用Generator封裝Symbol中的iterator方法：

註意：Generator的function後必須寫*

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);
            }
        })
    })
}

當前的叠代方法：
    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
        }

改寫後的方法:
        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
        }


    

用promise解決回調地獄問題與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);
})

以上缺點調用太多次then方法

那麽我們用Generator方法解決一下回調地獄

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：與回調地獄無區別

Generator優化：

註意：必須引入fs模塊才能運行

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)
})

node有co模塊，在npm下載使用，效果一樣

ES6-Generator使用與改寫