I've seen some npm packages (vue for example) have a module field in their package.json.
But module is not included in the package.json documentation - is this a convention of some kind? Is there documentation for this somewhere?
The module field is not officially defined by Node.js and support is not planned. Instead, the Node.js community settled on package exports which they believe is more versatile.
For practical reasons JavaScript bundlers will continue to be support the module field. The esbuild docs explain when to use module as well as related fields main and browser.
Is
"module"an official npm property or is this a convention of some kind?
It's a proposal, but likely to be supported due to de facto usage.
Is there documentation for this somewhere?
There is, in fact, and it can be found right here and later removed here.
What's it for?
ES6 Module Interoperability in Node. Additional discussion can be found here and here. And here's a blog post from Rich Harris talking more about it.
This is used by bundler tools for ESM (ECMAScript Module) detection. The Rollup documentation says it pretty well:
If your
package.jsonfile also has amodulefield, ES6-aware tools like Rollup and webpack 2 will import the ES6 module version directly.
This article on Rollup 1.0 says it another way:
The
mainfield makes sure that Node users usingrequirewill be served the UMD version. Themodulefield is not an official npm feature but a common convention among bundlers to designate how to import an ESM version of our library.
Further discussion of pkg.module is on the Rollup Github Wiki and the webpack Docs.