(myFunction);

function myFunction(value) {
  return value > 18;
}
Try it Yourself »

Learn more in JS Array Iteration Methods.

Array some()

This example checks if some values are over 18:

Example

var numbers = [45, 4, 9, 16, 25];
var allOver18 = numbers.some(myFunction);

function myFunction(value) {
  return value > 18;
}
Try it Yourself »

Learn more in JS Array Iteration Methods.

Array indexOf()

Search an array for an element value and returns its position.

Example

var fruits = ["Apple", "Orange", "Apple", "Mango"];
var a = fruits.indexOf("Apple");
Try it Yourself »

Learn more in JS Array Iteration Methods.

Array lastIndexOf()

lastIndexOf() is the same as indexOf(), but searches from the end of the array.

Example

var fruits = ["Apple", "Orange", "Apple", "Mango"];
var a = fruits.lastIndexOf("Apple");
Try it Yourself »

Learn more in JS Array Iteration Methods.

JSON.parse()

A common use of JSON is to receive data from a web server.

Imagine you received this text string from a web server:

'{"name":"John", "age":30, "city":"New York"}'

The JavaScript function JSON.parse() is used to convert the text into a JavaScript object:

var obj = JSON.parse('{"name":"John", "age":30, "city":"New York"}');
Try it Yourself »

Read more in our JSON Tutorial.

JSON.stringify()

A common use of JSON is to send data to a web server.

When sending data to a web server, the data has to be a string.

Imagine we have this object in JavaScript:

var obj = {name:"John", age:30, city:"New York"};

Use the JavaScript function JSON.stringify() to convert it into a string.

var myJSON = JSON.stringify(obj);

The result will be a string following the JSON notation.

myJSON is now a string, and ready to be sent to a server:

Example

var obj = {name:"John", age:30, city:"New York"};
var myJSON = JSON.stringify(obj);
document.getElementById("demo").innerHTML = myJSON;
Try it Yourself »

Read more in our JSON Tutorial.

Date.now()

Date.now() returns the number of milliseconds since zero date (January 1. 1970 00:00:00 UTC).

Example

var timInMSs = Date.now();
Try it Yourself »

Date.now() returns the same as getTime() performed on a Date object.

Learn more in JS Dates.

Date toISOString()

The toISOString() method converts a Date object to a string, using the ISO standard format:

Example

const d = new Date();
document.getElementById("demo").innerHTML = d.toISOString();
Try it Yourself »

Date toJSON()

toJSON() converts a Date object into a string, formatted as a JSON date.

JSON dates have the same format as the ISO-8601 standard: YYYY-MM-DDTHH:mm:ss.sssZ:

Example

d = new Date();
document.getElementById("demo").innerHTML = d.toJSON();
Try it Yourself »

Property Getters and Setters

ES5 lets you define object methods with a syntax that looks like getting or setting a property.

This example creates a getter for a property called fullName:

Example

// Create an object:
var person = {
  firstName: "John",
  lastName : "Doe",
  get fullName() {
    return this.firstName + " " + this.lastName;
  }
};

// Display data from the object using a getter:
document.getElementById("demo").innerHTML = person.fullName;
Try it Yourself »

This example creates a setter and a getter for the language property:

Example

var person = {
  firstName: "John",
  lastName : "Doe",
  language : "NO",
  get lang() {
    return this.language;
  },
  set lang(value) {
    this.language = value;
  }
};

// Set an object property using a setter:
person.lang = "en";

// Display data from the object using a getter:
document.getElementById("demo").innerHTML = person.lang;
Try it Yourself »

This example uses a setter to secure upper case updates of language:

Example

var person = {
  firstName: "John",
  lastName : "Doe",
  language : "NO",
  set lang(value) {
    this.language = value.toUpperCase();
  }
};

// Set an object property using a setter:
person.lang = "en";

// Display data from the object:
document.getElementById("demo").innerHTML = person.language;
Try it Yourself »

Learn more about Gettes and Setters in JS Object Accessors

Object.defineProperty()

Object.defineProperty() is a new Object method in ES5.

It lets you define an object property and/or change a property's value and/or metadata.

Example

// Create an Object:
var person = {
  firstName: "John",
  lastName : "Doe",
  language : "NO",
};

// Change a Property:
Object.defineProperty(person, "language", {
  value: "EN",
  writable : true,
  enumerable : true,
  configurable : true
});

// Enumerate Properties
var txt = "";
for (var x in person) {
  txt += person[x] + "<br>";
}
document.getElementById("demo").innerHTML = txt;
Try it Yourself »

Next example is the same code, except it hides the language property from enumeration:

Example

// Create an Object:
var person = {
  firstName: "John",
  lastName : "Doe",
  language : "NO",
};

// Change a Property:
Object.defineProperty(person, "language", {
  value: "EN",
  writable : true,
  enumerable : false,
  configurable : true
});

// Enumerate Properties
var txt = "";
for (var x in person) {
  txt += person[x] + "<br>";
}
document.getElementById("demo").innerHTML = txt;
Try it Yourself »

This example creates a setter and a getter to secure upper case updates of language:

Example

// Create an Object:
var person = {
  firstName: "John",
  lastName : "Doe",
  language : "NO"
};

// Change a Property:
Object.defineProperty(person, "language", {
  get : function() { return language },
  set : function(value) { language = value.toUpperCase()}
});

// Change Language
person.language = "en";

// Display Language
document.getElementById("demo").innerHTML = person.language;
Try it Yourself »

E5 Object Methods

ES5 added a lot of new Object Methods to JavaScript:

Managing Objects

// Create object with an existing object as prototype
Object.create(parent, donor)

// Adding or changing an object property
Object.defineProperty(object, property, descriptor)

// Adding or changing object properties
Object.defineProperties(object, descriptors)

// Accessing Properties
Object.getOwnPropertyDescriptor(object, property)

// Returns all properties as an array
Object.getOwnPropertyNames(object)

// Accessing the prototype
Object.getPrototypeOf(object)

// Returns enumerable properties as an array
Object.keys(object)

Protecting Objects

// Prevents adding properties to an object
Object.preventExtensions(object)

// Returns true if properties can be added to an object
Object.isExtensible(object)

// Prevents changes of object properties (not values)
Object.seal(object)

// Returns true if object is sealed
Object.isSealed(object)

// Prevents any changes to an object
Object.freeze(object)

// Returns true if object is frozen
Object.isFrozen(object)

Learn more in Object ECMAScript5.

Function Bind()

With the bind() method, an object can borrow a method from another object.

This example creates 2 objects (person and member).

The member object borrows the fullname method from the person object:

Example

const person = {
  firstName:"John",
  lastName: "Doe",
  fullName: function () {
    return this.firstName + " " + this.lastName;
  }
}

const member = {
  firstName:"Hege",
  lastName: "Nilsen",
}

let fullName = person.fullName.bind(member);
Try it Yourself »

Learn more in Function bind().

Trailing Commas

ES5 allows trailing commas in object and array definitions:

Object Example

person = {
  firstName: "John",
  lastName: " Doe",
  age: 46,
}

Array Example

points = [
  1,
  5,
  10,
  25,
  40,
  100,
];

WARNING !!!

JSON does not allow trailing commas.

JSON Objects:

// Allowed:
var person = '{"firstName":"John", "lastName":"Doe", "age":46}'
JSON.parse(person)

// Not allowed:
var person = '{"firstName":"John", "lastName":"Doe", "age":46,}'
JSON.parse(person)

JSON Arrays:

// Allowed:
points = [40, 100, 1, 5, 25, 10]

// Not allowed:
points = [40, 100, 1, 5, 25, 10,]

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