es5实现map/filter

// ES5循环循环实现filter

const selfFilter = function (fn, context) {

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

    let filteredArr = []

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

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

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

    }

    return filteredArr

}

// 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)]

    }, [])

}