Conditional types

A conditional type is a kind of generic type which passes the generic through a condition before resolving to one of two types.

Syntax

type Conditional<T> = T extends Condition ? TrueType : FalseType;

This happens at compile-time.

Say, we have a Success type which contains some data, and we want to get the type of that data at compile time.

type Success = { success: true; data: unknown };
type Failure = { success: false; error: Error };

// Error! Type "data" cannot be used to index type T.
type SuccessData<T> = T["data"];

To fix this issue, you could restrict that SuccessData only takes in a Success object.

// This is happy now :)
type SuccessData<T extends Success> = T["data"];

// However, if we try to use this in practice it'll end up less good.
const response: Success | Failure = goGetAnAPI();

// Error! The response object might not have a data key.
const data: SuccessData<typeof response> = response.data;

However you run into that last issue. No bueno. A solution here would be to let SuccessData take in anything, and only unwrap if it's required. Otherwise, default to something like never to indicate that its value is not to be used.

type SuccessData<T> = T extends Success ? T["data"] : never;

// never | unknown
const data: SuccessData<typeof response>;