Detect Whether Es Module Is Run From Command Line In Node

When using CommonJS modules in Node, you can detect whether a script is being run from the command line using require.main === module. What is an equivalent way to detect whether a

Solution 1:

Use

if (import.meta.url === `file://${process.argv[1]}`) {
  // module was not imported but called directly
}

See the MDN docs on import.meta for details.

Update Sep 27, 2021

Perhaps more robust, but involving an extra import (via Rich Harris)

import url from'url'if (import.meta.url === url.pathToFileURL(process.argv[1]).href) {
  // module was not imported but called directly
}

Solution 2:

There is none - yet (it's still experimental!). Although the prevailing opinion is that such a check is a bad practice anyway and you should just provide separate scripts for the library and the executable, there is an idea to provide a boolean import.meta.main property for this purpose.

Solution 3:

The module global variable will be defined in CommonJS, but won’t exist at all in an ES module. Yes, there is an inconsistency there, that ES modules are the things that don’t have module variables.

You can check for an undefined variable by seeing if typeof v is the string (not value!) 'undefined'.

That turns into:

const inCommonJs = typeofmodule !== 'undefined';

console.log(`inCommonJs = ${inCommonJs}`);

If we put that exact code into both .cjs and .mjs files, we get the correct answers:

$ node foo.mjs
inCommonJs = false$ cp foo.mjs foo.cjs
$ node foo.cjs
inCommonJs = true

Solution 4:

I like import.meta.url === `file://${process.argv[1]}` , but it does not work in Windows inside bash shell. This is the alternative that is only checking the basename:

const runningAsScript = import.meta.url.endsWith(path.basename(process.argv[1]));

Solution 5:

It looks like there is a documented way to do this now:

if (require.main === module) {
    console.log('executed directly');
    . . .
}