Conditional Types and infer

Conditional types allow you to express non-trivial type decisions based on inheritance relationships. The syntax resembles JavaScript's ternary operator:

T extends U ? X : Y

If type T is assignable to U, then the resulting type will be X, otherwise it will be Y.

Basic Conditional Types

A conditional type evaluates a condition at compile time:

type IsNumber<T> = T extends number ? true : false;

type A = IsNumber<number>; // true
type B = IsNumber<string>; // false

This pattern is extremely powerful when combined with generics to create flexible and dynamic utility types.

Type Extraction with infer

Inside the extends clause of a conditional type, we can use the infer keyword to declare a type variable to be inferred by the compiler.

For example, if we want to extract the return type of a function:

type GetReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

type ExampleFunction = () => string;
type Return = GetReturnType<ExampleFunction>; // string

In this example, infer R tells TypeScript to automatically determine the function's return type and make it available as R in the positive branch of the conditional.

Try it yourself

Exercise 1: The IsString Type

Exercise 2: Extracting a type from an Array with infer