eslint-plugin-toplevel

An eslint plugin for disallow side effect at module toplevel

Why?

Side effect in toplevel of a module are hard to reason, and behave diffrently in CommonJS and ES6 modules. And there is no way to import a module without it's side effects. Solution is simple: Don't make side effect at top level.

Example

Bad

console.log('hello world');
let s=0;
for (let i=0;i<10;i++) {
  s += i;
}
console.log(s);
fetch('/api').then(res=>res.text()).then(console.log);

Good

export async function main() {
  console.log('hello world');
  let s=0;
  for (let i=0;i<10;i++) {
    s += i;
  }
  console.log(s);
  const res = await fetch('/api');
  console.log(await res.text());
}

And then in html:

<script type="module">
  import { main } from "/js/main.mjs";
  main();
</script>

Or in nodejs make a runner.mjs file:

import { main } from "./main.mjs";
main();

Rules

There are three rules in this plugin:

no-toplevel-var

Disallow var usage at toplevel module.
Goal of this rule is to prevent modules to have internal state.

no-toplevel-let

similiar to no-toplevel-var

no-toplevel-side-effect

Disallow any side effect at top level

Contributing

I'd love to accept your patches and contributions to this project.
There are many ways you can contribute. For example:

• Add tests
• Fix bugs
• Add option to ignore module.exports assigns in no-toplevel-side-effect rule
• Add good documention