@quad/invariant

TypeScript assertion functions.

• Statement form (invariant) and expression form (nonnull)
• Narrows types
• Throws a typed error class (InvariantError)
• Supports lazy messages — only evaluated on failure
• Clean stack traces
• Works in browser and on the server
• Zero dependencies

Install

deno add jsr:@quad/invariant

For other runtimes:

bunx jsr add @quad/invariant   # bun
npx jsr add @quad/invariant    # npm
pnpm add jsr:@quad/invariant   # pnpm
yarn add jsr:@quad/invariant   # yarn

Examples

In the browser

import { invariant, nonnull } from "@quad/invariant";

const node: Node = getNode();
invariant(node instanceof HTMLElement, "expected an element"); // `node` narrowed to `HTMLElement`
const parent: HTMLElement = nonnull(node.parentElement, "expected a parent");

On the server

import { invariant, nonnull } from "@quad/invariant";

const result: Success | Failure = await rpc.call(req);
invariant(result.ok, "rpc failed"); // `result` narrowed to `Success`
const token: string = nonnull(
  result.session?.token,
  "expected a session token",
);

API

invariant(condition, message?) — assert condition is truthy, narrow via asserts condition.

const value: string | number = parse(input);
invariant(typeof value === "string", "expected a string");
value.toUpperCase(); // `value` narrowed to `string`

nonnull<T>(value, message?) — assert value is non-null, narrow and return it.

const name: string = nonnull(user?.name, "expected a name");

message: string | (() => string) — lazy form only evaluates on failure.

invariant(condition, () => `expected ${expensive()}`);

InvariantError — an exported Error subclass, thrown by both. Stack trace points at the call site, not the library internals.

try {
  invariant(condition);
} catch (e) {
  if (e instanceof InvariantError) reportBug(e);
  throw e;
}

Prior art

See PRIOR_ART.md for a comparison of 20+ existing libraries and the design decisions behind this one.

License

Copyright 2026 Substrate AI, Inc.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

https://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.