用Generator封裝Symbol中的iterator方法：

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注意：Generator的function后必須寫*??

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config:分別有3個txt文件，兩個文件寫路徑，一個文件寫要輸出的內(nèi)容

前置寫法，node引入fs:

let fs = require('fs');function read(path){return new Promise((res, rej) => {fs.readFile(path, 'utf-8', (err,data) => {if(data){res(data);}else{rej(err);}})}) }

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當(dāng)前的迭代方法：let obj = {0: 'a',1: 'b',2: 'c',length:4,[Symbol.iterator]: function (){let index = 0;let next = () => {return {value: this[index],done: this.length == ++index}}return {next}}}console.log([...obj]);//(3)?["a", "b", "c"]for(let p of obj){console.log(p);//a b c }改寫后的方法:let obj = {0: 'a',1: 'b',2: 'c',length:3,[Symbol.iterator]: function* (){let index = 0;while(index != this.length){yield this[index++]}}}console.log([...obj]);//(2)?["a", "b"]for(let p of obj){console.log(p);//a b }

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用promise解決回調(diào)地獄問題與Generator方法對比

let fs = require('fs');function read(path){return new Promise((res, rej) => {fs.readFile(path, 'utf-8', (err,data) => {if(data){res(data);}else{rej(err);}})}) }read('./number.txt').then((value) => {return read(value); }).then((value) => {return read(value); }).then((value) => {console.log(value); })以上缺點(diǎn)調(diào)用太多次then方法

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那么我們用Generator方法解決一下回調(diào)地獄

function * readA() {let val1 = yield read('./number.txt');let val2 = yield read(val1);let val3 = yield read(val2);return val3; } //返回Generator對象 let oG = readA();let {value, done} = oG.next();value.then((val) => {let {value, done} = oG.next(val);value.then((val) => {let {value, done} = oG.next(val);value.then((val) => {console.log(val)})}) })

缺點(diǎn)：與回調(diào)地獄無區(qū)別

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Generator優(yōu)化：

注意：必須引入fs模塊才能運(yùn)行

function * readA() {let val1 = yield read('./number.txt');let val2 = yield read(val1);let val3 = yield read(val2);return val3; }function Co(oIt){return new Promise((res, rej) => {let next = (data) => {let {value, done} = oIt.next(data);if(done != true){value.then((val) => {next(val);})}else{res(value);}}next();}) }Co(readA()).then((val) => {console.log(val) })

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node有co模塊，在npm下載使用，效果一樣

轉(zhuǎn)載于:https://www.cnblogs.com/set-promise/p/ES6.html

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