Common JS UMD Pattern Detection Removes IF checks for module exports

Recent compilers miss emiting a condition

Repro:

cat <<EOF >index.js
'use strict';
// Stripped down demo from react-dom/index.js (16.2.0)
function checkDCE() {
  if (process.env.NODE_ENV !== 'production') {
    throw new Error('^_^');
  }
}

if (process.env.NODE_ENV === 'production') {
  checkDCE();
  module.exports = function() { return "foo"; };
} else {
  module.exports = function() { return "bar"; };
}
EOF

curl -O http://repo1.maven.org/maven2/com/google/javascript/closure-compiler/v20170910/closure-compiler-v20170910.jar
curl -O http://repo1.maven.org/maven2/com/google/javascript/closure-compiler/v20180204/closure-compiler-v20180204.jar
java -jar closure-compiler-v20180204.jar --js index.js --process_common_js_modules --js_output_file failure.js
java -jar closure-compiler-v20170910.jar --js index.js --process_common_js_modules --js_output_file success.js

Output failure.js of the recent compiler unconditionally keeps the process.env.NODE_ENV === 'production' branch “optimizing” away the else. 😉

[ChadKillingsworth]

It’s the same thing. The logic looks for exports nested inside an if or else block.

[ChadKillingsworth]

I’ve known this could be a potential problem for quite some time: https://github.com/google/closure-compiler/blob/master/src/com/google/javascript/jscomp/ProcessCommonJSModules.java#L577-L580