数组reduce方法的使用

1.reduce 求数组中数值之和

/**
 * 1. reduce 求和 
 */
let arr = [1, 2, 3, 4, 5];
// let sum = arr.reduce(function (pre, cur, index) {
//     console.log(pre, cur, index);
//     return pre + cur;
// })
let sum = arr.reduce((x, y) => x + y)
console.log(sum);   // 15

2.reduce 统计数组中相同项出现的次数,用对象方式表达出来

/**
 * 2. reduce 统计数组中相同项出现的次数,用对象方式表达出来
 * ["Booy","Jiexi","Tio","Kori","Jiexi"]
 * {"Booy":1, "Jiexi":2, "Tio":1, "Kori":1} 
 */
let names = ["Booy", "Jiexi", "Tio", "Kori", "Jiexi"];
let obj = names.reduce((pre, cur, curIndex) => {
    if (cur in pre) {
        pre[cur]++;
    } else {
        pre[cur] = 1;
    }
    return pre
}, {})
console.log(obj);   //{Booy: 1, Jiexi: 2, Tio: 1, Kori: 1}

3.reduce 数组去重

/**
 * 3. reduce 数组去重 
 */
let num = [2, 5, 3, 6, 2, 3, 9, 3];
let newArr = num.reduce((pre, cur) => {
    if (pre.includes(cur)) {
        return pre;
    } else {
        return pre.concat(cur)
    }
}, [])
console.log(newArr);    //(5) [2, 5, 3, 6, 9]

4.reduce 将多维数组 转换为 一维数组

/**
 * 4. reduce 将多维数组 转换为 一维数组
 */
// 4.1 二维数组 ---->  一维数组
let arr1 = [1, [2, 3], [4, 5, 6]];
let newArr1 = arr1.reduce((pre, cur) => {
    return pre.concat(cur);
}, [])
console.log(newArr1);   //(6) [1, 2, 3, 4, 5, 6]

// 4.2 多维数组 ---->  一维数组
let arr2 = [1, [2, [3, 4]], [5, [6, [7]]]];
let newArr2 = function (arr) {
    return arr.reduce((pre, cur) => {
        return pre.concat(Array.isArray(cur) ? newArr2(cur) : cur)
    }, [])
}
console.log(newArr2(arr2)); //(7) [1, 2, 3, 4, 5, 6, 7]

5.reduce 将数组中的对象里的属性求和

/**
 * 5. reduce 对象里的属性求和
 */
let result = [
    {
        subject: "math",
        score: 10
    },
    {
        subject: "chinese",
        score: 20
    },
    {
        subject: "english",
        score: 30
    }
]
let sum1 = result.reduce((pre, cur) => {
    return pre + cur.score
}, 0)
console.log(sum1);  //60