js里面数组map方法的实现源码

if (!Array.prototype.map) {
    Array.prototype.map = function(callback) {
        var T, A, k;
        if (this == null) {
            throw new TypeError('this is null or not defined');
        }
        var O = Object(this);
        var len = O.length >>> 0;
        if (typeof callback !== 'function') {
            throw new TypeError(callback + ' is not a function');
        }
        if (arguments.length > 1) {
            T = arguments[1];
        }
        A = new Array(len);
        k = 0;
        while (k < len) {
            var kValue, mappedValue;
            if (k in O) {
                mappedValue = callback.call(T, kValue, k, O);
                A[k] = mappedValue;
            }
            k++;
        }
        return A;
    };
}

转载自：https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Array/map#polyfill