assert-never

Helper function for exhaustive checks of discriminated unions in TypeScript

Usage no npm install needed!

<script type="module">
  import assertNever from 'https://cdn.skypack.dev/assert-never';
</script>

README

Assert Never npm version

Helper function for exhaustive checks of discriminated unions in TypeScript.

Installation

npm install --save assert-never

Usage

import {assertNever} from "assert-never";

type A = {type: 'a'};
type B = {type: 'b'};
type Union = A | B;

function doSomething(arg: Union) {
  if (arg.type === 'a') {
    return something;
  }

  if (arg.type === 'b') {
    return somethingElse;
  }

  // TS will error if there are other types in the union
  // Will throw an Error when called at runtime. Use `assertNever(arg, true)`
  // instead to fail silently.
  return assertNever(arg);
}