The most efficient way to group object arrays

Question

What is the most efficient way to group objects in an array?

For example, given the following array of objects:

[
    { Phase: "Phase 1", Step: "Step 1", Task: "Task 1", Value: "5" },
    { Phase: "Phase 1", Step: "Step 1", Task: "Task 2", Value: "10" },
    { Phase: "Phase 1", Step: "Step 2", Task: "Task 1", Value: "15" },
    { Phase: "Phase 1", Step: "Step 2", Task: "Task 2", Value: "20" },
    { Phase: "Phase 2", Step: "Step 1", Task: "Task 1", Value: "25" },
    { Phase: "Phase 2", Step: "Step 1", Task: "Task 2", Value: "30" },
    { Phase: "Phase 2", Step: "Step 2", Task: "Task 1", Value: "35" },
    { Phase: "Phase 2", Step: "Step 2", Task: "Task 2", Value: "40" }
]

I am displaying this information in a table. I want to group by different methods but I want to sum the values.

I'm using the groupby function of Underscore.js, which is helpful, but doesn't quite satisfy the need because I don't want to "separate" them but "merge" them, more like SQL's group by method.

What I want is to be able to sum specific values ​​if needed.

So, if I group by Phase, I want to get:

[
    { Phase: "Phase 1", Value: 50 },
    { Phase: "Phase 2", Value: 130 }
]

If I group by Phase / Step, I want to get:

[
    { Phase: "Phase 1", Step: "Step 1", Value: 15 },
    { Phase: "Phase 1", Step: "Step 2", Value: 35 },
    { Phase: "Phase 2", Step: "Step 1", Value: 55 },
    { Phase: "Phase 2", Step: "Step 2", Value: 75 }
]

Is there a useful script that can do this, or should I just keep using Underscore.js and do the summing by looping over the result objects?

Answer 1

Use ES6 Map object:

/**
 * @description
 * 接受一个类型为V的数组和一个分组函数，返回按照分组函数分组的数组的Map。
 *
 * @param list 类型为V的数组。
 * @param keyGetter 一个函数，接受类型为V的数组作为输入，并返回类型为K的值。
 *                  K通常是V的属性键。
 *
 * @returns 按照分组函数分组的数组的Map。
 */
//export function groupBy<K, V>(list: Array<V>, keyGetter: (input: V) => K): Map<K, Array<V>> {
//    const map = new Map<K, Array<V>>();
function groupBy(list, keyGetter) {
    const map = new Map();
    list.forEach((item) => {
         const key = keyGetter(item);
         const collection = map.get(key);
         if (!collection) {
             map.set(key, [item]);
         } else {
             collection.push(item);
         }
    });
    return map;
}


// 使用示例

const pets = [
    {type:"Dog", name:"Spot"},
    {type:"Cat", name:"Tiger"},
    {type:"Dog", name:"Rover"}, 
    {type:"Cat", name:"Leo"}
];
    
const grouped = groupBy(pets, pet => pet.type);
    
console.log(grouped.get("Dog")); // -> [{type:"Dog", name:"Spot"}, {type:"Dog", name:"Rover"}]
console.log(grouped.get("Cat")); // -> [{type:"Cat", name:"Tiger"}, {type:"Cat", name:"Leo"}]

const odd = Symbol();
const even = Symbol();
const numbers = [1,2,3,4,5,6,7];

const oddEven = groupBy(numbers, x => (x % 2 === 1 ? odd : even));
    
console.log(oddEven.get(odd)); // -> [1,3,5,7]
console.log(oddEven.get(even)); // -> [2,4,6]

About Map: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map

Answer 2

If you want to avoid using an external library, you can simply implement a native version of groupBy() like this:

var groupBy = function(xs, key) {
  return xs.reduce(function(rv, x) {
    (rv[x[key]] = rv[x[key]] || []).push(x);
    return rv;
  }, {});
};

console.log(groupBy(['one', 'two', 'three'], 'length'));

// => {"3": ["one", "two"], "5": ["three"]}