JS中的groupBy⽅法
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，⾃⼰也进⾏修改简化。代码如下：
function groupBy( array , id ) {
let groups = {};
array.forEach( function( o ) {
let group = JSON.stringify( o[id] );
groups[group] = groups[group] || [];
groups[group].push( o );
});
return Object.values(groups);
}
let name = "name"
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, name);
console.log(sorted);
1. 函数groupBy有两个形参，⼀为对象数组，⼆为指定分类⽅式的key；
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；
js 二维数组7. 最后，Object.values(groups)是取出groups对象中的所有values，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 } ] ]

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