TypeScript Callable
Examples
Basic
interface ReturnString {
(): string
}
declare const foo: ReturnString
const bar = foo() // bar is inferred as a string
interface Complex {
(foo: string, bar?: number, ...others: boolean[]): number
}
Overloading
interface Overloaded {
(foo: string): string
(foo: number): number
}
// example implementation
function stringOrNumber(foo: number): number
function stringOrNumber(foo: string): string
function stringOrNumber(foo: any): any {
if (typeof foo === "number") {
return foo * foo
} else if (typeof foo === "string") {
return `hello ${foo}`
}
}
const overloaded: Overloaded = stringOrNumber
// example usage
const str = overloaded("") // type of `str` is inferred as `string`
const num = overloaded(123) // type of `num` is inferred as `number`
const overloaded: {
(foo: string): string
(foo: number): number
} = (foo: any) => foo
Arrow Syntax
const simple: (foo: number) => string = (foo) => foo.toString()
Newable
Newable is just a special type of callable type annotation with the prefix new. It simply means that you need to invoke with new:
interface CallMeWithNewToGetString {
new (): string
}
// Usage
declare const Foo: CallMeWithNewToGetString
const bar = new Foo() // bar is inferred to be of type string