Lesson
Tuesday
## Additional Resources

## Terminology

## Example

We just learned how to use the `forEach()`

function to loop through every element of an array. Now let's briefly explore `map()`

, a frequently-used array method that allows us to loop through every element of an array and *do something* to each element. Conveniently, the `map()`

method does not actually change the array it's called on, but it returns a new transformed array.

Let's say that we want to double every number in an array. We could do it with `forEach()`

this way:

```
var numbers = [1,2,3,4,5];
var doubledNumbers = [];
numbers.forEach(function(number) {
doubledNumbers.push(number * 2);
});
alert(doubledNumbers);
```

This works fine, but by using `.map()`

we can make the intent clearer, and our code even DRY-er:

```
var numbers = [1,2,3,4,5];
var doubledNumbers = numbers.map(function(number) {
return number * 2;
});
alert(doubledNumbers);
```

Here instead of looping through an array, we're simply saying that we want to transform one array into another. Note that the original array has not been changed:

```
> doubledNumbers;
[2, 4, 6, 8, 10]
> numbers;
[1, 2, 3, 4, 5]
```

The logic inside of `.map()`

can be as simple or as complicated as you want. Whatever you return will become the corresponding element of the new array. That is, when we say `return number * 2;`

in the example above, we're saying that each individual element of the *new* array will be equivalent multiplying the corresponding element in the *old* array by two.

So, continuing with this example, the first element in the original array is 1. Since we're multiplying each element by 2 in the `map()`

function, the first element in the *new* array will be 2. The second element in the *original* array is 2. So the second element in the *new* array will be 4, and so on.

For more information, check out the `map()`

entry in the Mozilla Developer Network's JavaScript documentation.

`map()`

: Is called upon an array. It creates a new array with the results of calling the provided function on every element of the original array.

The following loops through each item in the `numbers`

array, multiplies it by 2, and places it in a *new* array called `doubleNumbers`

:

```
var numbers = [1,2,3,4,5];
var doubledNumbers = numbers.map(function(number) {
return number * 2;
});
```