JS基础---手写数组多个方法的底层实现

Array.prototype.push = function(...items) {
    let O = Object(this); // ecma 中提到的先转换为对象
    let len = this.length >>> 0;
    let argCount = items.length >>> 0;
    // 2 ^ 53 - 1为JS能表示的最大正整数
    if (len + argCount > 2 ** 53 - 1) {
        throw new TypeError('The number of array is over the max value')
    }
    for (let i = 0; i < argCount; i++) {
        O[len + i] = items[i];
    }
    let newLength = len + argCount;
    O.length = newLength;
    return newLength;
}

Array.prototype.pop = function() {
    let O = Object(this);
    let len = this.length >>> 0;
    if (len === 0) {
        O.length = 0;
        return undefined;
    }
    len--;
    let value = O[len];
    delete O[len];
    O.length = len;
    return value;
}

Array.prototype.map = function(callbackFn, thisArg) {
    if (this === null || this === undefined) {
        throw new TypeError('Cannot read property 'map' of null');
    }
    if (Object.prototype.toString.call(callbackFn) != '[object Function]') {
        throw new TypeError(callbackFn + 'is not a function');
    }
    let O = Object(this);
    let T = thisArg;

    let len = O.length >>> 0;
    let A = new Array(len);
    for (let k = 0; k < len; k++) {
        if (k in O) {
            let kValue = O[k];
            // 依次传入this，当前项，当前索引，整个数组
            let mappedValue = callbackFn.call(T, kValue, k, O);
            A[k] = mappedValue;
        }
    }
    return A;
}

Array.prototype.reduce = function(callbackFn, initialValue) {
    // 异常处理， 和map类似
    if (this === null || this === undefined) {
        throw new TypeError('Cannot read property 'reduce' of null');
    }
    // 处理回调类型异常
    if (Object.prototype.toString.call(callbackFn) !== '[Object Function]') {
        throw new TypeError(callbackFn + 'is not a function');
    }
    let O = Object(this);
    let len = O.length >>> 0;
    let k = 0;
    let accumulator = initialValue; // reduce 方法第二个参数作为累加器的初始值
    if (accumulator === undefined) { // 初始值不传的处理
        for (; k < len ; k++) {
            if (k in O) {
                accumulator = O[k];
                k++;
                break;
            }
        }
        throw new Error('Each element of the array is empty');
    }
    for (; k < len; k++) {
        if (k in O) {
            // 注意 reduce 的核心累加器
            accumulator = callbackFn.call(undefined, accumulator, O[k], O);
        }
    }
    return accumulator;
}