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';