How can I set the language_in option for the Closure compiler?

Question

I need to set the language_in option on the Closure compiler to prevent the IE8 parse error:

ERROR - Parse error. IE8 (and below) will parse trailing commas in array an

Answer 1

Run the Closure Compiler Application with the --help flag to see a description of each flag.

java -jar compiler.jar --help

CommandLineRunner defines the set of allowed values for --language_in:

--language_in
Sets what language spec that input sources conform.
Options: ECMASCRIPT3, ECMASCRIPT5, ECMASCRIPT5_STRICT, ECMASCRIPT6_TYPED (experimental), ECMASCRIPT_2015, ECMASCRIPT_2016, ECMASCRIPT_2017, ECMASCRIPT_NEXT

The LanguageMode enum has a bit more detail about these values.

Using Closure Builder, the --language_in flag would be passed as an argument to
--compiler_flags as in:

--compiler_flags="--language_in=ECMASCRIPT5"