Composing TypeScript type guards

type-guard-composition.ts

type TypeGuard<T = any, U extends T = any> = (candidate: T) => candidate is U;

type From<T extends TypeGuard> =
  T extends TypeGuard<infer U, any>
   ? U
   : never;

type To<T extends TypeGuard> =
  T extends TypeGuard<any, infer U>
   ? U
   : never;

declare function or<T extends TypeGuard, U extends TypeGuard>(f: T, g: U): TypeGuard<From<T | U>, To<T> | To<U>>;

declare function isString(candidate: any): candidate is string;
declare function isNumber(candidate: any): candidate is number;

const isStringOrNumber = or(isString, isNumber);

declare const input: any;

if (isStringOrNumber(input)) {
    console.log(input.toString());
}

Implementation:

type TypeGuard<T = any, U extends T = any> = (candidate: T) => candidate is U;

type From<T extends TypeGuard> =
  T extends TypeGuard<infer U, any>
   ? U
   : never;

type To<T extends TypeGuard> =
  T extends TypeGuard<any, infer U>
   ? U
   : never;

function or<T extends TypeGuard, U extends TypeGuard>(f: T, g: U) {
  return (candidate => f(candidate) || g(candidate)) as TypeGuard<From<T | U>, To<T> | To<U>>
}

function isString(candidate: any): candidate is string {
  return typeof candidate === 'string';
}

function isNumber(candidate: any): candidate is number {
  return typeof candidate === 'number'
}

const isStringOrNumber = or(isString, isNumber);

const input = 'foo';

if (isStringOrNumber(input)) {
    console.log(`"${input}" is a string or a number`);
}

type TypeGuard<T, U extends T> = (candidate: T) => candidate is U;

type GetInputType<T extends TypeGuard<any, any>> =
  T extends TypeGuard<infer U, any>
   ? U
   : never;

type GetRefinementType<T extends TypeGuard<any, any>> =
  T extends TypeGuard<any, infer U>
   ? U
   : never;

declare function or<T extends TypeGuard<any, any>, U extends TypeGuard<any, any>>(
  f: T, g: U
): TypeGuard<GetInputType<T | U>, GetRefinementType<T> | GetRefinementType<U>>;

declare function isString(candidate: any): candidate is string;
declare function isNumber(candidate: any): candidate is number;

const isStringOrNumber = or(isString, isNumber);

declare const input: any;

if (isStringOrNumber(input)) {
    console.log(input.toString());
}

Here is another alternative if someone is interested https://gist.github.com/safareli/8e480d72f7e2665b6030b9a08fe93f40