In JavaScript, we can invert Boolean values with an exclamation mark. That doesn’t work for function names we use as shorthand in array methods like Array.prototype.filter() and Array.prototype.map(). 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]
Top comments (0)