I have a few questions regarding the “Where do I belong” problem.

First, why does the sort function has to be a call back function.

What is the sort (call back) function doing there. i.e.

```
arr.sort(function(a, b) {
return a - b;
});
```

why do we have to “return a-b”

While applying the sort function just like this:

```
arr.sort();
```

it does not sort this array

`[5, 3, 20, 3]`

but sorts every other array just fine, why is that?

here is my complete solution.

```
function getIndexToIns(arr, num) {
// console.log(arr.sort());
arr.sort(function(a, b) {
return a - b;
});
console.log(arr)
let i=0;
for(i=0; i<arr.length; i++){
if(num<=arr[i]){
console.log(i);
return i;
}
}
return i;
}
getIndexToIns([5, 3, 20, 3], 5);
```