/** * @description * Takes an Array
             
              , 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
              
               (list: Array
               
                , keyGetter: (input: V) => K): Map
                
                 > { // const map = new Map
                 
                  >(); 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 usage 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 the map:https://developer.mozilla.org/en-US /docs/Web/JavaScript/Reference/Global_Objects/Map

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"]}