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)