Issue with Code in Salesforce Interview Questions

Question

I'm having a problem with a code snippet on the Salesforce Interview Questions page online: https://www.interviewbit.com/salesforce-interview-questions/.
I've been attempting to solve the problem but I'm having difficulty understanding the logic behind the code. The code I'm working on is:

//Create a function to take in an array of integers and return an object with the count of each integer
function countIntegers(arr) {
let resultObj = {};

for (let i = 0; i < arr.length; i++) {
let currNum = arr[i];
if (resultObj[currNum] === undefined) {
resultObj[currNum] = 1;
} else {
resultObj[currNum] += 1;
}
}
return resultObj;
}
I understand the code but I'm having difficulty understanding how the logic works, and how to apply it to the problem. I would really appreciate any help or advice on how to approach the problem. Thanks!

Answer 1

The loop checks if the nth element of resultObj exists. If yes, the program adds 1 to that nth element. Else, it assigns the number one to the previously undefined nth element.

Answer 2

For example,
Input: 25382618
Output: [undefined, 1, 2, 1, undefined, 1, 1, undefined, 2]
^ ^ ^
No 0 in the input No 4 in the input No 7 in the input
Do you understand?