Most efficient method to groupby on an array of objects
If you want to avoid external libraries, you can concisely implement a vanilla version of groupBy() like so:
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"]}
Using ES6 Map object:
/** * @description * Takes an Array<V>, and a grouping function, * and returns a Map of the array grouped by the grouping function. * * @param list An array of type V. * @param keyGetter A Function that takes the the Array type V as an input, and returns a value of type K. * K is generally intended to be a property key of V. * * @returns Map of the array grouped by the grouping function. *///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;}// example usageconst 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
with ES6:
const groupBy = (items, key) => items.reduce( (result, item) => ({ ...result, [item[key]]: [ ...(result[item[key]] || []), item, ], }), {},);