Callable

You can annotate callables as a part of a type or an interface as follows

interface ReturnString {
  (): string
}

An instance of such an interface would be a function that returns a string e.g.

declare const foo: ReturnString;
const bar = foo(); // bar is inferred as a string

Obvious examples

Of course such a callable annotation can also specify any arguments / optional arguments / rest arguments as needed. e.g. here is a complex example:

interface Complex {
  (foo: string, bar?: number, ...others: boolean[]): number;
}

An interface can provide multiple callable annotations to specify function overloading. For example:

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`

Of course, like the body of any interface, you can use the body of a callable interface as a type annotation for a variable. For example:

const overloaded: {
  (foo: string): string
  (foo: number): number
} = (foo: any) => foo;

Arrow Syntax

To make it easy to specify callable signatures, TypeScript also allows simple arrow type annotations. For example, a function that takes a number and returns a string can be annotated as:

const simple: (foo: number) => string
    = (foo) => foo.toString();

Only limitation of the arrow syntax: You can't specify overloads. For overloads you must use the full bodied { (someArgs): someReturn } syntax.

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 e.g.

interface CallMeWithNewToGetString {
  new(): string
}
// Usage
declare const Foo: CallMeWithNewToGetString;
const bar = new Foo(); // bar is inferred to be of type string