# map()

### Description

`map()` is used to perform the same action on each individual value in a list.

That way, it lets you transform a list of values into another list, which comes in especially handy when working with user-provided values like `nationalities` or `sources_of_income`.

Also see [MAP](https://docs.atfinity.io/rule-language/operators/list-operators/map-1) for a non-method version.

### Example: Double some numbers

```
SUM(
  [1, 2, 3].map(number => number * 2)
)
```

* We call map on a list, here the list of numbers `[1, 2, 3]`.
* The argument within map is the function we use on each element, here `number * 2`. It takes each number from that array and multiplies it by 2. With each calculation, the variable `number` is replaced by the next value from the list, until the last value has been dealt with. We chose the name to be `number` as the variable before `=>`. We could have also chosen `n` or `x` if we would have wanted to.
* Since a SUM operator encloses the MAP function, the final output will be the added total of all doubled values - in this case: 12.

The calculations step-by-step:

* 1 \* 2 = 2
* 2 \* 2 = 4
* 3 \* 2 = 6
* The intermediate result of MAP is the new list \[2, 4, 6]
* SUM adds these new values: 2 + 4 + 6 = 12.