reduce的常用方式，并且尝试自己实现一个reduce

 实现一个支持串行执行promise的函数

function seqPromise (arr, initValue) {

    return arr.reduce((lastPromise, currentPromise) => {

        return lastPromise.then(currentPromise)

    }, Promise.resolve(initValue))

}

var p1 = res => new Promise((resolve, reject) => {

    console.log('p1:', res)

    resolve('p1' + res)

})

var p2 = res => new Promise((resolve, reject) => {

    console.log('p2', res)

    resolve('p2' + res)

})

seqPromise([p1, p2], 'init')

// p1: init

// p2 p1init

// Promise {<resolved>: "p2p1init"} 

实现一个pipe

需求：参数为f1,f2,f3，arg，调用结果为 f3(f2(f1(arg)))

function pipeFunc (arr, arg) {

    return arr.reduce((lastValue, currentFunc) => {

        return currentFunc(lastValue)

    }, arg)

}

var f1 = res => {

    return 'f1' + res

}

var f2 = res => {

    return 'f2' + res

}

pipeFunc([f1, f2], 'init') // "f2f1init"

自己实现一个reduce

Array.prototype.reduce1 = function (handle, initValue) {

    var arr = this

    var index = initValue ? 0 : 1

    var result = initValue ? initValue : arr[0]

    arr.slice(index).forEach((item, idx) => {

        result = handle(result, item, idx + index, arr)

    })

    return result

}

var arr = ['a', 'b', 'c']

arr.reduce1((last, current, index, all) => {

    console.log(last, current, index, all)

    return last + current

})

// a b 1 (3) ["a", "b", "c"]

// ab c 2 (3) ["a", "b", "c"]

// "abc"