Common Expectations

A small collection of common typescript functions for asserting and guarding types.

Installation

# using npm
npm install common-expectations
# or using yarn
yarn add common-expectations

Usages

exists<T>(value: T, message: string): void

A utility for asserting existence of a value when the type may be undefined or null.

import { exists } from "common-expectations";

// where use params returns a partial type where `id` maybe undefined.
const { id } = useParams<{ id: string }>();
exists(value, 'Expected id to be defined.')

This is intended only to be used in those rare cases where you know based on context that a value exists but the type system is unable to infer the fact. A good example of this is when using params in React Router.

isUnreachable(value: never, message?: string): void

A utility for helping check type exhaustiveness. It will raise typescript type error when a values type is anything other than never, and will throw an error at runtime if a value is encountered.

import { isUnreachable } from "common-expectations";

type AnimalType = 'cat' | 'dog' | 'monkey';

function doSomthing(type: AnimalType) {
  switch (type) {
    case "cat":
    case "dog":
    case "monkey":
      break;
    default:
      isUnreachable(type, `Encountered unexpected animal type: ${type}`);
  }
}