ECMAscript6(ECMAscript 2015 ,ES5,ES2016)技術已經在前端圈子很流行了,他給前端開發人員帶來了很多驚喜,提供的語法糖使復雜的操作變得簡單。
本文沒有詳細描述這些新特性,因為網上都已經有很多相關的介紹了。主要針對ES6 新特性收集了相關范例代碼,他可以讓你快速了解這個新的javascript規范。
function()
函數的簡寫表示法,但它不綁定 this
。
var odds = evens.map(v => v + 1); // no parentes and no brackets var nums = evens.map((v, i) => v + i); var pairs = evens.map(v => ({even: v, odd: v + 1})); // Statement bodies nums.forEach(v => { if (v % 5 === 0) fives.push(v); });
this
是如何工作的?
var object = { name: "Name", arrowGetName: () => this.name, regularGetName: function() { return this.name }, arrowGetThis: () => this, regularGetThis: function() { return this } } console.log(this.name) console.log(object.arrowGetName()); console.log(object.arrowGetThis()); console.log(this) console.log(object.regularGetName()); console.log(object.regularGetThis());
結果:
this.name -> object.arrowGetName() -> object.arrowGetThis() -> [object Window] this -> [object Window] object.regularGetName() -> Name object.regularGetThis() -> {"name":"Name"}
我們知道“真正”語言中的類(Classes)。在 ES6 中類(Classes)其實是原型繼承的語法糖。
class SkinnedMesh extends THREE.Mesh { constructor(geometry, materials) { super(geometry, materials); this.idMatrix = SkinnedMesh.defaultMatrix(); this.bones = []; this.boneMatrices = []; //... } update(camera) { //... super.update(); } get boneCount() { return this.bones.length; } set matrixType(matrixType) { this.idMatrix = SkinnedMesh[matrixType](); } static defaultMatrix() { return new THREE.Matrix4(); } }
Lebab.io
var theProtoObj = { toString: function() { return "The ProtoOBject To string" } } var handler = () => "handler" var obj = { // __proto__ __proto__: theProtoObj, // Shorthand for ‘handler: handler' handler, // Methods toString() { // Super calls return "d " + super.toString(); }, // Computed (dynamic) property names [ "prop_" + (() => 42)() ]: 42 }; console.log(obj.handler) console.log(obj.handler()) console.log(obj.toString()) console.log(obj.prop_42)
結果:
obj.handler -> () => "handler" obj.handler() -> handler obj.toString() -> d The ProtoOBject To string obj.prop_42 -> 42
字符串插值的好語法
var name = "Bob", time = "today"; var multiLine = `This Line Spans Multiple Lines` console.log(`Hello ${name},how are you ${time}?`) console.log(multiLine)
結果:
`Hello ${name},how are you ${time}?` -> Hello Bob,how are you today? multiLine -> This Line Spans Multiple Lines
愚人碼頭注:列表匹配
// list "matching" var [a, , b] = [1,2,3]; console.log(a) console.log(b)
結果:
a -> 1 b -> 3
對象也能很好的解構
nodes = () => { return {op: "a", lhs: "b", rhs: "c"}} var { op: a, lhs: b , rhs: c } = nodes() console.log(a) console.log(b) console.log(c)
結果:
a -> a b -> b c -> c
使用速記表示法。
nodes = () => { return {lhs: "a", op: "b", rhs: "c"}} // binds `op`, `lhs` and `rhs` in scope var {op, lhs, rhs} = nodes() console.log(op) console.log(lhs) console.log(rhs)
結果:
op -> b lhs -> a rhs -> c
可在參數位置使用
function g({name: x}) { return x } function m({name}) { return name } console.log(g({name: 5})) console.log(m({name: 5}))
結果:
g({name: 5}) -> 5 m({name: 5}) -> 5
var [a] = [] var [b = 1] = [] var c = []; console.log(a) console.log(b); console.log(c);
結果:
a -> undefined b -> 1 c -> []
function f(x, y=12) { return x + y; } console.log(f(3)) console.log(f(3,2))
結果:
f(3) -> 15 f(3,2) -> 5
在函數中:
function f(x, y, z) { return x + y + z; } // 傳遞數組的每個元素作為參數 console.log(f(...[1,2,3]))
結果:
f(...[1,2,3]) -> 6
在數組中:
var parts = ["shoulders", "knees"]; var lyrics = ["head", ...parts, "and", "toes"]; console.log(lyrics)
結果:
lyrics -> ["head","shoulders","knees","and","toes"]
我們可以使用這個創造很酷的對象。
let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 }; console.log(x); // 1 console.log(y); // 2 console.log(z); // { a: 3, b: 4 } // Spread properties let n = { x, y, ...z }; console.log(n); // { x: 1, y: 2, a: 3, b: 4 } console.log(obj)
可惜的是它還不支持:
npm install --save-dev babel-plugin-transform-object-rest-spread
我們可以使用 rest 操作符來允許無限參數。
function demo(part1, ...part2) { return {part1, part2} } console.log(demo(1,2,3,4,5,6))
結果:
demo(1,2,3,4,5,6) -> {"part1":1,"part2":[2,3,4,5,6]}
let
是新的var
。 因為它有塊級作用域。
{ var globalVar = "from demo1" } { let globalLet = "from demo2"; } console.log(globalVar) console.log(globalLet)
結果:
globalVar -> from demo1 globalLet -> ReferenceError: globalLet is not defined
但是,它不會向window
分配任何內容:
let me = "go"; // 全局作用域 var i = "able"; // 全局作用域 console.log(window.me); console.log(window.i);
結果:
window.me -> undefined window.i -> able
不能使用let
重新聲明一個變量:
let me = "foo"; let me = "bar"; console.log(me);
結果:
SyntaxError: Identifier 'me' has already been declared
var me = "foo"; var me = "bar"; console.log(me)
結果:
me -> bar
const
是只讀變量。
const a = "b" a = "a"
結果:
TypeError: Assignment to constant variable.
應該注意,const 對象仍然可以被改變的。
const a = { a: "a" } a.a = "b" console.log(a)
結果:
a -> {"a":"b"}
迭代器的新類型,可以替代for..in
。 它返回的是值而不是keys
。
let list = [4, 5, 6]; console.log(list) for (let i in list) { console.log(i); }
結果:
list -> [4,5,6] i -> 0 i -> 1 i -> 2
let list = [4, 5, 6]; console.log(list) for (let i of list) { console.log(i); }
結果:
list -> [4,5,6] i -> 4 i -> 5 i -> 6
迭代器是一個比數組更動態的類型。
let infinite = { [Symbol.iterator]() { let c = 0; return { next() { c++; return { done: false, value: c } } } } } console.log("start"); for (var n of infinite) { // truncate the sequence at 1000 if (n > 10) break; console.log(n); }
結果:
"start" -> start n -> 1 n -> 2 n -> 3 n -> 4 n -> 5 n -> 6 n -> 7 n -> 8 n -> 9 n -> 10
使用Typescript,我們可以看到它接口的樣子:
interface IteratorResult { done: boolean; value: any; } interface Iterator { next(): IteratorResult; } interface Iterable { [Symbol.iterator](): Iterator }
生成器創建迭代器,並且比迭代器更具動態性。他們不必以相同的方式跟蹤狀態 並不支持 done
的概念。
var infinity = { [Symbol.iterator]: function*() { var c = 1; for (;;) { yield c++; } } } console.log("start") for (var n of infinity) { // truncate the sequence at 1000 if (n > 10) break; console.log(n); }
結果:
"start" -> start n -> 1 n -> 2 n -> 3 n -> 4 n -> 5 n -> 6 n -> 7 n -> 8 n -> 9 n -> 10
使用Typescript 再次顯示接口:
interface Generator extends Iterator { next(value?: any): IteratorResult; throw(exception: any); }
function* Iterators and generator
一個產量的例子*
function* anotherGenerator(i) { yield i + 1; yield i + 2; yield i + 3; } function* generator(i) { yield i; yield* anotherGenerator(i); yield i + 10; } var gen = generator(10); console.log(gen.next().value); console.log(gen.next().value); console.log(gen.next().value); console.log(gen.next().value); console.log(gen.next().value);
結果:
gen.next().value -> 10 gen.next().value -> 11 gen.next().value -> 12 gen.next().value -> 13 gen.next().value -> 20
ES6 為Unicode 提供了更好的支持。
var regex = new RegExp('\u{61}', 'u'); console.log(regex.unicode) console.log("\uD842\uDFD7") console.log("\uD842\uDFD7".codePointAt())
結果:
regex.unicode -> true "" -> "".codePointAt() -> 134103
原生支持模塊。
import defaultMember from "module-name"; import * as name from "module-name"; import { member } from "module-name"; import { member as alias } from "module-name"; import { member1 , member2 } from "module-name"; import { member1 , member2 as alias2 , [...] } from "module-name"; import defaultMember, { member [ , [...] ] } from "module-name"; import defaultMember, * as name from "module-name"; import "module-name";
export { name1, name2, …, nameN }; export { variable1 as name1, variable2 as name2, …, nameN }; export let name1, name2, …, nameN; // also var export let name1 = …, name2 = …, …, nameN; // also var, const export expression; export default expression; export default function (…) { … } // also class, function* export default function name1(…) { … } // also class, function* export { name1 as default, … }; export * from …; export { name1, name2, …, nameN } from …; export { import1 as name1, import2 as name2, …, nameN } from …;
Import Export
Set 為數學對應,其中所有項目都是唯一的。對於知道SQL的人來說,這相當於distinct
。
var set = new Set(); set.add("Potato").add("Tomato").add("Tomato"); console.log(set.size) console.log(set.has("Tomato")) for(var item of set) { console.log(item) }
結果:
set.size -> 2 set.has("Tomato") -> true item -> Potato item -> Tomato
Set
WeakSet對象允許您在集合中存儲弱持有的對象。沒有引用的對象將被垃圾回收。
var item = { a:"Potato"} var set = new WeakSet(); set.add({ a:"Potato"}).add(item).add({ a:"Tomato"}).add({ a:"Tomato"}); console.log(set.size) console.log(set.has({a:"Tomato"})) console.log(set.has(item)) for(let item of set) { console.log(item) }
結果:
set.size -> undefined set.has({a:"Tomato"}) -> false set.has(item) -> true TypeError: set[Symbol.iterator] is not a function
WeakSet
Map 也稱為詞典。
var map = new Map(); map.set("Potato", 12); map.set("Tomato", 34); console.log(map.get("Potato")) for(let item of map) { console.log(item) } for(let item in map) { console.log(item) }
結果:
map.get("Potato") -> 12 item -> ["Potato",12] item -> ["Tomato",34]
可以使用除字符串之外的其他類型。
var map = new Map(); var key = {a: "a"} map.set(key, 12); console.log(map.get(key)) console.log(map.get({a: "a"}))
結果:
map.get(key) -> 12 map.get({a: "a"}) -> undefined
Map
使用鍵的對象,並且只保留對鍵的弱引用。
var wm = new WeakMap(); var o1 = {} var o2 = {} var o3 = {} wm.set(o1, 1); wm.set(o2, 2); wm.set(o3, {a: "a"}); wm.set({}, 4); console.log(wm.get(o2)); console.log(wm.has({})) delete o2; console.log(wm.get(o3)); for(let item in wm) { console.log(item) } for(let item of wm) { console.log(item) }
結果:
wm.get(o2) -> 2 wm.has({}) -> false wm.get(o3) -> {"a":"a"} TypeError: wm[Symbol.iterator] is not a function
WeakMap
代理可以用來改變對象的行為。 它們允許我們定義 trap 。
var obj = function ProfanityGenerator() { return { words: "Horrible words" } }() var handler = function CensoringHandler() { return { get: function (target, key) { return target[key].replace("Horrible", "Nice"); }, } }() var proxy = new Proxy(obj, handler); console.log(proxy.words);
結果:
proxy.words -> Nice words
提供以下 trap :
var handler = { get:..., set:..., has:..., deleteProperty:..., apply:..., construct:..., getOwnPropertyDescriptor:..., defineProperty:..., getPrototypeOf:..., setPrototypeOf:..., enumerate:..., ownKeys:..., preventExtensions:..., isExtensible:... }
Proxy
Symbols 是一個新類型。 可用於創建匿名屬性。
var typeSymbol = Symbol("type"); class Pet { constructor(type) { this[typeSymbol] = type; } getType() { return this[typeSymbol]; } } var a = new Pet("dog"); console.log(a.getType()); console.log(Object.getOwnPropertyNames(a)) console.log(Symbol("a") === Symbol("a"))
結果:
a.getType() -> dog Object.getOwnPropertyNames(a) -> [] Symbol("a") === Symbol("a") -> false
更多信息
我們現在可以繼承原生類。
class CustomArray extends Array { } var a = new CustomArray(); a[0] = 2 console.log(a[0])
結果:
a[0] -> 2
不能使用數組的代理(Proxy)來覆蓋getter函數。
各種新的方法和常量。
console.log(Number.EPSILON) console.log(Number.isInteger(Infinity)) console.log(Number.isNaN("NaN")) console.log(Math.acosh(3)) console.log(Math.hypot(3, 4)) console.log(Math.imul(Math.pow(2, 32) - 1, Math.pow(2, 32) - 2)) console.log("abcde".includes("cd") ) console.log("abc".repeat(3) ) console.log(Array.of(1, 2, 3) ) console.log([0, 0, 0].fill(7, 1) ) console.log([1, 2, 3].find(x => x == 3) ) console.log([1, 2, 3].findIndex(x => x == 2)) console.log([1, 2, 3, 4, 5].copyWithin(3, 0)) console.log(["a", "b", "c"].entries() ) console.log(["a", "b", "c"].keys() ) console.log(["a", "b", "c"].values() ) console.log(Object.assign({}, { origin: new Point(0,0) }))
結果:
Number.EPSILON -> 2.220446049250313e-16 Number.isInteger(Infinity) -> false Number.isNaN("NaN") -> false Math.acosh(3) -> 1.7627471740390859 Math.hypot(3, 4) -> 5 Math.imul(Math.pow(2, 32) - 1, Math.pow(2, 32) - 2) -> 2 "abcde".includes("cd") -> true "abc".repeat(3) -> abcabcabc Array.of(1, 2, 3) -> [1,2,3] [0, 0, 0].fill(7, 1) -> [0,7,7] [1, 2, 3].find(x => x == 3) -> 3 [1, 2, 3].findIndex(x => x == 2) -> 1 [1, 2, 3, 4, 5].copyWithin(3, 0) -> [1,2,3,1,2] ["a", "b", "c"].entries() -> {} ["a", "b", "c"].keys() -> {} ["a", "b", "c"].values() -> TypeError: ["a","b","c"].values is not a function Object.assign({}, { origin: new Point(0,0) }) -> ReferenceError: Point is not defined
文檔: Number, Math, Array.from, Array.of, Array.prototype.copyWithin, Object.assign
二進制和八進制數字的字面量。
console.log(0b11111) console.log(0o2342) console.log(0xff); // also in es5
結果:
0b11111 -> 31 0o2342 -> 1250 0xff -> 255
異步編程。
var p1 = new Promise((resolve, reject) => { setTimeout(() => resolve("1"), 101) }) var p2 = new Promise((resolve, reject) => { setTimeout(() => resolve("2"), 100) }) Promise.race([p1, p2]).then((res) => { console.log(res) }) Promise.all([p1, p2]).then((res) => { console.log(res) })
結果:
res -> 2 res -> ["1","2"]
var p1 = Promise.resolve("1") var p2 = Promise.reject("2") Promise.race([p1, p2]).then((res) => { console.log(res) })
結果:
res -> 1
如果一個 promise 失敗,all
和race
也將 reject(拒絕)。
var p1 = new Promise((resolve, reject) => { setTimeout(() => resolve("1"), 1001) }) var p2 = new Promise((resolve, reject) => { setTimeout(() => reject("2"), 1) }) Promise.race([p1, p2]).then((res) => { console.log("success" + res) }, res => { console.log("error " + res) }) Promise.all([p1, p2]).then((res) => { console.log("success" + res) }, res => { console.log("error " + res) })
結果:
"error " + res -> error 2 "error " + res -> error 2
更多信息
新類型的元編程與新的API現有的還有一些新的方法。
var z = {w: "Super Hello"} var y = {x: "hello", __proto__: z}; console.log(Reflect.getOwnPropertyDescriptor(y, "x")); console.log(Reflect.has(y, "w")); console.log(Reflect.ownKeys(y, "w")); console.log(Reflect.has(y, "x")); console.log(Reflect.deleteProperty(y,"x")) console.log(Reflect.has(y, "x"));
結果:
Reflect.getOwnPropertyDescriptor(y, "x") -> {"value":"hello","writable":true,"enumerable":true,"configurable":true} Reflect.has(y, "w") -> true Reflect.ownKeys(y, "w") -> ["x"] Reflect.has(y, "x") -> true Reflect.deleteProperty(y,"x") -> true Reflect.has(y, "x") -> false
更多信息
尾調用的概念非常簡單,一句話就能說清楚,就是指某個函數的最後一步是調用另一個函數。
ES6可以確保尾調用不會造成堆棧溢出。 (不是所有的實現工作)。
function factorial(n, acc = 1) { if (n <= 1) return acc; return factorial(n - 1, n * acc); } console.log(factorial(10)) console.log(factorial(100)) console.log(factorial(1000)) console.log(factorial(10000)) console.log(factorial(100000)) console.log(factorial(1000000))
結果:
factorial(10) -> 3628800 factorial(100) -> 9.332621544394418e+157 factorial(1000) -> Infinity factorial(10000) -> Infinity factorial(100000) -> RangeError: Maximum call stack size exceeded factorial(1000000) -> RangeError: Maximum call stack size exceeded
原文:ES6 Features