ES2015 (ES6) 新特性
http://babeljs.io/docs/learn-es2015/
Learn ES2015
A detailed overview of ECMAScript 2015 features. Based on Luke Hoban's es6features repo.
es6features
This document was originally taken from Luke Hoban's excellent es6features repo. Go give it a star on GitHub!
REPL
Be sure to try these features out in the online REPL.
1
Introduction
ECMAScript 2015 is the newest version of the ECMAScript standard. This standard was ratified in June 2015. ES2015 is a significant update to the language, and the first major update to the language since ES5 was standardized in 2009. Implementation of these features in major JavaScript engines is underway now.
See the ES2015 standard for full specification of the ECMAScript 2015 language.
ECMAScript 2015 Features
Arrows and Lexical This
Arrows are a function shorthand using the => syntax. They are syntactically similar to the related feature in C#, Java 8 and CoffeeScript. They support both expression and statement bodies. Unlike functions, arrows share the same lexical this as their surrounding code.
// Expression bodies var odds = evens.map(v => v + 1); var nums = evens.map((v, i) => v + i); // Statement bodies nums.forEach(v => { if (v % 5 === 0) fives.push(v); }); // Lexical this var bob = { _name: "Bob", _friends: [], printFriends() { this._friends.forEach(f => console.log(this._name + " knows " + f)); } };
Classes
ES2015 classes are a simple sugar over the prototype-based OO pattern. Having a single convenient declarative form makes class patterns easier to use, and encourages interoperability. Classes support prototype-based inheritance, super calls, instance and static methods and constructors.
class SkinnedMesh extends THREE.Mesh {
constructor(geometry, materials) {
super(geometry, materials);
this.idMatrix = SkinnedMesh.defaultMatrix();
this.bones = [];
this.boneMatrices = [];
//...
}
update(camera) {
//...
super.update();
}
static defaultMatrix() {
return new THREE.Matrix4();
}
}
Enhanced Object Literals
Object literals are extended to support setting the prototype at construction, shorthand for foo: foo assignments, defining methods and making super calls. Together, these also bring object literals and class declarations closer together, and let object-based design benefit from some of the same conveniences.
var obj = {
// Sets the prototype. "__proto__" or '__proto__' would also work.
__proto__: theProtoObj,
// Computed property name does not set prototype or trigger early error for
// duplicate __proto__ properties.
['__proto__']: somethingElse,
// Shorthand for ‘handler: handler’
handler,
// Methods
toString() {
// Super calls
return "d " + super.toString();
},
// Computed (dynamic) property names
[ "prop_" + (() => 42)() ]: 42
};
The
__proto__property requires native support, and was deprecated in previous ECMAScript versions. Most engines now support the property, but some do not. Also, note that only web browsers are required to implement it, as it's in Annex B. It is available in Node.
Template Strings
Template strings provide syntactic sugar for constructing strings. This is similar to string interpolation features in Perl, Python and more. Optionally, a tag can be added to allow the string construction to be customized, avoiding injection attacks or constructing higher level data structures from string contents.
// Basic literal string creation
`This is a pretty little template string.`
// Multiline strings
`In ES5 this is
not legal.`
// Interpolate variable bindings
var name = "Bob", time = "today";
`Hello ${name}, how are you ${time}?`
// Unescaped template strings
String.raw`In ES5 "\n" is a line-feed.`
// Construct an HTTP request prefix is used to interpret the replacements and construction
GET`http://foo.org/bar?a=${a}&b=${b}
Content-Type: application/json
X-Credentials: ${credentials}
{ "foo": ${foo},
"bar": ${bar}}`(myOnReadyStateChangeHandler);
Destructuring
Destructuring allows binding using pattern matching, with support for matching arrays and objects. Destructuring is fail-soft, similar to standard object lookup foo["bar"], producing undefined values when not found.
// list matching
var [a, ,b] = [1,2,3];
a === 1;
b === 3;
// object matching
var { op: a, lhs: { op: b }, rhs: c }
= getASTNode()
// object matching shorthand
// binds `op`, `lhs` and `rhs` in scope
var {op, lhs, rhs} = getASTNode()
// Can be used in parameter position
function g({name: x}) {
console.log(x);
}
g({name: 5})
// Fail-soft destructuring
var [a] = [];
a === undefined;
// Fail-soft destructuring with defaults
var [a = 1] = [];
a === 1;
// Destructuring + defaults arguments
function r({x, y, w = 10, h = 10}) {
return x + y + w + h;
}
r({x:1, y:2}) === 23
Default + Rest + Spread
Callee-evaluated default parameter values. Turn an array into consecutive arguments in a function call. Bind trailing parameters to an array. Rest replaces the need for arguments and addresses common cases more directly.
function f(x, y=12) {
// y is 12 if not passed (or passed as undefined)
return x + y;
}
f(3) == 15
function f(x, ...y) {
// y is an Array
return x * y.length;
}
f(3, "hello", true) == 6
function f(x, y, z) {
return x + y + z;
}
// Pass each elem of array as argument
f(...[1,2,3]) == 6
Let + Const
Block-scoped binding constructs. let is the new var. const is single-assignment. Static restrictions prevent use before assignment.
function f() {
{
let x;
{
// okay, block scoped name
const x = "sneaky";
// error, const
x = "foo";
}
// okay, declared with `let`
x = "bar";
// error, already declared in block
let x = "inner";
}
}
Iterators + For..Of
Iterator objects enable custom iteration like CLR IEnumerable or Java Iterable. Generalize for..in to custom iterator-based iteration with for..of. Don’t require realizing an array, enabling lazy design patterns like LINQ.
let fibonacci = {
[Symbol.iterator]() {
let pre = 0, cur = 1;
return {
next() {
[pre, cur] = [cur, pre + cur];
return { done: false, value: cur }
}
}
}
}
for (var n of fibonacci) {
// truncate the sequence at 1000
if (n > 1000)
break;
console.log(n);
}
Iteration is based on these duck-typed interfaces (using TypeScript type syntax for exposition only):
interface IteratorResult {
done: boolean;
value: any;
}
interface Iterator {
next(): IteratorResult;
}
interface Iterable {
[Symbol.iterator](): Iterator
}
Support via polyfill
In order to use Iterators you must include the Babel polyfill.
Generators
Generators simplify iterator-authoring using function* and yield. A function declared as function* returns a Generator instance. Generators are subtypes of iterators which include additional next and throw. These enable values to flow back into the generator, so yield is an expression form which returns a value (or throws).
Note: Can also be used to enable ‘await’-like async programming, see also ES7 await proposal.
var fibonacci = {
[Symbol.iterator]: function*() {
var pre = 0, cur = 1;
for (;;) {
var temp = pre;
pre = cur;
cur += temp;
yield cur;
}
}
}
for (var n of fibonacci) {
// truncate the sequence at 1000
if (n > 1000)
break;
console.log(n);
}
The generator interface is (using TypeScript type syntax for exposition only):
interface Generator extends Iterator {
next(value?: any): IteratorResult;
throw(exception: any);
}
Support via polyfill
In order to use Generators you must include the Babel polyfill.
Comprehensions
Removed in Babel 6.0
Unicode
Non-breaking additions to support full Unicode, including new unicode literal form in strings and new RegExp u mode to handle code points, as well as new APIs to process strings at the 21bit code points level. These additions support building global apps in JavaScript.
// same as ES5.1
"