Options

These are typical options available to the types and what do they mean.

🎭 Predicate

🎭 :performing_arts:

Predicate is also known as validate or logical. When the input satisfies the predicate, the type returns true. Otherwise, false.

It is one of the $Selection options (selection: 'predicate'). Typically, it is the default option.

(other icons considered: ⭕)

🌪️ Filter

🌪️ :tornados:

Filter is a type or function that filters the input. If the input passes the filter, it is returned unchanged. Otherwise, it returns never.

Filter is also known as pares, as in Parse, don’t validate.

It is one of the $Selection options (selection: 'filter').

The returned input can be narrowed if 🔀 distributive is also enabled. This means it is better to infer the return type instead of reusing the input type:

type IsUndefined<T> = T extends undefined ? T : never

// yes, these are silly, but just an example
type Bad<T> = IsUndefined<T> extends T ? T : never
type Good<T> = IsUndefined<T> extends infer R ? R : never

type R1 = Bad<undefined | number> // undefined | number
type R2 = Good<undefined | number> // undefined

(other icons considered: ↪️👉🚋⏩🐾🔑🚪💂🧲🙅‍♂️🪚)

🔀 Distributive

🔀 :twisted_rightwards_arrows:

Distributive means each value in a union type will be evaluated separately in conditional types, so both branches may be executed.

type R = IsUndefined<string | undefined> // true | false -> boolean

Typically, most types are distributive by default.

📌 Exact

📌 :pushpin:

Exact means type comparison will be performed strictly, treating subtype as separate types.

type R1 = IsString<'a', { exact: true }> // false

🔱 Branching

🔱 :trident:

Branching allows you to control the behavior of the types with type branching.

type R = IsNever<Input, {
  $any: 1,
  $unknown: 2,
  $then: 3,
  $else: 4,
}>