实现一个原生的数组Map

[lynhao]

Array.prototype._map = function(callbackfn, thisArg) {
    let O = new Object(this);
    if (this === null || this === undefined) {
        throw new TypeError("Cannot read property 'map' of null or undefined");
    }
    if (Object.prototype.toString.call(O) !== "[object Array]") {
        throw new TypeError("this is not a Array!")
    }
    let len = O.length >>> 0; // 向右移动0位, 相当于Math.floor
    if (Object.prototype.toString.call(callbackfn) !== "[object Function]") {
        throw new TypeError(callbackfn + "is not a function!")
    }
    let T = thisArg ? thisArg : undefined
    let A = new Array(len);
    let k = 0;
    while(k < len) {
        if (k in O) {
            let kValue = O[k];
            let mappedValue = callbackfn.call(T, kValue, O);
            A[k] = mappedValue;
        }
        k++;
    }
    return A;
    
}

Test

[2,3,4]._map(r => r * 2) // [4,6,8]

[{b:1},{b:2}]._map(r => {
    return {
        ...r,
        c: r.b
    }
}) 
//[{b:1, c:1},{b:2, c:2}]