Grouping elements of an array of objects using a key from the object

groupBy.javascript.js

/**
 * Will return an object which contains all the elements from the `items` array grouped according to the `key` field
 * Eg. items = [{name: "ABC", age:2},{name: "DEF", age:2},{name: "GHI", age:2},{name: "JKL", age:3},{name: "MNO", age:3}]
 * key = "age"
 * result = {2: [{name: "ABC", age:2},{name: "DEF", age:2},{name: "GHI", age:2}], 3: [{name: "JKL", age:3},{name: "MNO", age:3}]}
 * @param {Array} items - array of items that needs to be grouped
 * @param {String} key - key in the object inside the items array for which the grouping is to be performed
 * @returns {*} - resulting object with grouped elements
 */
const groupBy = (items, key) =>
    items.reduce(
        (result, item) => ({
          ...result,
          [item[key]]: [...(result[item[key]] || []), item]
        }),
        {}
    );