Ryan KesPGP keyI build stuffEmailGithubTwitterLast.fmMastodonMatrixTypeScript String Literal Type
Description
Allows a String to be used as a literal type.
Syntax
let foo: "Hello"
foo = "Bar" // Error: "Bar" is not assignable to type "Hello"Examples
Unions
This is useful when uses with Unions:
type CardinalDirection = "North" | "East" | "South" | "West"
function move(distance: number, direction: CardinalDirection) {
// ...
}
move(1, "North") // Okay
move(1, "Nurth") // Error!Enums
If you want to go nuts you can mock a string based Enum:
/** Utility function to create a K:V from a list of strings */
function strEnum<T extends string>(o: Array<T>): { [K in T]: K } {
return o.reduce((res, key) => {
res[key] = key
return res
}, Object.create(null))
}
/**
* Sample create a string enum
*/
/** Create a K:V */
const Direction = strEnum(["North", "South", "East", "West"])
/** Create a Type */
type Direction = keyof typeof Direction
/**
* Sample using a string enum
*/
let sample: Direction
sample = Direction.North // Okay
sample = "North" // Okay
sample = "AnythingElse" // ERROR!Property names
String types can be uses as property names in mapped types.
type Options = {
[K in "noImplicitAny" | "strictNullChecks" | "strictFunctionTypes"]?: boolean
}
// same as
// type Options = {
// noImplicitAny?: boolean,
// strictNullChecks?: boolean,
// strictFunctionTypes?: boolean
// };