JavaScript 2009 (ES5)
ECMAScript 2009
The first major revision to JavaScript.
ECMAScript 2009 is also known as ES5.
ES5 Features
| Feature | Description |
|---|---|
| "use strict" | Allows code to bee executed in "strict mode" |
| String [] access | Returns the character at a specified index in a string |
| Multiline strings | Aallows strings over multiple lines if escaped with \ |
| String.trim() | Removes whitespace from both sides of a string |
| Array.isArray() | Returns true if a variable is an array |
| Array forEach() | Calls a function for each array element |
| Array map() | Creates a new array from a function on each element |
| Array filter() | Creates an array from array elements that passes a test |
| Array reduce() | Reduces an array to a single value (from left) |
| Array reduceRight() | Reduces an array to a single value (from right) |
| Array every() | Checks if all array values pass a test |
| Array some() | Checks if some values pass a test |
| Array indexOf() | Search for an element value and returns its position |
| Array lastIndexOf() | Search for an element value and returns its position |
| JSON.parse() | Convert JSON into a JavaScript object |
| JSON.stringify() | Convert JSON into a string |
| Date.now() | Returns the number of milliseconds since zero date |
| Date toISOString() | Converts a date object into to an ISO string |
| Date toJSON() | Converts a date object into to a JSON string |
| Property getters | Allows for defining how a property value is retrieved |
| Property setters | Allows for defining how a property value is set |
| Reserved names | Allows reserved names as property names |
| Object.create() | Creates an object from an existing object |
| Object.keys() | Returns an array with the keys of an object |
| Object management | Object management methods |
| Object protection | Object protection methods |
| Object defineProperty() | Allows for defining or changing object properties |
| Function bind() | Let objects borrow methods from other objects |
| Trailing commas | allows trailing commas in object and array definitions: |
Browser Support
JavaScript 2009 is supported in all modern browsers since July 2013:
| Chrome 23 |
IE/Edge 10 |
Firefox 21 |
Safari 6 |
Opera 15 |
| Sep 2012 | Sep 2012 | Apr 2013 | Jul 2012 | Jul 2013 |
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:
ES5 allows property access on strings:
Property access on string might be a little unpredictable.
Read more in JS String Methods.
Strings Over Multiple Lines
ES5 allows string literals over multiple lines if escaped with a backslash: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:
Reserved Words as Property Names
ES5 allows reserved words as property names:
String trim()
The trim() method removes whitespace from both sides of a string.
Read more in JS String Methods.
Array.isArray()
The isArray() method checks whether an object is an array.
Example
const fruits = ["Banana", "Orange", "Apple", "Mango"];
result = Array.isArray(fruits);
Try it Yourself »
Read more in JS Arrays.
Array forEach()
The forEach() method calls a function once for each array element.
Learn more in JS Array Iteration Methods.
Array map()
The map() method creates a new array by performing a function on each array element.
Example
Multiply each array value by 2:
const numbers1 = [45, 4, 9, 16, 25];
const numbers2 = numbers1.map(myFunction);
function myFunction(value) {
return value * 2;
}
Try it Yourself »
Learn more in JS Array Iteration Methods.
Array filter()
The filter() method creates a new array from array elements that passes a test.
Example
Create a new array from elements with a value larger than 18:
const numbers = [45, 4, 9, 16, 25];
const over18 = numbers.filter(myFunction);
function myFunction(value) {
return value > 18;
}
Try it Yourself »
Learn more in JS Array Iteration Methods.
Array reduce()
The reduce() method reduces an array to a single value.
Example
Find the sum of all numbers in an array:
const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction);
function myFunction(total, value) {
return total + value;
}
Try it Yourself »
Learn more in JS Array Iteration Methods.
Array reduceRight()
The reduceRight() method reduces an array to a single value (from right to left).
Example
Find the sum of all numbers in an array:
const numbers1 = [45, 4, 9, 16, 25];
let sum = numbers1.reduceRight(myFunction);
function myFunction(total, value) {
return total + value;
}
Try it Yourself »
Learn more in JS Array Iteration Methods.
Array every()
The every() method checks if all array values pass a test.
Example
Check if all values are over 18:
const numbers = [45, 4, 9, 16, 25];
let allOver18 = numbers.every(myFunction);
function myFunction(value) {
return value > 18;
}
Try it Yourself »
Learn more in JS Array Iteration Methods.
Array some()
The some() method checks if some array values pass a test.
Example
Check if some values are over 18:
const numbers = [45, 4, 9, 16, 25];
let allOver18 = numbers.some(myFunction);
function myFunction(value) {
return value > 18;
}
Try it Yourself »
Learn more in JS Array Iteration Methods.
Array indexOf()
The indexOf() method searches for an element value and returns its position.
Example
Search an array for an element value:
const fruits = ["Apple", "Orange", "Apple", "Mango"];
let position = fruits.indexOf("Apple") + 1;
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
const fruits = ["Apple", "Orange", "Apple", "Mango"];
let position = fruits.lastIndexOf("Apple") + 1;
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:
Example
const txt = '{"name":"John", "age":30, "city":"New York"}'
const myObj = JSON.parse(txt);
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:
const myObj = {name:"John", age:30, city:"New York"};
Use the JavaScript function JSON.stringify() to convert it into a string.
const myJSON = JSON.stringify(myObj);
The result will be a string following the JSON notation.
myJSON is now a string, and ready to be sent to a server:
Example
const myObj = {name:"John", age:30, city:"New York"};
const myJSON = JSON.stringify(myObj);
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).
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:
Property Getters
A property getter is a method that allows you to define how a property value is retrieved when it is accessed.
This example creates a getter for a property called fullName:
Example
// Create an object:
const 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 »
Property Setters
A property setter is a method that allows you to define how a property value is set.
This example uses a setter to secure upper case updates of language:
Example
const 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 »
This example creates a setter and a getter for the language property:
Example
const 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 »
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:
const person = {
firstName:
"John",
lastName : "Doe",
language : "NO",
};
// Change a Property:
Object.defineProperty(person, "language", {
value: "EN",
writable : true,
enumerable : true,
configurable : true
});
//
Enumerate Properties
let txt = "";
for (let x in person) {
txt += person[x] + "<br>";
}
// Display Properties
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:
const person = {
firstName:
"John",
lastName : "Doe",
language : "NO",
};
// Change a Property:
Object.defineProperty(person, "language", {
value: "EN",
writable : true,
enumerable : false,
configurable : true
});
//
Enumerate Properties
let txt = "";
for (let 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:
const 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 »
Object.create()
The Object.create() method creates an object from an existing object.
Example
// Create an Object:
const person = {
firstName: "John",
lastName: "Doe"
};
// Create new Object
const man = Object.create(person);
man.firstName = "Peter";
Try it Yourself »
Object.keys()
The Object.keys() method returns an array with the keys of an object.
Example
// Create an Object
const person = {
firstName: "John",
lastName: "Doe",
age: 50,
eyeColor: "blue"
};
// Get the Keys
const keys = Object.keys(person);
Try it Yourself »
Object Management
ES5 added new Object management methods to JavaScript:
Managing Objects
// Adding or changing an object property
Object.defineProperty(object, property, descriptor)
// Adding or changing object properties
Object.defineProperties(object, descriptors)
// Accessing a Property
Object.getOwnPropertyDescriptor(object, property)
// Accessing Properties
Object.getOwnPropertyDescriptors(object)
// Returns all properties as an array
Object.getOwnPropertyNames(object)
// Accessing the prototype
Object.getPrototypeOf(object)
Learn more in Object Properties.
Object Protection
ES5 added Object protection methods to JavaScript:
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 Protection.
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,]
