bun/no-side-effects-on-import

A file that does something at its top level does it for everyone who imports it. A test that imports one helper from the file also starts the server, and a bundler has to keep the file even if nothing from it is used. Export a function instead, and let the importer decide when to call it.

The rule reports a statement that isn't inside a function if it calls something, uses new, import(), or a tagged template, or assigns to, deletes, or increments a property.

Configure#

Off by default. To turn it on:

[lint.rules]
"bun/no-side-effects-on-import" = "error" # or "warn"

Every option, set to its default. The ones that are commented out are unset by default.

[lint.rules]
"bun/no-side-effects-on-import" = ["error", {
  # allow = <list of strings>,
}]

Examples#

import { createServer } from "./server.js";

createServer().listen(3000);

globalThis.appVersion = "1.4.0";

import { createServer } from "./server.js";

export function start() {
  createServer().listen(3000);
}

Declarations aren't checked, so a module can still build its constants. require() and assigning to module.exports or exports are allowed too:

require("./polyfills.js");

const cache = new Map();

module.exports = { cache };
exports.size = 100;

A call in the assigned value is still reported:

module.exports = createApp();

Options#

The default list covers the functions that declare tests, hooks, and mocks, so test files pass:

import { expect, mock, test } from "bun:test";

mock.module("./database.js", () => ({ connect: mock() }));

test.each([1, 2])("is positive", value => {
  expect(value).toBeGreaterThan(0);
});

A plain name such as "test" allows everything that starts with it, like test() and test.each()(). A name with a dot such as "customElements.define" only allows that call:

customElements.define("user-card", UserCard);

customElements.upgrade(document.body);

When not to use it#

An entry point, a script, or a polyfill exists to do something when it runs. Turn the rule off for those files.