Conditional Types

In TypeScript, conditional types are a powerful feature that allows you to create types that depend on a condition. Conditional types are used to create types that change based on the properties of other types. They are often used in generic types to create more flexible and reusable code.

// Define a type that checks if a type is an array
type IsArray<T> = T extends any[] ? "yes" : "no";

// Test the IsArray type
type Test1 = IsArray<number>; // "no"
type Test2 = IsArray<string[]>; // "yes"

Using Infer

Prerequisites

• Template Literal Types

Infer is a keyword used in conditional types to infer the type of a variable based on a condition. Infer is often used in conjunction with the extends keyword to extract the type of a variable that matches a specific condition.

type FindPrefix<T> = 
    T extends `${infer Prefix}/${string}`
    ? Prefix
    : 'Not Found';


type Test = FindPrefix<'api/users'>;

// End of Example

For example above, the FindPrefix type extracts the prefix of a string that is separated by a /. If the string matches the pattern ${infer Prefix}/${string}, the type Prefix is inferred as the prefix of the string. Otherwise, the type is set to 'Not Found'.