# Functional JavaScript: Write your own negate/not function

Let’s say you want to filter odd numbers from a list of numbers, like `[1,2,3,4,5,6]`

should become `[1,3,5]`

.

One way to solve this problem in a functional fashion is to provide an `odd`

filter function, that is internally composed by a negation of an `even`

filter on those numbers.

So, let’s define these functions step by step:

```
function odd(number){
return not(even)
}
function not(predicate){
return function(){
return !predicate.apply(this, arguments)
}
}
function even(number){
return number % 2 == 0
}
```

The `even`

function needs no explaination, where the `functional magic ®`

happens is in the `odd`

and `not`

functions.

`odd`

takes a number and uses the `even`

filter, by negating it with `not`

, to determine whether a number is odd or not.

`not`

just wraps the `predicate`

function passed in and negates its application. Note that this application makes sense with functions that return boolean values, thus the name `predicate`

.

One use case could be in combination with `Array.prototype.filter`

:

```
[1,2,3,4,5,6].filter(not(even))
// or
[1,2,3,4,5,6].filter(odd)
```