关于javascript:reduce方法高级使用

前端小伙伴儿应该都听过 reduce 这个数组的办法，总结一下我在开发过程中遇到的 reduce 的一些好玩儿的用法

老规矩，上 MDN：reduce-MDN

简略介绍一下一些重要的点

定义：reduce() 办法对数组中的每个元素执行一个由您提供的 reducer 函数(升序执行)，将其后果汇总为单个返回值。

示例：

const array1 = [1, 2, 3, 4];
const reducer = (accumulator, currentValue) => accumulator + currentValue;

// 1 + 2 + 3 + 4
console.log(array1.reduce(reducer));
// expected output: 10

// 5 + 1 + 2 + 3 + 4
console.log(array1.reduce(reducer, 5));
// expected output: 15

reducer 函数接管 4 个参数:

Accumulator (acc) (累计器)
CurrentValue (cur) (以后值)
CurrentIndex (idx) (以后索引)
SourceArray (src) (源数组)

您的 reducer 函数的返回值调配给累计器，该返回值在数组的每个迭代中被记住，并最初成为最终的单个后果值

arr.reduce(callback(accumulator, currentValue, index, array), initialValue)

callback执行数组中每个值 (如果没有提供 initialValue 则第一个值除外 ) 的函数，蕴含四个参数：

`accumulator`：累计器累计回调的返回值; 它是上一次调用回调时返回的累积值，或 `initialValue`。`currentValue`：数组中正在解决的元素。`index`(可选)：数组中正在解决的以后元素的索引。如果提供了 `initialValue`，则起始索引号为 0，否则从索引 1 起始。`array`(可选)：调用 `reduce()` 的数组

reduce为数组中的每一个元素顺次执行 callback 函数，不包含数组中被删除或从未被赋值的元素，承受四个参数：

• accumulator 累计器
• currentValue 以后值
• currentIndex 以后索引
• array 数组

回调函数第一次执行时，accumulator 和 currentValue 的取值有两种状况：
1、如果调用 reduce() 时提供了 initialValue，accumulator 取值为 initialValue，currentValue 取数组中的第一个值；
2、如果没有提供 initialValue，那么 accumulator 取数组中的第一个值，currentValue取数组中的第二个值。

留神：官网举荐在 reduce 应用时提供 initialValue，为了防止谬误，更多的具体了解请查阅 MDN，上面就用理论的例子来玩儿一下

1、根底的累加累乘

var  arr = [1, 2, 3, 4];
var sum = arr.reduce((prev, cur) => prev + cur, 0)
var mul = arr.reduce((prev, cur) => prev * cur, 1)
console.log(sum); //    10
console.log(mul); //    24

2、对象内的操作

var result = [
    {
        subject: 'math',
        score: 10
    },
    {
        subject: 'chinese',
        score: 20
    },
    {
        subject: 'english',
        score: 30
    }
];

var sum = result.reduce((prev, cur) => {return cur.score + prev;}, 0);
console.log(sum) //60

3、统计数组中元素呈现次数

let names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];

let nameNum = names.reduce((prev, cur)=>{if(cur in prev){prev[cur]++
  }else{prev[cur] = 1 
  }
  return pre
}, {})
console.log(nameNum); //{Alice: 2, Bob: 1, Tiff: 1, Bruce: 1}

4、数组去重

let arr = [1,2,3,4,4,1]
let newArr = arr.reduce((prev, cur)=>{if(!prev.includes(cur)){return prev.concat(cur)
    }else{return prev}
}, [])
console.log(newArr);// [1, 2, 3, 4]

5、多维数组降一维数组

let arr = [[0, 1], [2, 3], [4,[5,6,7]]]
const newArr = function(arr){return arr.reduce((prev, cur) => prev.concat(Array.isArray(cur) ? newArr(cur) : cur), [])
}
console.log(newArr(arr)); //[0, 1, 2, 3, 4, 5, 6, 7]

6、数组和对象深克隆

const deepClone = param => {if (typeof param !== 'object') return
    if (Array.isArray(param)) {param.reduce((prev, cur) => (cur instanceof Array ? [...prev, deepClone(cur)] : [...prev, cur]), [])
    } else {Object.entries(param).reduce((prev, [key, value]) => (typeof value === 'object' ? { ...prev, [key]: deepClone(value) } : {...prev, [key]: value }),
            {})
    }
    return param
}

7、封装一个同步程序执行函数，并返回后果

let fn1 = () => {
    return {
        name: 'lsd',
        age: 18
    }
}
let fn2 = () => {
    return {
        name: 'lbb',
        age: 19
    }
}
let fn3 = () => {
    return {
        name: 'whh',
        age: 20
    }
}

let fnlist = [fn1, fn2, fn3]

let res = fnlist.reduce((prev, cur) => {let t = cur()
    if (t) {prev.push(t)
    }
    return prev
}, [])
console.log(res)

8、基于 7 封装异步申请程序执行，并解决申请后果

let fn1 = () => {return new Promise((resolve, reject) => resolve(1))
}
let fn2 = () => {return new Promise((resolve, reject) => reject())
}
let fn3 = () => {return new Promise((resolve, reject) => resolve(2))
}
let fnlist = [fn1, fn2, fn3]
 let res = fnlist.reduce((prev, cur) => {cur().then(
        data => {if (data) {prev.push(data)
            }
        },
        reason => {prev.push('失败')
        }
    )
    return prev
}, [])
console.log(res)
// 应该在 then 函数中定义 onResolve 和 onRejct 函数，如果应用 catch 捕捉谬误，会进入下一次事件循环，不是同步执行；此处如果须要异步执行，请自行批改

9、模仿 koa 洋葱模型

// 每个中间件都能接管到 core
function receiveMiddleware(middlewareList) {
    // 将中间件队列革新为函数层层嵌套模式
    //[a,b,c,d] => a(b(c(d(core)))) By reduce
    let tiggerPipe = middlewareList.reduce((a, b) => core => a(b(core)))

    let tiggerPipeWitchCore = tiggerPipe(() => {console.log('我是外围操作')
    })

    return tiggerPipeWitchCore
}
const VerfiyCsrfToekn = next => lastMDarg => {console.log('验证 csrf Token')

    next(lastMDarg)

    console.log('验证 csrf Token end')
}

const VerfiyAuth = next => lastMDarg => {console.log('验证是否登录')

    next(lastMDarg)

    console.log('验证是否登录 end')
}

const VerfiyRoutes = next => lastMDarg => {console.log('验证路由匹配')

    next(lastMDarg)

    console.log('验证路由匹配 end')
}
let dispatch = receiveMiddleware([VerfiyCsrfToekn, VerfiyAuth, VerfiyRoutes])
dispatch()

10、带异步控制的中间件

const store = {status: { name: '固态空气'},
    getState: () => {return this.status},
    dispatch: arg => {console.log(` 我是外围操作, 参数 =${arg}`)
    }
}

function receiveMiddleware(middlewareList) {
    // 拿到中间件队列
    let dispatch = store.dispatch
    let middlewareAPI = {
        dispatch: arg => {dispatch(arg)
        },
        getState: store.getState
    }

    // 判断中间件数量
    if (middlewareList.length === 0) {return dispatch}
    // 将外围操作当作参数赋予每个中间件
    middlewareList = middlewareList.map(middleware => middleware(middlewareAPI))
    // 将中间件队列革新为函数层层嵌套模式
    //[a,b,c,d] => a(b(c(d(core)))) By reduce
    let tiggerPipe = middlewareList.reduce((prev, cur) => reallyDispatch => prev(cur(reallyDispatch)))

    // 重写 dispatch
    dispatch = tiggerPipe(store.dispatch)
    return dispatch
}

const VerfiyCsrfToekn = middlewareAPI => next => lastMDarg => {console.log('验证 csrf Token')
    next(lastMDarg)
    console.log('验证 csrf Token end')
}

const VerfiyAuth = middlewareAPI => next => lastMDarg => {console.log('验证是否登录')
    next(lastMDarg)
    console.log('验证是否登录 end')
}

const VerfiyRoutes = middlewareAPI => next => lastMDarg => {console.log('验证路由匹配')
    next(lastMDarg)
    console.log('验证路由匹配 end')
}

const asyncMiddleware = middlewareAPI => next => lastMDarg => {console.log('异步中间件 -start')
    if (typeof lastMDarg === 'function') {lastMDarg(middlewareAPI)
    } else {next(lastMDarg)
        console.log('异步中间件 -end')
    }
}

let dispatch = receiveMiddleware([VerfiyCsrfToekn, VerfiyAuth, VerfiyRoutes, asyncMiddleware])

let asyncFun = middlewareAPI => {setTimeout(() => {
        let test = '我是固态空气'
        middlewareAPI.dispatch(test)
        console.log(middlewareAPI.getState())
    }, 3000)
}

dispatch(asyncFun)