String Literal Types in TypeScript

In TypeScript, string literal types let you define types that only accept a defined string literal. They are useful to limit the possible string values of variables. It’s better explained with short code samples, so here are examples of how to define and use string literal types:

let pet: 'cat';

pet = 'cat'; // Ok
pet = 'dog'; // Compiler error

On their own, they don’t have that much use, but when combined with Union Types, things start to get interesting:

let pet: 'cat' | 'dog';

pet = 'cat'; // Ok
pet = 'dog'; // Ok
pet = 'zebra'; // Compiler error

As your string literal types start to get pretty long or when you use them at multiple places in your code, type aliases can become useful:

type Pet = 'cat' | 'dog' | 'hamster';

let pet: Pet;

pet = 'cat'; // Ok
pet = 'dog'; // Ok
pet = 'hamster'; // Ok
pet = 'alligator'; // Compiler error