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"ReferenceError: Invalid left-hand side expression in postfix operation" in production build

See original GitHub issue

Update: if you see this, you have a buggy version of UglifyJS (`2.8.0`).

Run rm -rf node_modules and then npm install (or yarn).

Then verify that node_modules/uglify-js/package.json has at least 2.8.1 in the version field. If not, you might need to clean your cache, or update your lockfile or shrinkwrap.

Original Post

I see this both in Windows and OS X tests.

  Integration
    Environment variables
Error: Uncaught [ReferenceError: Invalid left-hand side expression in postfix operation]
    at reportException (/tmp/tmp.sLfC3ipnAc/test-kitchensink/node_modules/jsdom/lib/jsdom/living/helpers/runtime-script-errors.js:58:24)
    at processJavaScript (/tmp/tmp.sLfC3ipnAc/test-kitchensink/node_modules/jsdom/lib/jsdom/living/nodes/HTMLScriptElement-impl.js:130:7)
    at HTMLScriptElementImpl._eval (/tmp/tmp.sLfC3ipnAc/test-kitchensink/node_modules/jsdom/lib/jsdom/living/nodes/HTMLScriptElement-impl.js:65:7)
    at /tmp/tmp.sLfC3ipnAc/test-kitchensink/node_modules/jsdom/lib/jsdom/browser/resource-loader.js:31:22
    at Object.check (/tmp/tmp.sLfC3ipnAc/test-kitchensink/node_modules/jsdom/lib/jsdom/living/nodes/Document-impl.js:85:11)
    at ResourceQueue.resume (/tmp/tmp.sLfC3ipnAc/test-kitchensink/node_modules/jsdom/lib/jsdom/living/nodes/Document-impl.js:119:12)
    at DocumentImpl.close (/tmp/tmp.sLfC3ipnAc/test-kitchensink/node_modules/jsdom/lib/jsdom/living/nodes/Document-impl.js:518:17)
    at Document.close (/tmp/tmp.sLfC3ipnAc/test-kitchensink/node_modules/jsdom/lib/jsdom/living/generated/Document.js:322:27)
    at _callee$ (/tmp/tmp.sLfC3ipnAc/test-kitchensink/integration/initDOM.js:55:7)
    at tryCatch (/home/travis/build/facebookincubator/create-react-app/packages/babel-preset-react-app/node_modules/babel-runtime/node_modules/regenerator-runtime/runtime.js:64:40)
    at GeneratorFunctionPrototype.invoke [as _invoke] (/home/travis/build/facebookincubator/create-react-app/packages/babel-preset-react-app/node_modules/babel-runtime/node_modules/regenerator-runtime/runtime.js:299:22)
    at GeneratorFunctionPrototype.prototype.(anonymous function) [as next] (/home/travis/build/facebookincubator/create-react-app/packages/babel-preset-react-app/node_modules/babel-runtime/node_modules/regenerator-runtime/runtime.js:116:21)
    at step (/tmp/tmp.sLfC3ipnAc/test-kitchensink/integration/initDOM.js:13:191)
    at /tmp/tmp.sLfC3ipnAc/test-kitchensink/integration/initDOM.js:13:361
    at run (/tmp/tmp.sLfC3ipnAc/test-kitchensink/node_modules/babel-polyfill/node_modules/core-js/modules/es6.promise.js:87:22)
    at /tmp/tmp.sLfC3ipnAc/test-kitchensink/node_modules/babel-polyfill/node_modules/core-js/modules/es6.promise.js:100:28
    at flush (/tmp/tmp.sLfC3ipnAc/test-kitchensink/node_modules/babel-polyfill/node_modules/core-js/modules/_microtask.js:18:9)
    at nextTickCallbackWith0Args (node.js:489:9)
    at process._tickCallback (node.js:418:13) [ReferenceError: Invalid left-hand side expression in postfix operation]