Guarding

In computing there's a pattern called "guard". Specifically, Haskell has a dedicated syntax for that, which fnts also implemented in a more JavaScripty way:

import guard from 'fnts/guard';

guard<(x: number) => number>(
  [(x) => x < 5, (x) => x + 1],
  [(x) => x === 5, (x) => x - 1], // we'll end up here eventually
  () => 1
)(5) // 4

The guard function accepts an arbitrary amount of arguments, each one, except for the last, should be a pair of a validator predicate (a function returning a boolean) and its corresponding expression. The qualifiers are checked one by one, and when encountering the validator that returns a truthy value, the expression next to it gets executed. All other qualifiers after it are ignored. If none of them is truthy, the last argument, which is a default expression, executes. All expressions must return the value of the same type.