# Trying to understand code that locates a unique number.

```javascript
let counts = {};

  for(let i = 0; i < arr.length; i++) {
    if(counts[arr[i]] === undefined) {
      counts[arr[i]] = 1;
    } else {
      counts[arr[i]] += 1;
    }
  }
```

`let counts = {};`

This line creates an empty object named `counts`. In JavaScript, an object is a collection of properties, which are essentially key-value pairs. Here, we'll use the `counts` object to keep track of how many times each number appears in the array.

`for(let i = 0; i < arr.length; i++) { ... }`

This `for` loop goes through each element in the array `arr`. `i` is the index of the current element.

`if(counts[arr[i]] === undefined) { ... }`

This `if` statement checks whether the `counts` object has a property for the current number `arr[i]`. If it doesn't (which is the case when `counts[arr[i]]` is `undefined`), this means it's the first time we've seen this number.

`counts[arr[i]] = 1;`

If it's the first time we've seen the number, we set its count to 1 in the `counts` object. `arr[i]` is the current number and acts as the key in our `counts` object, and the value is the count of that number.

`else { ... }`

This `else` clause runs if the `counts` object already has a property for the current number `arr[i]` (in other words, if we've seen this number before).

`counts[arr[i]] += 1;`

If we've seen the number before, we increment its count by 1.

So, in conclusion, these lines of code go through each number in the array, count how many times each number appears, and store those counts in the `counts` object. After running this code, `counts` will look something like `{1: 2, 2: 1, 3: 1}` for an input array `[1, 1, 2, 3]`, which means the number `1` appears `2` times, and numbers `2` and `3` appear `1` time each.