typescript-conditional-types

Helpers for typescript generic types

Table of Contents

• Motivation
• Install
• Type Helper List
• Usage Example

Motivation

Creating complex types with conditional types ( T extends U ? X : Y ) could be a little verbose. This package aims to simplify code and make it more readable.

Instead of

type ComplexType<A> = A extends boolean
  ? number
  : A extends number ? number : string
  : string

You could write

type ComplexType<A> = If<
  Or<Extends<A, number>, Extends<A, boolean>>,
  number,
  string
>;

Install

$ npm install typescript-conditional-types

You'll probably want to save it in the devDependencies

Type Helper List

• If<Condition, Then, Else>: If Condition is true resulting type is Then else Else
• And<A, B>: true if A and B are both true else false
• Or<A, B>: true if A or B are true else false
• Not: Negate A
• Extends<A, B>: true if A extends B like in A extends B ? true : false
• Extends<A, B, Then, Else: Equivalent to If<Extends<A, B>, Then, Else>

Usage Example

import { If } from "typescript-conditional-types";

type BooleanToString<T extends boolean> = If<T, "true", "false">

BooleanToString<true> // "true"
BooleanToString<false> // "false"