数组分组: 

含义: 数据按照某个特性归类   

1. reduce+fn(cur, index)作为对象的key,值为按照fn筛选出来的数据

// 利用reduce分组
function group(arr, fn) {
    // 不是数组
    if (!Array.isArray(arr)) {
        return arr
    }
    // 不是函数
    if (typeof fn !== 'function') {
        throw new TypeError('fn必须是一个函数')
    }
    var v
    return arr.reduce((obj, cur, index) => {
        v = fn(cur, index)
        if (!Reflect.hasOwnProperty.call(obj, v)) {
            obj[v] = []
        }
        obj[v].push(cur)
        return obj
    }, {})
}


// 按照字符串长度分组
let products = ["apple", "pear", "orange", "peach"];
const f1 = v => v.length
console.log(
    group(products, f1), 
);

// 按照分数分组
result = [{
    name: "tom",
    score: 60
}, {
    name: "Jim",
    score: 40
}, {
    name: "Nick",
    score: 88
}]
const fn = v => v.score >= 60

console.log(
    group(result, fn), 
);

2.filter+fn(value, index)作为对象的key,值为按照fn筛选出来的数据

// 利用forEach和filter分组
function group(arr, fn) {
    // 不是数组
    if (!Array.isArray(arr)) {
        return arr
    }
    // 不是函数
    if (typeof fn !== 'function') {
        throw new TypeError('fn必须是一个函数')
    }
    let obj = {}
    arr.forEach((item, key) => {
        const v = fn(item, key)
        obj[v] = arr.filter((ee, ix) => fn(ee, ix) === v)
    });
    return obj
}

// 按照字符串长度分组
let products = ["apple", "pear", "orange", "peach"];
const f1 = v => v.length
console.log(
    group(products, f1), 
);

// 按照分数分组
result = [{
    name: "tom",
    score: 60
}, {
    name: "Jim",
    score: 40
}, {
    name: "Nick",
    score: 88
}]
const fn = v => v.score >= 60

console.log(
    group(result, fn), 
);