SOURCE

Array.prototype.reduce = Array.prototype.reduce || function(callfun) {
    var arr = this;
    var len = arr.length;
    if (typeof callfun !== 'function') {
        throw new TypeError('type error');
    }
    var accumulate = null;
    var i = 0;
    var kPresent = false;
    var initialVal = arguments.length > 1 ? arguments[1] : null;
    if (len === 0 && arguments.length < 2) {
        throw new TypeError('Reduce of empty array with no initial value');
    }

    if (arguments.length > 1) {
        accumulate = initialVal;
    } else {
        accumulate = arr[i];
        ++1;
    }
    while(k < len) {
        // 判断是否为 empty [,,,]
        kPresent = arr.hasOwnProperty(k);
        if (kPresent) {
            var val = arr[k];
            accumulate = callfun.apply(undefined, [accumulate, val, k, arr]);
        }
        k++;
    }
    return accumulate;

}