How do I overcome "object literals cannot contain duplicate keys in ES5 strict mode" error with Maven minify plugin? - maven

Question

I'm using the Maven minify plugin with Maven 3.5. I have the below configuration ...
<plugin>
<groupId>com.samaxes.maven</groupId>
<artifactId>minify-maven-plugin</artifactId>
<version>1.7.6</version>
<executions>
<execution>
<id>default-minify</id>
<phase>verify</phase>
<configuration>
<cssSourceIncludes>
<cssSourceInclude>**/*.css</cssSourceInclude>
</cssSourceIncludes>
<cssTargetDir>../${project.artifactId}/css</cssTargetDir>
<jsSourceExcludes>
<jsSourceExclude>lib/pdf.js</jsSourceExclude>
<jsSourceExclude>lib/pdf.worker.js</jsSourceExclude>
<jsSourceExclude>ckeditor_4.2/**</jsSourceExclude>
<jsSourceExclude>geogebra/**</jsSourceExclude>
<jsSourceExclude>contextMenu/**</jsSourceExclude>
<jsSourceExclude>wPaint/**</jsSourceExclude>
</jsSourceExcludes>
<jsSourceIncludes>
<jsSourceInclude>**/*.js</jsSourceInclude>
</jsSourceIncludes>
<jsTargetDir>../${project.artifactId}/js</jsTargetDir>
<jsEngine>CLOSURE</jsEngine>
<webappTargetDir>${project.build.outputDirectory}</webappTargetDir>
<skipMerge>true</skipMerge>
<nosuffix>true</nosuffix>
</configuration>
<goals>
<goal>minify</goal>
</goals>
</execution>
</executions>
</plugin>
However, I'm getting this error on the below file ...
SEVERE: panel.js:845: ERROR - with
SIDE_PANEL_BEGIN_CLOSE_EVENT:SIDE_PANEL_BEGIN_CLOSE_EVENT,
^
Nov 27, 2018 2:57:06 PM com.google.javascript.jscomp.LoggerErrorManager printSummary
WARNING: 1 error(s), 0 warning(s)
Everything works in my application fine with my uncompressed file so I'm thinking I just made a mistake with my configuration but I can't see what. How can I correct the configuration so that the optimization occurs without the error without affecting the underlying functionality of the code?

Answer 1

While Google's Closure Compiler (which is used under the hood by the minify plugin to optimize the source code) by default expects ECMAScript 6 as input language it seems to run additional diagnostic checks which do not take the language level into account.
These diagnostic checks include one for ECMAScript 5 strict mode compliance which forbids duplicate keys in objects.
As described in this answer by Sebstian Häger you can turn this diagnostic check of in the minify configuration by adding
<closureWarningLevels>
<es5Strict>OFF</es5Script>
</closureWarningLevels>