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1 Answer

Thomas Nilsen
Thomas Nilsen
14,957 Points

I would do something like this:

(I added some comments. Let me know if something is still unclear. )

var list = [{id: "102", name: "Alice"}, {id: "205", name: "Bob", title: "Dr."}, {id: "592", name: "Clyde", age: 32}];

function groupBy(arr, callback) {
    //Object that will contain the key/val pairs
    var result = {};
    //Loop through the arr we passed in
    arr.forEach(obj => {
        //When we execute our callback function
        //We get back the value we want to groupBy
        var key = callback(obj);
        //If the key does NOT exist in the object,
        //then create it and set it equal an empty array
        if(!result.hasOwnProperty(key)) {
            result[key] = [];
        //Push the object that corresponds to our key

    //We have our answer. 
    return result;

console.log(groupBy(list, function(obj) {
    return obj.id;

This prints:

{ '102': [ { id: '102', name: 'Alice' } ],
  '205': [ { id: '205', name: 'Bob', title: 'Dr.' } ],
  '592': [ { id: '592', name: 'Clyde', age: 32 } ] }