SOURCE

Array.prototype.map = function (callbackFn, thisArg) {
    // 处理数组异常
    if (this === null || this === undefined) {
        throw new TypeError('Cannot read property "map" of null or undefined')
    }
    // 处理回调类型异常
    if (Object.prototype.toString.call(callbackFn) != '[object Function]') {
        throw new TypeError(callbackFn + 'is not a function')
    }
    // 草案中提到要先转换为对象
    let O = Object(this)
    let T = thisArg

    let len = O.length >>> 0
    let A = new Array(len)
    for (let k = 0; k < len; k++) {
        // 还记得原型链那一节提到的in吗？表示在原型链查找
        // 如果用 hasOwnProperty 是有问题的， 它只能找私有属性
        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
    }
}

[1,2,3].map(item => {return item})