'Tis Inference System

Typescript has robust support for type guards, but writing functions that return is expressions is tedious. 'Tis aims to make it easy to add type guards.

is

import { is } from 'tis';

const isString = is('string');
const isTruthy = is('truthy');
const isInstance = is(Constructor);

is(typeof)

This form of is will narrow a type to only strings. It preserves constant string types if present.

is("truthy" / "falsey")

This form of is will narrow a type to only truthy or falsey values. It will provide no type information when called on types of string or number which contain both truthy and falsey values.

is(Constructor)

This form of is will narrow a type to types that are instances of a constructor.