Returning a subarray group in JS

Return an grouped version of an array by calling array.groupBy(fn) on any array. A grouped array with an object where each key is the output of fn(arr[i]) and each key is that array of items in the original array.

Consider:

Input: 
array = [
  {"id":"1"},
  {"id":"1"},
  {"id":"2"}
], 
fn = function (item) { 
  return item.id; 
}
Output: 
{ 
  "1": [{"id": "1"}, {"id": "1"}],   
  "2": [{"id": "2"}] 
}

To solve this, we firstly create a tempObj to return later.

We add the groupBy method to the Array prototype. Now, all arrays can use this method when it is invoked.

We use this inside the block because this will refer to the context in which a function is called - this would be the array that groupBy is invoked on.

We map over the array, and if the key doesn’t exist, we create an new array and with the individual el in. Otherwise, we push el to an existing key returned by fn(el). Then we return tempObj once the entire array is iterated through.

Array.prototype.groupBy = function(fn) {
    let tempObj = {}
    
    this.map((el) => {
        if (tempObj[fn(el)]) {
            tempObj[fn(el)].push(el)
        } else {
            tempObj[fn(el)] = [el]
        }
    } )

    return tempObj
};

/**
 * [1,2,3].groupBy(String) // {"1":[1],"2":[2],"3":[3]}
 */