Very unsure as to why my answer is returning in a string and not as an array. Is it because of concatenation?

The sum function should add up an array of numbers into a single number. For example:

sum([]) // 0

sum([ 1 ]) // 1

sum([ 1, 2 ]) // 3

sum([ 1, 2, 3 ]) // 6

```
function sum (numbers) {
let arr = [];
for(let i=0; i < arr.length; i++){
arr += numbers[i];
}
return arr;
}
```