no-undef

Using a name that no import, declaration, or parameter defines throws a ReferenceError when that line runs. It's usually a typo or a missing import, and you only find out when that code path runs.

Without a config file, bun lint knows the browser, service worker and Node.js globals, and Bun. In test files such as math.test.js it also knows test, expect, describe and the like.

This rule is on by default in JavaScript files. It's off in TypeScript files, where the type checker reports the same thing.

Configure

On by default as an error. To turn it off:

[lint.rules]
"no-undef" = "off" # or "warn"

Every option, set to its default:

[lint.rules]
"no-undef" = ["error", {
  typeof = false,
}]

Examples

Reported:

function total(items) {
  return items.reduce((sum, item) => sum + item.price, 0) * taxRate;
}

Not reported:

import { taxRate } from "./config.js";

function total(items) {
  return items.reduce((sum, item) => sum + item.price, 0) * taxRate;
}

In math.test.js:

test("adds", () => {
  expect(1 + 1).toBe(2);
});

If a script tag or the host defines a global, declare it. For one file, use a comment:

/* global analytics */

analytics.track("signup");

An eslint-env comment from an older ESLint setup still works. It declares all the globals of an environment such as jquery, mocha or webextensions. With an ESLint config file it's an error, as in ESLint 9 and later.

/* eslint-env jquery */

$(".menu").hide();

For the whole project, use bunfig.toml:

[lint.globals]
analytics = "readonly"

A name that the file tests with typeof isn't reported anywhere in that file. You've shown that you know it may be missing. With an ESLint config file only the typeof itself is allowed, as in ESLint.

if (typeof define === "function" && define.amd) {
  define(["jquery"], () => 1);
}

bun lint --infer-globals also accepts the globals that your project's types declare.

Options

Option	Default	What it does
typeof	false	Also report an undeclared name after typeof.

typeof never throws, so typeof name === "undefined" is a safe way to test for a global that may be missing. That's why it's allowed by default:

if (typeof analytics !== "undefined") {
  console.log("analytics is loaded");
}

if (typeof analytics !== "undefined") {
  console.log("analytics is loaded");
}