version 1.0.2 breaks webpack: 'return' outside of function

[zaggino]

Using 1.0.1 works but 1.0.2 breaks webpack build with:

Creating an optimized production build...
Failed to compile.

./node_modules/is-map/index.js
SyntaxError: /usr/app/node_modules/is-map/index.js: 'return' outside of function (12:1)

  10 | \t\treturn false;
  11 | \t};
> 12 | \treturn;
     | \t^
  13 | }
  14 | 
  15 | var $mapHas = $Map ? Map.prototype.has : null;

[ljharb]

Top-level return has always been valid in node CJS modules. Webpack v4.31+ should have support for this pattern; what version are you using?

[ljharb]

v2.0.1 of both is-map and is-set have been released.

[zaggino]

@ljharb we're using create-react-app, version is react-scripts@3.3.0 which references webpack 4.41.2 https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/package.json#L79

[ljharb]

@zaggino in CRA's case, the problem is caused by two things: 1) transpiling node_modules, which is highly unsafe, and 2) babel does not enable the allowTopLevelReturn by default, nor does CRA enable it, which makes transpiling all valid CJS modules impossible.