Tóm Tắt
Javascript ES6
ECMAScript 2015 was the second major revision to JavaScript.
ECMAScript 2015 is also known as ES6 and ECMAScript 6.
This chapter describes the most important features of ES6.
New Features in ES6
Browser Support for ES6 (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.
Example
var x = 10;
// Here x is 10
{
let x = 2;
// Here x is 2
}
// Here x is 10
Try it Yourself »
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:
Example
const x = (x, y) => { return x * y };
Try it Yourself »
Learn more about Arrow Functions in the chapter: JavaScript Arrow Function.
The Spread (…) Operator
The … operator expands an iterable (like an array) into more elements:
Example
const cars1 = [“Saab”, “Volvo”, …”BMW”];
const cars2 = [“Fiat”, “Toyota”];
const combined = [cars1, …cars2];
Try it Yourself »
The … operator can be used to expand an iterable into more arguments for function calls:
Example
const numbers = [23,55,21,87,56];
let maxValue = Math.max(…numbers);
Try it Yourself »
The For/Of Loop
The JavaScript for/of statement loops
through the values of an iterable objects.
for/of lets you loop over data structures
that are iterable such as Arrays, Strings, Maps, NodeLists, and more.
The for/of loop has the following syntax:
for (variable of iterable) {
// code block to be executed
}
variable – For every iteration the value of the next property is
assigned to the variable. Variable can be declared with
const, let, or var.
iterable – An object that has iterable properties.
Looping over an Array
Example
const cars = [“BMW”, “Volvo”, “Mini”];
let text = “”;
for (let x of cars) {
text += x + ” “;
}
Try it Yourself »
Looping over a String
Example
let language = “JavaScript”;
let text = “”;
for (let x of language) {
text += x + ” “;
}
Try it Yourself »
Learn more in the chapter: JavaScript Loop For/In/Of.
JavaScript Maps
Being able to use an Object as a key is an important Map feature.
Example
const fruits = new Map([
[“apples”, 500],
[“bananas”, 300],
[“oranges”, 200]
]);
Try it Yourself »
Learn more about Map objects, and the difference between a Map and an Array, in the the chapter: JavaScript Maps.
JavaScript Sets
Example
// Create a Set
const letters = new Set();
// Add some values to the Set
letters.add(“a”);
letters.add(“b”);
letters.add(“c”);
Try it Yourself »
Learn more about Set objects in the the chapter: JavaScript Sets.
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:
Example
const myCar1 = new Car(“Ford”, 2014);
const myCar2 = new Car(“Audi”, 2019);
Try it Yourself »
Learn more about classes in the the chapter: JavaScript Classes.
JavaScript Promises
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
const 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
const myPromise = new Promise(function(myResolve, myReject) {
setTimeout(function() { myResolve(“I love You !!”); }, 3000);
});
myPromise.then(function(value) {
document.getElementById(“demo”).innerHTML = value;
});
Try it Yourself »
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 instance, 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;
// Now person[id] = 140353
// but person.id is still undefined
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 »
String.includes()
The includes() method returns true if a string contains a specified value,
otherwise false:
Example
let text = “Hello world, welcome to the universe.”;
text.includes(“world”) // Returns true
Try it Yourself »
String.startsWith()
The startsWith() method returns true
if a string begins with a specified value, otherwise false:
Example
let text = “Hello world, welcome to the universe.”;
text.startsWith(“Hello”) // Returns true
Try it Yourself »
String.endsWith()
The endsWith() method returns true
if a string ends with a specified value, otherwise false:
Example
var text = “John Doe”;
text.endsWith(“Doe”) // Returns true
Try it Yourself »
Array.from()
The Array.from() method returns an Array object from any object with a length
property or any iterable object.
Example
Create an Array from a String:
Array.from(“ABCDEFG”) // Returns [A,B,C,D,E,F,G]
Try it Yourself »
Array keys()
The keys() method returns an Array Iterator object with the keys of an array.
Example
Create an Array Iterator object, containing the keys of the array:
const fruits = [“Banana”, “Orange”, “Apple”, “Mango”];
const keys = fruits.keys();
let text = “”;
for (let x of keys) {
text += x + “<br>”;
}
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
const numbers = [4, 9, 16, 25, 29];
let 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
const numbers = [4, 9, 16, 25, 29];
let 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 Math Methods
ES6 added the following methods to the Math object:
Math.trunc()Math.sign()Math.cbrt()Math.log2()Math.log10()
The Math.trunc() Method
Math.trunc(x) returns the integer part of x:
Example
Math.trunc(4.9); // returns 4
Math.trunc(4.7); // returns 4
Math.trunc(4.4); // returns 4
Math.trunc(4.2); // returns 4
Math.trunc(-4.2); // returns -4
Try it Yourself »
The Math.sign() Method
Math.sign(x) returns if x is negative, null or positive:
Example
Math.sign(-4); // returns -1
Math.sign(0); // returns 0
Math.sign(4); // returns 1
Try it Yourself »
The Math.cbrt() Method
Math.cbrt(x) returns the cube root of x:
Example
Math.cbrt(8); // returns 2
Math.cbrt(64); // returns 4
Math.cbrt(125); // returns 5
Try it Yourself »
The Math.log2() Method
Math.log2(x) returns the base 2 logarithm of x:
Example
Math.log2(2); // returns 1
Try it Yourself »
The Math.log10() Method
Math.log10(x) returns the base 10 logarithm of x:
Example
Math.log10(10); // returns 1
Try it Yourself »
New Number Properties
ES6 added the following properties to the Number object:
EPSILONMIN_SAFE_INTEGERMAX_SAFE_INTEGER
Example
let x = Number.EPSILON;
Try it Yourself »
Example
let x = Number.MIN_SAFE_INTEGER;
Try it Yourself »
Example
let x = Number.MAX_SAFE_INTEGER;
Try it Yourself »
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:
Example
isFinite(10/0); // returns false
isFinite(10/1); // returns true
Try it Yourself »
The isNaN() Method
The global isNaN() method returns true if the argument is NaN. Otherwise it returns false:
Example
isNaN(“Hello”); // returns true
Try it Yourself »
Object entries()
Example
Create an Array Iterator, and then iterate over the key/value pairs:
const fruits = [“Banana”, “Orange”, “Apple”, “Mango”];
const f = fruits.entries();
for (let x of f) {
document.getElementById(“demo”).innerHTML += x;
}
Try it Yourself »
The entries() method returns an Array Iterator object with key/value pairs:
[0, “Banana”]
[1, “Orange”]
[2, “Apple”]
[3, “Mango”]
The entries() method does not change the original array.
Modules
Modules are imported in two differen ways:
Import from named exports
Import named exports from the file person.js:
import { name, age } from “./person.js”;
Try it Yourself »
Import from default exports
Import a default export from the file message.js:
import message from “./message.js”;
Try it Yourself »
Learn more about Modules in: JavaScript Modules.
