使用 reduce 统计一个数组中各个元素出现的次数

countElements.js

const arr = [4, 5, 3, 1, 6, 3, 4, 1]
let res = arr.reduce((countObj, ele) => {
    if (ele in countObj) {
        countObj[ele]++;
    } else {
        countObj[ele] = 1;
    }
    return countObj;
}, {});

// res: {1: 2, 3: 2, 4: 2, 5: 1, 6: 1}