全網最完整的JavaScript迭代器（遍歷器）整理長文，強烈建議收藏！！！

    let arr = ['hello','been']
    const iterator = arr[Symbol.iterator]();
    console.log(iterator.next()); // {value: "hello", done: false}
    console.log(iterator.next()); // {value: "been", done: false}
    console.log(iterator.next()); // {value: undefined, done: true}

    Object.prototype.writeBlog = function () {
        console.log("hello world");
    }
    let obj = {
        age: 22,
        name: 'been'
    };
    for ( key in obj) {
        console.log(key);
    }

    for (let key in obj) {
        if (obj.hasOwnProperty(key)) {
            console.log(key);
        }
    }
    // 輸出：age  name

    let arr = ['hello','been'];
    for (let val of arr) {
        console.log(val);
    }
    //輸出：
    // hello
    // been

    let set = new Set();
    set.add('hello');
    set.add('been');
    for (let val of set) {
        console.log(val);
    }
    //輸出：
    // hello
    // been

    let map = new Map();
    map.set(0, "zero");
    map.set(1, "one");
    for (let item of map) {
        console.log(item);
    }
    // 輸出:
    // [0,"zero"]
    // [1,"one"]

    for (let [key,val] of map) {
        console.log(`${key}:${val}`);
    }
    // 輸出:
    // 0:"zero"
    // 1:"one"

    let obj = {
        age: 22,
        name: 'been'
    };
    for (let key of obj) {
        console.log(key);
    }
    // 報錯 Uncaught TypeError: obj is not iterable

    Object.prototype[Symbol.iterator] = function*() {
        let index = 0;
        let arr = Object.entries(this);
        let length = arr.length;
        while (true) {
            if (index >= length) {
                return false
            } else {
                let key = arr[index] && arr[index][0];
                let val = arr[index] && arr[index][1];
                let result = { [key]: val };
                index++;
                yield result 
            }
        }
    };

    // 創建異步可迭代物件
    const asyncIterable = {
        [Symbol.asyncIterator]() {
            return {
                i: 0,
                next() {
                    if (this.i < 2) {
                        return Promise.resolve({
                            value: this.i++,
                            done: false
                        });
                    }

                    return Promise.resolve({
                        done: true
                    });
                }
            };
        }
    };
    // 執行迭代
    (async function () {
        for await (num of asyncIterable) {
            console.log(num);
        }
    })();

    // 輸出:
    // 0
    // 1

    // 創建異步生成器
    async function* asyncGenerator() {
        var i = 0;
        while (i < 2) {
            yield i++;
        }
    }
    // 執行迭代
    (async function() {
        for await (num of asyncGenerator()) {
            console.log(num);
        }
    })();
    // 輸出:
    // 0
    // 1

    const arr = [1,2,3,4];
    const res = arr.forEach((v,i,a)=>{
        arr[i] = v*2;
    })
    console.log(arr,res);
    // 輸出：[2, 4, 6, 8] undefined

    const arr = [1,2,3,4];
    arr.forEach((val,i,eachArr)=>{
        if(val===2){
            return
        }
        eachArr[i] = val*2;
    })
    console.log(arr);
    // 輸出：[2, 2, 6, 8]

    const arr = [1,2,3,4];
    const res = arr.map(v=> v*2)
    console.log(arr,res);
    // 輸出:
    // [1, 2, 3, 4] 
    // [2, 4, 6, 8] 

    const arr = [1, 2, 3, 4];
    const res = arr.map(v => {
        if(v === 2){
            return
        }
        return v * 2
    })
    console.log(res);
    // 輸出: [2, undefined, 6, 8]

    const result1 = arr.every(v=>{
        console.log(v);
        return v<=4
    })
    console.log(result1);
    // 輸出：1 2 3 4 true


    const result2 = arr.every(v=>{
        console.log(v);
        return v>2
    })
    console.log(result2);
    // 輸出：1 false

    const arr = [1,2,3,4];
    const res = arr.every(v=>{
        v=v*2
        return v>1
    })
    console.log(res,arr);
    // 輸出:
    // true  [1, 2, 3, 4]

    const arr = [1,2,3,4];
    const result1 = arr.some(v=>{
        console.log(v);
        return v<=4
    })
    console.log(result1);
    // 輸出:
    // 1 true

    const result2 = arr.some(v=>{
        console.log(v);
        return v>4
    })
    console.log(result2);
    // 輸出:
    // 1 2 3 4 false

    const res = arr.some((v,i,a)=>{
        v=v*2
        return v<2
    })
    console.log(res,arr);
    // 輸出:
    // false [1, 2, 3, 4]