reduce 收敛

语法

函数的语法在API整理中已经收集了：《Array.prototype.reduce() 方法》。这边只介绍下它的应用。

应用

求和

let arr = [1, 2, 3, 4, 5];
let plus = arr.reduce((total, current) => total + current, 0);
console.log(plus); //15

let arr = [{ price: 100, count: 1 }, { price: 200, count: 2 }, { price: 3, count: 3 }];
let plus = arr.reduce(function (total, current) {
    return total + current.price * current.count
}, 0);
console.log(plus);//509

数据合并

let keys = ['name', 'age'];
let values = ['jw', 18]; // =>{name:'jw', age:18}
let obj = keys.reduce(function (total, current, index, arr) {
    total[current] = values[index];
    return total
}, {});
console.log(obj);//{ name: 'jw', age: 18 }
//简化写法
let objSimple = keys.reduce((total, current, index, arr) => (total[current] = values[index], total), {});
console.log(objSimple);//{ name: 'jw', age: 18 }

组合多个函数 compose

reduce() 作为一个高阶函数，用于函数的compose。

//组合多个函数
function sum(a, b) {
    return a + b;
}
function toUpper(str) {
    return str.toUpperCase();
}
function add(str) {
    return '***' + str + '***';
}
console.log(add(toUpper(sum('rui', 'jie'))));//***RUIJIE***
function compose(...fns) {
    return fns.reduce(function (total, current, index, arr) {
        return function (...args) {
            return total(current(...args));
        }
    })
}
console.log(compose(add, toUpper, sum)('rui', 'jie'));//***RUIJIE***
// 化简 
const composeSimple = (...fns) => fns.reduce((total, current) => (...args) => total(current(...args)));
console.log(composeSimple(add, toUpper, sum)('rui', 'jie'));

实现 reduce 源码

Array.prototype.reduce = function (callback, prev) {
    for (let i = 0; i < this.length; i++) {
        if (typeof prev === 'undefined') {
            prev = callback(this[i], this[i + 1], i + 1, this);
            i++;
        } else {
            prev = callback(prev, this[i], i, this);
        }
    }
    return prev;
}