ECMAScript 2015 - ES6
ECMAScript 6, also known as ES6 and ECMAScript 2015, was the second major revision to JavaScript.
This chapter describes the most important features of ES6.
New Features in ES6
- The
letkeyword - The
constkeyword - JavaScript Arrow Functions
- JavaScript Class
- JavaScript Promise
- JavaScript Symbol
- Default Parameter Values
- Function Rest Parameter
Array.find()Array.findIndex()- New Number Properties
- New Number Methods
- New Global Methods
Browser Support for ES6 (ECMAScript 2015)
Safari 10 and Edge 14 were the first browsers to fully support ES6:
| Chrome 58 | Edge 14 | Firefox 54 | Safari 10 | Opera 55 |
| Jan 2017 | Aug 2016 | Mar 2017 | Jul 2016 | Aug 2018 |
JavaScript let
The let keyword allows you to declare a variable with
block scope.
Read more about let in the chapter: JavaScript Let.
JavaScript const
The const keyword allows you to declare a constant (a
JavaScript variable with a constant value).
Constants are similar to let variables, except that the value cannot be changed.
Example
var x = 10;
// Here x is 10
{
const x = 2;
// Here x is 2
}
// Here x is 10
Try it Yourself »
Read more about const in the chapter: JavaScript Const.
Arrow Functions
Arrow functions allows a short syntax for writing function expressions.
You don't need the function keyword, the return keyword, and the
curly brackets.
Example
// ES5
var x = function(x, y) {
return x * y;
}
// ES6
const x = (x, y) => x * y;
Try it Yourself »
Arrow functions do not have their own this.
They are not well suited for defining object methods.
Arrow functions are not hoisted. They must be defined before they are used.
Using const
is safer than using var, because a function expression is
always a constant value.
You can only omit the return keyword and the curly brackets if the function is a single statement.
Because of this, it might be a good habit to always keep them:
Learn more about Arrow Functions in the chapter: JavaScript Arrow Function.
JavaScript Classes
JavaScript Classes are templates for JavaScript Objects.
Use the keyword class to create a class.
Always add a method named constructor():
Syntax
class ClassName {
constructor() { ... }
}
Example
class Car {
constructor(name, year) {
this.name = name;
this.year = year;
}
}
The example above creates a class named "Car".
The class has two initial properties: "name" and "year".
A JavaScript class is not an object.
It is a template for JavaScript objects.
Using a Class
When you have a class, you can use the class to create objects:
Learn more about classes in the the chapter: JavaScript Classes.
JavaScript Promise Object
A Promise is a JavaScript object that links "Producing Code" and "Consuming Code".
"Producing Code" can take some time and "Consuming Code" must wait for the result.
Promise Syntax
let myPromise = new Promise(function(myResolve, myReject) {
// "Producing Code" (May take some time)
myResolve(); // when successful
myReject(); // when error
});
// "Consuming Code" (Must wait for a fulfilled Promise).
myPromise.then(
function(value) { /* code if successful */ },
function(error) { /* code if some error */ }
);
Example Using a Promise
let myPromise = new Promise(function(myResolve, myReject) {
setTimeout(function() { myResolve("I love You !!"); }, 3000);
});
myPromise.then(function(value) {
document.getElementById("demo").innerHTML = value;
});
Learn more about Promises in the the chapter: JavaScript Promises.
The Symbol Type
A JavaScript Symbol is a primitive datatype just like Number, String, or Boolean.
It represents a unique "hidden" identifier that no other code can accidentally access.
For instace, if different coders want to add a person.id property to a person object belonging to a third-party code, they could mix each others values.
Using Symbol() to create a unique identifiers, solves this problem:
Example
const person = {
firstName: "John",
lastName: "Doe",
age: 50,
eyeColor: "blue"
};
let id = Symbol('id');
person.id = 140353;
Try it Yourself »
Symbols are always unique.
If you create two symbols with the same description they will have different values.
Symbol("id") == Symbol("id") // false
Default Parameter Values
ES6 allows function parameters to have default values.
Example
function myFunction(x, y = 10) {
// y is 10 if not passed or undefined
return x + y;
}
myFunction(5); // will return 15
Try it Yourself »
Function Rest Parameter
The rest parameter (...) allows a function to treat an indefinite number of arguments as an array:
Example
function sum(...args) {
let sum = 0;
for (let arg of args) sum += arg;
return sum;
}
let x = sum(4, 9, 16, 25, 29, 100, 66, 77);
Try it Yourself »
Array.find()
The find() method returns the value of the first array element that passes a
test function.
This example finds (returns the value of ) the first element that is larger than 18:
Example
var numbers = [4, 9, 16, 25, 29];
var first =
numbers.find(myFunction);
function myFunction(value, index, array) {
return
value > 18;
}
Try it Yourself »
Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
Array.findIndex()
The findIndex() method returns the index of the first array element that
passes a test function.
This example finds the index of the first element that is larger than 18:
Example
var numbers = [4, 9, 16, 25, 29];
var first =
numbers.findIndex(myFunction);
function myFunction(value, index, array) {
return
value > 18;
}
Try it Yourself »
Note that the function takes 3 arguments:
- The item value
- The item index
- The array itself
New Number Properties
ES6 added the following properties to the Number object:
EPSILONMIN_SAFE_INTEGERMAX_SAFE_INTEGER
New Number Methods
ES6 added 2 new methods to the Number object:
Number.isInteger()Number.isSafeInteger()
The Number.isInteger() Method
The Number.isInteger() method returns true if the argument is an integer.
Example
Number.isInteger(10); // returns true
Number.isInteger(10.5); // returns false
Try it Yourself »
The Number.isSafeInteger() Method
A safe integer is an integer that can be exactly represented as a double precision number.
The Number.isSafeInteger() method returns true if the argument is a safe integer.
Example
Number.isSafeInteger(10); // returns true
Number.isSafeInteger(12345678901234567890); // returns false
Try it Yourself »
Safe integers are all integers from -(253 - 1) to +(253 - 1).
This is safe: 9007199254740991. This is not safe: 9007199254740992.
New Global Methods
ES6 added 2 new global number methods:
isFinite()isNaN()
The isFinite() Method
The global isFinite() method returns false if the argument is Infinity or NaN.
Otherwise it returns true:
The isNaN() Method
The global isNaN() method returns true if the argument is NaN. Otherwise it returns false:
