How do I detect if an ES Module is the main module?

Question

How can I detect if an ECMAScript module is the main module? This is easy for CommonJS modules (see Detect if called through require or directly by command line).

What do you mean by main module? And in what context? browser? node.js? — Luka, Oct 03 '20 at 03:47
take a look at this https://stackoverflow.com/questions/57838022/detect-whether-es-module-is-run-from-command-line-in-node — Luka, Oct 03 '20 at 03:48
@Luka The main module is the module directly invoked. It's node.js, as noted in the tags. — Dietrich Epp, Oct 03 '20 at 03:52
@Take-Some-Bytes: That sounds like a proper answer, not a comment — Dietrich Epp, Oct 03 '20 at 03:53
Possible duplicate of [`if __name__ == '__main__'` equivalent in javascript es6 modules](https://stackoverflow.com/q/34842738/1048572) — Bergi, Aug 06 '21 at 22:44

score 9 · Accepted Answer · answered Oct 03 '20 at 04:07

9

You could use es-main.

From the package README:

import esMain from 'es-main';
 
if (esMain(import.meta)) {
  // Module run directly.
}

NOTE: This module will only work in a Node.JS environment, because the module code uses native Node.JS modules.

answered Oct 03 '20 at 04:07

Take-Some-Bytes

1

`es-main` [does](https://github.com/tschaub/es-main/blob/8a569c0ca2345221e62a1dedeb00f9a29301178f/main.js#L20) `fileURLToPath(import.meta.url) === process.argv[1]`, possibly stripping the extension from the module path if it's missing from the script name. – Boris Verkhovskiy Sep 29 '21 at 19:49

score 3 · Answer 2 · answered May 11 '22 at 18:43

3

In the browser I don't know, but in node with .mjs module the following seems to work :

const isMainModule = import.meta.url.endsWith(process.argv[1])

Explanation:

import.meta.url begins with the protocole file:// eg:

file:///path/to/my/module.mjs

but in node, process.argv[1] is shorter, eg:

/path/to/my/module.mjs

so endsWith is useful here.

answered May 11 '22 at 18:43

Joseph Merdrignac

I suspect this doesn't work if the path contains any characters that are quoted in URLs, and may not work on Windows at all. – Dietrich Epp May 12 '22 at 04:43
Worked perfectly on linux with TypeScript + [@tsconfig/node16-strictest-esm/tsconfig.json](https://github.com/tsconfig/bases#node-16--esm--strictest-tsconfigjson) + `moduleResolution: "node"`. – Jan Święcki Jun 27 '22 at 13:56

2 Answers2