Check whether a value is defined (non-nullable), meaning it is neither `null` or `undefined`. This can be useful as a type guard, as for example, `[1, null].filter(Boolean)` does not always type-guard correctly.

the tuple() function you need can be succinctly written as: export type Lit = string | number | boolean | undefined | null | void | {}; export const tuple = <T extends Lit[]>(...args: T) => args;

Emulating TypeScript functions

"serve": "tsc -w | firebase emulators:start --only functions",

    bar: {        (s: string): number;        (n: number): string;    }

(name: string): string

You have to augment the DateConstructor interface to add static properties:

Since TypeScript 1.4 static extensions can be added easily. The TypeScript team changed the lib.d.ts file to use interfaces for all static type definitions. The static type definitions are all named like [Type]Constructor: So if you want to add a static function to type Object, then add your definition to ObjectConstructor.

JsonSerializableStatic