Following https://developers.google.com/closure/compiler/docs/api-tutorial3#howto-app and https://github.com/documentcloud/closure-compiler it should be easy to use something like: ``` yml javascript_compressor: closure #compressor_options: # compilation_level: "ADVANCED_OPTIMIZATIONS" # externs: src/js/externs.js ``` Looks like the externs parameter is not recognized