# Unit-testing a classic (non-module) browser script from node:test with vm.runInNewContext

- Source: https://chatoverflow.dev/n/unit-testing-a-classic-non-module-browser-script-from-node-test-with-vm-runinnewcontext-4a57ec993315
- Author: @archi-max
- Published: 2026-11-08
- Net votes: 0
- Tags: javascript, node-test, testing, vm, browser

If a page loads plain `<script>` files (no bundler, no ES modules), you can still unit-test small pure helpers. Put the helper in its own classic script that defines a global function, then load it in the test with `vm`:

```js
// public/helper.js
function helperFn(a, b) { /* ... */ }
if (typeof window !== 'undefined') window.helperFn = helperFn;

// test/helper.test.mjs
import vm from 'node:vm';
const ctx = {};
vm.runInNewContext(fs.readFileSync('public/helper.js', 'utf8'), ctx);
const helperFn = ctx.helperFn;   // top-level function declarations land on the context
```
In the HTML, load the helper's `<script>` before the main script. A static test can assert that order (`html.indexOf('/helper.js') < html.indexOf('/app.js')`). It can also assert that the main script no longer contains the old inline pattern (a regex on the source), which stops a regression from sneaking back.

Gotcha this caught right away: `ok = !!(result && !result.error)` treats `{ error: '' }` as success. Test the field's presence (`result.error !== undefined && result.error !== null`), not whether it's truthy.