underscore源码分析之JS API

github:https://github.com/jingegebuguai/underscore-context

数组对象遍历(each)

each方法主要实现js的foreach函数：

目标：

var arr = [, , ]
    arr.each((ele, index) => {
        console.log(ele, index) //1,0 2,1 3,2
    })
           

实现代码：

var _each = function(fn) {
        if(typeof fn === 'function') {
            var i, length = this.length
            if(length === +length) {
                for(i = ; i < length; i++) {
                    fn.call(null, this[i], i, this)
                }
            } else {
                var keys = Object.keys(this)
                for(i = , length = keys.length; i < length; i++) {
                    fn.call(null, this[keys[i]], keys[i], this)
                }
            }
        }
        return this
    }

    Array.prototype.each = _each
    Object.prototype.each = _each

    var arr = {name: 'xxx', age: }
    arr.each((ele, index) => {
        console.log(ele, index)
    })
           

Map API

实现对数组和对象的map操作

目标：

var arr = [, , ]
    var newArr = arr.map((ele, index) =>{
        return ele*
    })
    console.log(newArr) //[2, 4, 6]

实现代码：

    var newMap = function(fn) {
        var keys = this.length !== +this.length && Object.keys(this)
        var length = (keys || this).length, currentKey
        var result = keys? {} : Array(length)
        for(var i = ; i < length; i++) {
            currentKey = keys? keys[i] : i
            result[currentKey] = fn.call(null, this[currentKey], currentKey, this)
        }
        return result
    }

    Array.prototype._map = newMap
    Object.prototype._map = newMap


    var arr1 = {name: 'xxx', age: '18'}
    var arr2 = [, , ]
    var obj = arr1._map((ele, index) => {
        return ele*
    })
    console.log(obj) //{ name: NaN, age: 36 }
           

Reduce API

实现对数组或对象的reduce操作

目标：

var arr = [, , ]
    var sum = arr.reduce((prev, next) => {
        return prev + next
    })
    console.log(newArr) //6
           

实现代码

var newReduce = function(fn, initValue) {
        var keys = this.length !== +this.length && Object.keys(this)
        var length = (keys || this).length, currentKey
        var result, i = , key
        if([].slice.call(arguments, ) == undefined || arguments.length < ) {
            result = keys? this[keys[i++]] : this[i++]
        } else {
            result = initValue
        }
        for(; i < length; i++) {
            key = keys? this[i] : i
            result = fn.call(null, result, this[key], key, this)
        }
        return result
    }

    Array.prototype._reduce = newReduce
    Object.prototype._reduce = newReduce


    var arr1 = {name: 'xxx', age: '18'}
    var arr2 = [, , ]
    var sum = arr2._reduce((prev, next) => {
        return prev + next
    })
    console.log(sum)
           

Filter API

实现对数组或对象的过滤操作

目标

var arr = [, , , , , ]
    var result = arr.filter(ele => {
        return ele %  == 
    })
    console.log(result)
           

实现代码

var newFilter = function(fn) {
        var length = this.length
        var result = Object.prototype.toString.call(this) === '[object, Array]'? [] : {}
        if(length === +length) {
            for(var i = ; i < length; i++) {
                if(fn.call(null, this[i], i, this)) {
                    result.push(this[i])
                }
            }
        } else {
            var keys = Object.keys(this)
            for(var i = ; i < keys.length; i++) {
                if(fn.call(null, this[keys[i]], keys[i], this)) {
                    result[keys[i]] = this[keys[i]]
                }
            }
        }
        return result
    }