# Inverting Boolean functions

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We can invert Boolean values with an exclamation mark. That does not work for functions. We can’t invert functions in the shorthand syntax of `Array.prototype.filter()` or `Array.prototype.map()`.

We can wrap those in a helper function to have them return the opposite of what they would return normally.

``````const numbers = [0, 1, 2, 3, 4, 5]
const isEven = n => n % 2 === 0

// the long and short form of this do the same
numbers.filter(number => isEven(number))  // ⇒ [0, 2, 4]
numbers.filter(isEven)                    // ⇒ [0, 2, 4]

// `!` can flip the Boolean value, but it only works with the long form
numbers.filter(number => !isEven(number))  // ⇒ [1, 3, 5]
numbers.filter(!isEven)                    // TypeError (not a function)

// this (curried) helper makes functions return a flipped result
const not = callback => value => !callback(value)

// we can use `not` like this, in both the long and short form
numbers.filter(number => not(isEven)(number))  // ⇒ [1, 3, 5]
numbers.filter(not(isEven))                    // ⇒ [1, 3, 5]``````
``````const numbers = [0, 1, 2, 3, 4, 5]
const isEven = n => n % 2 === 0

// the long and short form of this do the same
numbers.filter(number => isEven(number))  // ⇒ [0, 2, 4]
numbers.filter(isEven)                    // ⇒ [0, 2, 4]

// `!` can flip the Boolean value, but it only works with the long form
numbers.filter(number => !isEven(number))  // ⇒ [1, 3, 5]
numbers.filter(!isEven)                    // TypeError (not a function)

// this (curried) helper makes functions return a flipped result
const not = callback => value => !callback(value)

// we can use `not` like this, in both the long and short form
numbers.filter(number => not(isEven)(number))  // ⇒ [1, 3, 5]
numbers.filter(not(isEven))                    // ⇒ [1, 3, 5]``````