Overriding export * causes TypeError

[klaussner]

• Rollup Version: 1.21.4
• Operating System (or Browser): macOS 10.14.6
• Node Version: 12.2

How Do We Reproduce?

REPL link.

export * from "fs";
export const existsSync = ":)";

Expected Behavior

The second export overrides existsSync of the first export.

Actual Behavior

A TypeError is thrown when the Rollup output is executed in Node:

TypeError: Cannot set property existsSync of #<Object> which has only a getter
    at Object.<anonymous> (repl.js:17:20)
    at Module._compile (internal/modules/cjs/loader.js:759:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:770:10)
    at Module.load (internal/modules/cjs/loader.js:628:32)
    at Function.Module._load (internal/modules/cjs/loader.js:555:12)
    at Function.Module.runMain (internal/modules/cjs/loader.js:822:10)
    at internal/main/run_main_module.js:17:11

[lukastaegert]

I see. So Rollup is basically interfering with its own logic here. I see at least two possible solutions:

• change the reexport namespace copying logic to skip named exports; does not work well for multiple reexports, though
• if a namespace is reexported, subsequent exports should not directly assign to the exports object but also use Object.defineProperty

[lukastaegert]

Should also probably check how Babel handles this.

[klaussner]

Babel creates an object with all named exports and skips the exports in the copying logic: Babel REPL link. But it looks like Babel doesn't handle overriding exports correctly, either. It works for named exports but if two export * statements are used, the code generated by Babel also fails with a TypeError if both modules have exports with the same name because the defined properties are not configurable and cannot be redefined: Babel REPL link (bug).

[lukastaegert]

Thanks for checking. Not sure what to make of the second scenario, but at least the first is better than what we have at the moment.

As far as I know, the correct behaviour for conflicting namespace reexports is to remove the conflicting bindings. So if the bindings should be non-configurable, we would need to check and omit the bindings of the other export in such a scenario. I.e.

var _module = require("module-1");
var _module2 = require("module-2");

Object.keys(_module).forEach(function (key) {
  if (key === "default" || key === "__esModule" ||
      Object.prototype.hasOwnProperty.call(_module2, key)) return;
  Object.defineProperty(exports, key, {
    enumerable: true,
    get: function get() {
      return _module[key];
    }
  });
});

Object.keys(_module2).forEach(function (key) {
  if (key === "default" || key === "__esModule" ||
      Object.prototype.hasOwnProperty.call(_module, key)) return;
  Object.defineProperty(exports, key, {
    enumerable: true,
    get: function get() {
      return _module2[key];
    }
  });
});

Combining this with Babel's approach for the first scenario, I guess this should work, or am I overlooking something here?

[salmanm]

As a temporary workaround I've done the following and it works.

import * as fs from "fs";
const existsSync = ":)";

export default { ...fs, existsSync }

This way it doesn't try to do Object.defineProperty.

But would be good to see this one fixed.

[aleclarson]

I'm bumping into this. Is there a better workaround than using export default?

edit: Using output: { externalLiveBindings: false } solves the problem for me :)

[lukastaegert]

Sorry for dropping the ball on this. Fix is here: #3959