Prepack: exports is not defined

[matheusml]

I'm running prepack, as a npm script:

"scripts": {
   "prepack": "prepack dist/pareto.cjs.js"
}

in a commonjs file, but I'm getting:

exports is not defined

I'm sorry if this is a stupid question, but I didn't read about anything related in the DOCs.

[deltaidea]

Prepack is still in development and only has definitions for basic objects described in ECMAScript.
exports is a Node-specific thing which is not specified anywhere in the language spec. Prepack assumes that everything unknown is undefined because reasons, therefore exports.foo is trying to access a property of an undefined global variable.

I've been trying to come up with a hack until we have CommonJS support, but it seems like there's a bug. I'm typing this into the repl:

__assumeDataProperty(global, "exports", {})
exports.foo = 5;

And I'm getting this weird result:

(function () {
  var _0 = this;

  if (_0.exports !== {
    foo: 5
  }) {
    throw new Error("Prepack model invariant violation: " + _0.exports);
  }
}).call(this);

Why is it checking if the exports contents match the stuff I'm assigning later in this file? That doesn't seem right.

If I use __abstract("object") instead of {} in the __assumeDataProperty() arguments, I get this error:

This operation is not yet supported on ::global.exports at foo
__IntrospectionError
    at repl:2:15

What are we missing?

Also, how do we stop Prepack from wrapping the whole file into an IIFE and translating exports into this.exports?

[matheusml]

Is this the same reason why I'm getting this error:

'import' and 'export' may appear only with 'sourceType: module' (112:0)
Syntax Error

when trying to prepack a ES module file?

[vkurchatkin]

Is this the same reason why I'm getting this error:

It' a different issue. Prepack parses files as scripts, which are not allowed to have import or export statements.

[jdalton]

Related to #451.

[leocavalcante]

which are not allowed to have import or export statements

I think its just not implemented yet. Hacked into sourceType argument, it is a hard-coded script string, but clearly an open space for module comes in eventually. There is already type declarations its missing an evaluator.

[leocavalcante]

Workaround for now seams to be calling __residual:

input

const foo = 'foo'
__residual('object', e => module.exports = e, {foo})

output (repl)

(function () {
  function _0(e) {
    return module.exports = e;
  }

  var _1 = {
    foo: "foo"
  };

  var _$0 = _0(_1);

  if (typeof _$0 !== "object" && typeof _$0 !== "function" || _$0 === null) {
    throw new Error("Prepack model invariant violation: " + _$0);
  }
})();

test

const { foo } = require('./foo.js')
console.log(foo) // foo

⚠️ Just a hacky way to play around, you shouldn't be using Prepack right now.

[deltaidea]

@leocavalcante
That works, but it also outputs way more code than the original module.exports = 'foo'.

[leocavalcante]

@deltaidea Yeah, definitely do not worth it for such small code like exporting a string.
IMHO, Prepack is only specially suited for bundles, because it can't predict what will be called from the public API of a module if it not gets the clients using this module. But, of couse, probably in the near future it would be able to static analyse import/require calls from a entry point (and maybe already generate the bundle? Good bye Webpack?) Let's see.