when

when() function

It returns the original value based on the condition of the first argument or the result of executing the function passed as the second argument.

Signature:

declare function when<T, R>(predicate: (input: T) => boolean, callback: (input: T) => R, a: T): T | R;

declare function when<T, R>(predicate: (input: T) => boolean, callback: (input: T) => R): (a: T) => T | R;

declare function when<T, S extends T, R>(predicate: (input: T) => input is S, callback: (input: S) => R, a: T): Exclude<T, S>;

declare function when<T, S extends T, R>(predicate: (input: T) => input is S, callback: (input: S) => R): (a: T) => Exclude<T, S>;

Example

when(
  isNumber,
  () => `This is number`
  100,
); // This is number

with pipe

pipe(
  100,
  when(
    isNumber,
    () => `This is number`
  )
) // This is number

pipe(
  100,
  when(
    isNumber,
    (value) => value * 2
  )
) // 200

pipe(
  100,
  when(
    isString,
    () => `This is number` // not work
  ),
) // 100

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