JavaScript在过去几年发生了很大的变化。这篇文章整理了 JS 的12个新特性，你今天就开始使用！ES6是JavaScript语言的下一个版本，也称作 ECMAScript 6 和 ES2015+。ES6中包含了许多新的语言特性，它们将使JS变得更加强大，更富表现力。

JavaScript 历史

The new additions to the language are called ECMAScript 6. It is also referred as ES6 or ES2015+.

Since JavaScript conception on 1995, it has been evolving slowly. New additions happened every few years. ECMAScript came to be in 1997 to guide the path of JavaScript. It has been releasing versions such as ES3, ES5, ES6 and so on.

As you can see, there are gaps of 10 and 6 years between the ES3, ES5, and ES6. The new model is to make small incremental changes every year. Instead of doing massive changes at once like happened with ES6.

浏览器兼容

All modern browser and environments support ES6 already!

source: https://kangax.github.io/compat-table/es6/

Chrome, MS Edge, Firefox, Safari, Node and many others have already built-in support for most of the features of JavaScript ES6. So, everything that you are going to learn in this tutorial you can start using it right now.

Let’s get started with ECMAScript 6!

ES6 核心特性

You can test all these code snippets on your browser console!

So don’t take my word and test every ES5 and ES6 example. Let’s dig in

Block scope variables

With ES6, we went from declaring variables with var to use let/const.

What was wrong with var?

The issue with var is the variable leaks into other code block such as for loops or ifblocks.

ES5

var x = ‘outer’;
function test(inner) {
if (inner) {
var x = ‘inner’; // scope whole function
return x;
}
return x; // gets redefined because line 4 declaration is hoisted
}
test(false); // undefined 
test(true); // inner

For test(false) you would expect to return outer, BUT NO, you get undefined.

Why?

Because even though the if-block is not executed, the expression var x in line 4 is hoisted.

var hoisting:

var is function scoped. It is availble in the whole function even before being declared.

Declarations are Hoisted. So you can use a variable before it has been declared.

Initializations are NOT hoisted. If you are using var ALWAYS declare your variables at the top.

After applying the rules of hoisting we can understand better what’s happening:

ES5

1

2

3

4

5

6

7

8

9

var x = ‘outer’;

function test(inner) {

var x; // HOISTED DECLARATION

if (inner) {

x = ‘inner’; // INITIALIZATION NOT HOISTED

return x;

}

return x;

}

ECMAScript 2015 comes to the rescue:

ES6

let x = ‘outer’;
function test(inner) {
if (inner) {
let x = ‘inner’;
return x;
}
return x; // gets result from line 1 as expected
}
test(false); // outer
test(true); // inner

Changing var for let makes things work as expected. If the if block is not called the variable x doesn’t get hoisted out of the block.

Let hoisting and “temporal dead zone”

In ES6, let will hoist the variable to the top of the block (NOT at the top of function like ES5).

However, referencing the variable in the block before the variable declaration results in aReferenceError.

let is blocked scoped. You cannot use it before it is declared.

“Temporal dead zone” is the zone from the start of the block until the variable is declared.

IIFE

Let’s show an example before explaining IIFE. Take a look here:

ES5

{
var private = 1;
}
console.log(private); // 1

As you can see, private leaks out. You need to use IIFE (immediately-invoked function expression) to contain it:

ES5

(function(){
var private2 = 1;
})();
console.log(private2); // Uncaught ReferenceError

If you take a look at jQuery/lodash or other open source projects you will notice they have IIFEto avoid polluting the global environment and just defining on global such as _, $ orjQuery.

On ES6 is much cleaner, We also don’t need to use IIFE anymore when we can just use blocks and let:

ES6

{
let private3 = 1;
}
console.log(private3); // Uncaught ReferenceError

Const

You can also use const if you don’t want a variable to change at all.

Bottom line: ditch var for let and const.

Use const for all your references; avoid using var.

If you must reassign references, use let instead of const.

Template Literals

We don’t have to do more nesting concatenations when we have template literals. Take a look:

ES5

1
2
3

var first = ‘Adrian’;
var last = ‘Mejia’;
console.log(‘Your name is ‘ + first + ‘ ‘ + last + ‘.’);

Now you can use backtick (`) and string interpolation ${}:

ES6

1
2
3

const first = ‘Adrian’;
const last = ‘Mejia’;
console.log(`Your name is ${first} ${last}.`);

Multi-line strings

We don’t have to concatenate strings + \n anymore like this:

ES5

var template = ‘<li *ngFor=”let todo of todos” [ngClass]=”{completed: todo.isDone}” >\n’ +
‘ <div class=”view”>\n’ +
‘ <input class=”toggle” type=”checkbox” [checked]=”todo.isDone”>\n’ +
‘ <label></label>\n’ +
‘ <button class=”destroy”></button>\n’ +
‘ </div>\n’ +
‘ <input class=”edit” value=””>\n’ +
‘</li>’;
console.log(template);

On ES6 we can use the backtick again to solve this:

ES6

const template = `<li *ngFor=”let todo of todos” [ngClass]=”{completed: todo.isDone}” >
<div class=”view”>
<input class=”toggle” type=”checkbox” [checked]=”todo.isDone”>
<label></label>
<button class=”destroy”></button>
</div>
<input class=”edit” value=””>
</li>`;
console.log(template);

Both pieces of code will have exactly the same result.

Destructuring Assignment

ES6 desctructing is very useful and consise. Follow this examples:

Getting elements from an arrays

ES5

var array = [1, 2, 3, 4];
var first = array[0];
var third = array[2];
console.log(first, third); // 1 3

Same as:

ES6

const array = [1, 2, 3, 4];
const [first, ,third] = array;
console.log(first, third); // 1 3

Swapping values

ES5

var a = 1;
var b = 2;
var tmp = a;
a = b;
b = tmp;
console.log(a, b); // 2 1

same as

ES6

let a = 1;
let b = 2;
[a, b] = [b, a];
console.log(a, b); // 2 1

Destructuring for multiple return values

ES5

function margin() {
var left=1, right=2, top=3, bottom=4;
return { left: left, right: right, top: top, bottom: bottom };
}
var data = margin();
var left = data.left;
var bottom = data.bottom;
console.log(left, bottom); // 1 4

In line 3, you could also return it in an array like this (and save some typing):

1	return [left, right, top, bottom];

but then, the caller needs to think about the order of return data.

1 2	var left = data[0]; var bottom = data[3];

With ES6, the caller selects only the data they need (line 6):

ES6

function margin() {
const left=1, right=2, top=3, bottom=4;
return { left, right, top, bottom };
}
const { left, bottom } = margin();
console.log(left, bottom); // 1 4

Notice: Line 3, we have some other ES6 features going on. We can compact { left: left } to just { left }. Look how much concise it is compare to the ES5 version. Isn’t that cool?

Destructuring for parameters matching

ES5

var user = {firstName: ‘Adrian’, lastName: ‘Mejia’};
function getFullName(user) {
var firstName = user.firstName;
var lastName = user.lastName;
return firstName + ‘ ‘ + lastName;
}
console.log(getFullName(user)); // Adrian Mejia

Same as (but more concise):

ES6

const user = {firstName: ‘Adrian’, lastName: ‘Mejia’};
function getFullName({ firstName, lastName }) {
return `${firstName} ${lastName}`;
}
console.log(getFullName(user)); // Adrian Mejia

Deep Matching

ES5

function settings() {
return { display: { color: ‘red’ }, keyboard: { layout: ‘querty’} };
}
var tmp = settings();
var displayColor = tmp.display.color;
var keyboardLayout = tmp.keyboard.layout;
console.log(displayColor, keyboardLayout); // red querty

Same as (but more concise):

ES6

function settings() {
return { display: { color: ‘red’ }, keyboard: { layout: ‘querty’} };
}
const { display: { color: displayColor }, keyboard: { layout: keyboardLayout }} = settings();
console.log(displayColor, keyboardLayout); // red querty

This is also called object destructing.

As you can see, destructing is very useful and encourages good coding styles.

Best practices:

Use array destructing to get elements out or swap variables. It saves you from creating temporary references.

Don’t use array destructuring for multiple return values, instead use object destructuring

Classes and Objects

With ECMAScript 6, We went from “constructor functions” to “classes” .

In JavaScript every single object has a prototype, which is another object. All JavaScript objects inherit their methods and properties from their prototype.

In ES5, we did Object Oriented programming (OOP) using constructor functions to create objects as follows:

ES5

var Animal = (function () {
function MyConstructor(name) {
this.name = name;
}
MyConstructor.prototype.speak = function speak() {
console.log(this.name + ‘ makes a noise.’);
};
return MyConstructor;
})();
var animal = new Animal(‘animal’);
animal.speak(); // animal makes a noise.

In ES6, we have some syntax sugar. We can do the same with less boiler plate and new keywords such as class and constructor. Also, notice how clearly we define methodsconstructor.prototype.speak = function () vs speak():

ES6

class Animal {
constructor(name) {
this.name = name;
}
speak() {
console.log(this.name + ‘ makes a noise.’);
}
}
const animal = new Animal(‘animal’);
animal.speak(); // animal makes a noise.

As we saw, both styles (ES5/6) produces the same results behind the scenes and are used in the same way.

Best practices:

Always use class syntax and avoid manipulating the prototype directly. Why? because it makes the code more concise and easier to understand.

Avoid having an empty constructor. Classes have a default constructor if one is not specified.

Inheritance

Building on the previous Animal class. Let’s say we want to extend it and define a Lionclass

In ES5, It’s a little more involved with prototypal inheritance.

ES5

var Lion = (function () {
function MyConstructor(name){
Animal.call(this, name);
}
// prototypal inheritance
MyConstructor.prototype = Object.create(Animal.prototype);
MyConstructor.prototype.constructor = Animal;
MyConstructor.prototype.speak = function speak() {
Animal.prototype.speak.call(this);
console.log(this.name + ‘ roars ‘);
};
return MyConstructor;
})();
var lion = new Lion(‘Simba’);
lion.speak(); // Simba makes a noise.
// Simba roars.

I won’t go over all details but notice:

Line 3, we explicitly call Animal constructor with the parameters.
Line 7-8, we assigned the Lion prototype to Animal‘s prototype.
Line 11, we call the speak method from the parent class Animal.

In ES6, we have a new keywords extends and super .

ES6

class Lion extends Animal {
speak() {
super.speak();
console.log(this.name + ‘ roars ‘);
}
}
const lion = new Lion(‘Simba’);
lion.speak(); // Simba makes a noise.
// Simba roars.

Looks how legible this ES6 code looks compared with ES5 and they do exactly the same. Win!

Best practices:

Use the built-in way for inherintance with extends.

Native Promises

We went from callback hell to promises

ES5

function printAfterTimeout(string, timeout, done){
setTimeout(function(){
done(string);
}, timeout);
}
printAfterTimeout(‘Hello ‘, 2e3, function(result){
console.log(result);
// nested callback
printAfterTimeout(result + ‘Reader’, 2e3, function(result){
console.log(result);
});
});

We have one function that receives a callback to execute when is done. We have to execute it twice one after another. That’s why we called the 2nd time printAfterTimeout in the callback.

This can get messy pretty quickly if you need a 3rd or 4th callback. Let’s see how we can do it with promises:

ES6

function printAfterTimeout(string, timeout){
return new Promise((resolve, reject) => {
setTimeout(function(){
resolve(string);
}, timeout);
});
}
printAfterTimeout(‘Hello ‘, 2e3).then((result) => {
console.log(result);
return printAfterTimeout(result + ‘Reader’, 2e3);
}).then((result) => {
console.log(result);
});

As you can see, with promises we can use then to do something after another function is done. No more need to keep nesting functions.

Arrow functions

ES6 didn’t remove the function expressions but it added a new one called arrow functions.

In ES5, we have some issues with this:

ES5

var _this = this; // need to hold a reference
$(‘.btn’).click(function(event){
_this.sendData(); // reference outer this
});
$(‘.input’).on(‘change’,function(event){
this.sendData(); // reference outer this
}.bind(this)); // bind to outer this

You need to use a temporary this to reference inside a function or use bind. In ES6, you can use the arrow function!

ES6

// this will reference the outer one
$(‘.btn’).click((event) => this.sendData());
// implicit returns
const ids = [291, 288, 984];
const messages = ids.map(value => `ID is ${value}`);

For…of

We went from for to forEach and then to for...of:

ES5

// for
var array = [‘a’, ‘b’, ‘c’, ‘d’];
for (var i = 0; i < array.length; i++) {
var element = array[i];
console.log(element);
}
// forEach
array.forEach(function (element) {
console.log(element);
});

The ES6 for…of also allow us to do iterations.

ES6

// for …of
const array = [‘a’, ‘b’, ‘c’, ‘d’];
for (const element of array) {
console.log(element);
}

Default parameters

We went from checking if the variable was defined to assign a value to default parameters. Have you done something like this before?

ES5

function point(x, y, isFlag){
x = x || 0;
y = y || -1;
isFlag = isFlag || true;
console.log(x,y, isFlag);
}
point(0, 0) // 0 -1 true 
point(0, 0, false) // 0 -1 true 
point(1) // 1 -1 true
point() // 0 -1 true

Probably yes, it’s a common pattern to check is the variable has a value or assign a default. Yet, notice there are some issues:

Line 8, we pass 0, 0 and get 0, -1
Line 9, we pass false but get true.

If you have a boolean as a default parameter or set the value to zero, it doesn’t work. Do you know why??? I’ll tell you after the ES6 example 😉

With ES6, Now you can do better with less code!

ES6

function point(x = 0, y = -1, isFlag = true){
console.log(x,y, isFlag);
}
point(0, 0) // 0 0 true
point(0, 0, false) // 0 0 false
point(1) // 1 -1 true
point() // 0 -1 true

Notice line 5 and 6 we get the expected results. The ES5 example didn’t work. We have to check for undefined first since false, null, undefined and 0 are falsy values. We can get away with numbers:

ES5

function point(x, y, isFlag){
x = x || 0;
y = typeof(y) === ‘undefined’ ? -1 : y;
isFlag = typeof(isFlag) === ‘undefined’ ? true : isFlag;
console.log(x,y, isFlag);
}
point(0, 0) // 0 0 true
point(0, 0, false) // 0 0 false
point(1) // 1 -1 true
point() // 0 -1 true

Now it works as expected when we check for undefined.

Rest parameters

We went from arguments to rest parameters and spread operator.

On ES5, it’s clumpsy to get an arbitrary number of arguments:

ES5

function printf(format) {
var params = [].slice.call(arguments, 1);
console.log(‘params: ‘, params);
console.log(‘format: ‘, format);
}
printf(‘%s %d %.2f’, ‘adrian’, 321, Math.PI);

We can do the same using the rest operator ....

ES6

function printf(format, …params) {
console.log(‘params: ‘, params);
console.log(‘format: ‘, format);
}
printf(‘%s %d %.2f’, ‘adrian’, 321, Math.PI);

Spread operator

We went from apply() to the spread operator. Again we have ... to the rescue:

Reminder: we use apply() to convert an array into a list of arguments. For instance,Math.max() takes a list of parameters, but if we have an array we can use apply to make it work.

As we saw in earlier, we can use apply to pass arrays as list of arguments:

ES5

1	Math.max.apply(Math, [2,100,1,6,43]) // 100

In ES6, you can use the spread operator:

ES6

1	Math.max(…[2,100,1,6,43]) // 100

Also, we went from concat arrays to use spread operator:

ES5

var array1 = [2,100,1,6,43];
var array2 = [‘a’, ‘b’, ‘c’, ‘d’];
var array3 = [false, true, null, undefined];
console.log(array1.concat(array2, array3));

In ES6, you can flatten nested arrays using the spread operator:

ES6

const array1 = [2,100,1,6,43];
const array2 = [‘a’, ‘b’, ‘c’, ‘d’];
const array3 = [false, true, null, undefined];
console.log([…array1, …array2, …array3]);

JavaScript ES6特性（ECMAScript 6 and ES2015+）