Implement the foreach method of array

Array of forEach Method is used to traverse the array , no return value .

let arr = [1, 2, 3, 4, 5];

arr.forEach((item, index, arr) => {
    console.log(index + ':' + item, arr);
})


0:1 (5) [1, 2, 3, 4, 5]
1:2 (5) [1, 2, 3, 4, 5]
2:3 (5) [1, 2, 3, 4, 5]
3:4 (5) [1, 2, 3, 4, 5]
4:5 (5) [1, 2, 3, 4, 5]

IE Medium Array There is no type forEach() Method , Then let's realize one forEach Method .

The first version

if (!Array.prototype.forEach1) {
    Array.prototype.forEach1 = function (callback, thisArg) {
        var _this, index;
        if (!Array.isArray(this)) {
            throw new TypeError(" Can only be used with arrays forEach Method ");
        }
        if (Object.prototype.toString.call(callback) != "[object Function]") {
            throw new TypeError('callback  It has to be a function ');
        }
        if (thisArg) {
            _this = thisArg;
        }
        for (index = 0; index < this.length; index++) {
            callback.call(_this, this[index], index, this);
        }
    }
}

But for loose arrays ,arr = [ , , , , ,] To use forEach You can't get results . Like we use above length To cycle , Yes, it will print out undefined Of , arr[i] Can't find it will return undefined.

So we need to change .

The second edition .

if (!Array.prototype.froEach) {
    Array.prototype.froEach = function(callback, thisArg) {
        let _this, k = 0;
        if (!Array.isArray(this)) {
            throw new TypeError(" Can only be used with arrays forEach Method ");
        }
        var O = Object(this);
        var len = this.length;
        if (Object.prototype.toString.call(callback) != "[object Function]") {
            throw new TypeError('callback  It has to be a function ');
        }
        if (thisArg) {
            _this = thisArg;
        }
        while (k < len) {
            var kValue;
            if (k in O) {
                kValue = O[k];
                callback.call(T, kValue, k, O);
            }
            k++;
        }
    };
}