# Group array elements based on function

JavaScript, Array · Nov 3, 2020

Creates an array of elements, grouped based on the position in the original arrays and using a function to specify how grouped values should be combined.

- Check if the last argument provided is a function.
- Use
`Math.max()`

to get the longest array in the arguments. - Use
`Array.from()`

to create an array with appropriate length and a mapping function to create array of grouped elements. - If lengths of the argument arrays vary,
`undefined`

is used where no value could be found. - The function is invoked with the elements of each group.

```
const zipWith = (...array) => {
const fn =
typeof array[array.length - 1] === 'function' ? array.pop() : undefined;
return Array.from({ length: Math.max(...array.map(a => a.length)) }, (_, i) =>
fn ? fn(...array.map(a => a[i])) : array.map(a => a[i])
);
};
```

```
zipWith([1, 2], [10, 20], [100, 200], (a, b, c) => a + b + c); // [111, 222]
zipWith(
[1, 2, 3],
[10, 20],
[100, 200],
(a, b, c) =>
(a != null ? a : 'a') + (b != null ? b : 'b') + (c != null ? c : 'c')
); // [111, 222, '3bc']
```