JS中的groupBy方法

Posted 2021-02-28 只争朝夕

最近工作中需要写一些运营数据报表的页面，后端返回的数据都是未处理过的json，所以，大量的运算任务交给了前端来做，这其中有一个功能就是对数据进行分组，一开始我自己也尝试写了一些，但代码量比较大，显得很繁琐，直到后来在stackoverflow上看到了某位大牛写的方法，非常简洁优雅，代码如下：

　　

function groupBy( array , f ) {
    let groups = {};
    array.forEach( function( o ) {
        let group = JSON.stringify( f(o) );
        groups[group] = groups[group] || [];
        groups[group].push( o );
    });
    return Object.keys(groups).map( function( group ) {
        return groups[group];
    });
}
 
let list = [
    {"name": "John","Average":15,"High":10,"DtmStamp":1358226000000},
    {"name": "Jane","Average":16,"High":92,"DtmStamp":1358226000000},
    {"name": "Jane","Average":17,"High":45,"DtmStamp":1358226000000},
    {"name": "John","Average":18,"High":87,"DtmStamp":1358226000000},
    {"name": "Jane","Average":15,"High":10,"DtmStamp":1358226060000},
    {"name": "John","Average":16,"High":87,"DtmStamp":1358226060000},
    {"name": "John","Average":17,"High":45,"DtmStamp":1358226060000},
    {"name": "Jane","Average":18,"High":92,"DtmStamp":1358226060000}
];
 
let sorted = groupBy(list, function(item){
    return [item.name];
});
console.log(sorted);

1. 函数groupBy有两个形参，一为对象数组，二为匿名函数（该函数功能：返回对象的某个指定属性的属性值并存放在数组中）；
2. groupBy函数内，先创建一个空对象；
3. 然后forEach遍历对象数组，遍历时要执行的函数中只有一个形参o（数组中的每个元素）；
4. 由于下面函数调用是想用name来分组，因此let group = JSON.stringify( f(o) )，相当于先获取到对象数组list中的name属性对应的属性值并放入数组中：["John"]，然后再将属性值转换为json字符串：‘["John"]‘；
5. groups[group] = groups[group] || []，在js中对象也是关联数组，因此这里相当于做了两件事，一是把group作为groups的key，二是将对应的value初始化，第一次执行为空数组，循环执行时找到相同的name时保持不变；
6. groups[group].push( o )，这句相当于把list中每个对象压入数组中作为value；
7. 最后，Object.keys(groups)是取出groups对象中的所有key，然后遍历一个个key组成的新数组，返回分好了组的二维数组，至此大功告成~~~

 

[ [ { name: ‘John‘, Average: 15, High: 10, DtmStamp: 1358226000000 },
    { name: ‘John‘, Average: 18, High: 87, DtmStamp: 1358226000000 },
    { name: ‘John‘, Average: 16, High: 87, DtmStamp: 1358226060000 },
    { name: ‘John‘, Average: 17, High: 45, DtmStamp: 1358226060000 } ],
  [ { name: ‘Jane‘, Average: 16, High: 92, DtmStamp: 1358226000000 },
    { name: ‘Jane‘, Average: 17, High: 45, DtmStamp: 1358226000000 },
    { name: ‘Jane‘, Average: 15, High: 10, DtmStamp: 1358226060000 },
    { name: ‘Jane‘, Average: 18, High: 92, DtmStamp: 1358226060000 } ] ]