Using Javascript is there a way to push information into an array that is apart of a key/value pair?

Question

I am having trouble with this algorithm and would love anyones support!

Essentially my trouble is coming on line 7.
Here I manage to create a key for each element while assigning a value that matches the current key we are working with.
But if you notice my output, instead of getting { 1: [1.3], 2: [2.1, 2.4] }
Im outputting { 1: [1.3], 2: [2.4] } .

function groupBy (array, callBack) {
    const myObj = {};
    let numsA = [];
    // iterate over array
    array.forEach(element => {
        //each element perform callBack
        myObj[callBack(element)] = [element];
        // console.log(myObj)
    })
    // return an object
    return myObj
};

const decimals = [1.3, 2.1, 2.4];
const floored = function(num) { return Math.floor(num); };

console.log(groupBy(decimals, floored)); 

// log: { 1: [1.3], 2: 2.4] }
// should log: { 1: [1.3], 2: [2.1, 2.4] } 

Is there a.push() feature for key/value pair objects as there is for arrays?
How do I add a value to the array of the 2nd key instead of overwriting it?

Answer 1

Try changing

myObj[callBack(element)] = [element];

To this:

if (myObj[callBack(element)] == undefined)
    myObj[callBack(element)] = [element];
else
    myObj[callBack(element)].push(element);

The idea is that if an array exists, the element will get added. Otherwise, a new array is created, with the element in it.

---

As @RobbyCornelissen pointed out, the callBack function is ran several times. Here is a more optimized version:

var output = callBack(element);
if (myObj[output] == undefined)
    myObj[output] = [element];
else
    myObj[output].push(element);

Answer 2

You can use the for in method to transverse the object and array or try array look up

Answer 3

You could capture everything in a simple reduce() operation:

 const groupBy = (array, callback) => { return array.reduce((a, v) => { const key = callback(v); a[key] = [...(a[key] || []), v]; return a; }, {}); }; const decimals = [1.3, 2.1, 2.4]; const floored = function(num) { return Math.floor(num); }; console.log(groupBy(decimals, floored));

Answer 4

to further Robby's contribution - you can pass the floor function right into groupBy

const groupBy = (array, callback) => array.reduce((a, v) => {
  const key = callback(v);
  a[key] = [...(a[key] || []), v];
  return a;
}, {});


const decimals = [1.3, 2.1, 2.4];

console.log(groupBy(decimals, Math.floor));