require.main is undefined when using --experimental-modules

[mgalakos]

• Version: node v10.3.0
• Platform: linux, windows, wsl

When running node with --experimental-modules flag, the global require object has a main property but its value is always undefined. Is this by design?

// index.js
console.log(require.main === undefined)

$ node index.js
false

$ node --experimental-modules index.js
true

[bnoordhuis]

What would you expect it to print? Without --experimental-modules it's the CJS Module object.

[mgalakos]

I was expecting to be no difference in behavior (using the flag or not).
I thought .js files are considered cjs (by default) even when using --experimental-modules.

p.s. all the other properties of require object have almost identical values.

[bnoordhuis]

I thought .js files are considered cjs (by default) even when using --experimental-modules.

Yes, but they don't use the traditional module loader where require.main === process.mainModule.

The new module loader doesn't really have a concept of "main-ness" right now and I don't know if that is something that ought to be fixed. One for @nodejs/modules to consider.

[mgalakos]

Oh i see, thank you for your quick response.

Maybe this should be solved before esm support drop experimental status, because it will break all cjs modules that rely on require.main

[devsnek]

I found this a few weeks ago but wasn't sure how to fix it, I'll take another look before friday.

[guybedford]

Agreed this is a bug, had just put on my list to fix too.