// ES5循环实现map

const selfMap = function (fn, context) {

let arr = Array.prototype.slice.call(this)

let mappedArr = []

for (let i = 0; i < arr.length; i++) {

// 判断稀疏数组的情况

if (!arr.hasOwnProperty(i)) continue;

mappedArr.push(fn.call(context, arr[i], i, this))

}

return mappedArr

}

// reduce实现map

const selfMap2 = function (fn, context) {

let arr = Array.prototype.slice.call(this)

return arr.reduce((pre, cur, index) => {

return [...pre, fn.call(context, cur, index, this)]

}, [])

}

Array.prototype.selfMap || (Object.defineProperty(Array.prototype, 'selfMap', {

value: selfMap,

enumerable: false,

configurable: true,

writable: true

}))

Array.prototype.selfMap2 || (Object.defineProperty(Array.prototype, 'selfMap2', {

value: selfMap2,

enumerable: false,

configurable: true,

writable: true

}))

let arr = ['z', 'h', 'l']

console.log(arr.selfMap(item => item + "1"))

console.log(selfMap2.call({0:'a',1:'b',length:2}, item => item + "1")) // map 方法同样支持类数组