String Literal Type

You can use a string literal as a type. For example:

let foo: 'Hello';

Here we have created a variable called foo that will only allow the literal value 'Hello' to be assigned to it. This is demonstrated below:

let foo: 'Hello';
foo = 'Bar'; // Error: "Bar" is not assignable to type "Hello"

They are not very useful on their own but can be combined in a type union to create a powerful (and useful) abstraction e.g.:

type CardinalDirection =
    "North"
    | "East"
    | "South"
    | "West";

function move(distance: number, direction: CardinalDirection) {
    // ...
}

move(1,"North"); // Okay
move(1,"Nurth"); // Error!

Use cases

Valid use cases for string literal types are:

String based enums

TypeScript enums are number based. You can use string literals with union types to mock a string based enum as we did in the CardinalDirection example above.

Modelling existing JavaScript APIs

e.g. CodeMirror editor has an option readOnly that can either be a boolean or the literal string "nocursor" (effective valid values true,false,"nocursor"). It can be declared as:

readOnly: boolean | 'nocursor';

Discriminated Unions

We will cover this later in the book.