This issue will be tested by CI as we build and lint all files anyway. To further test, you can: install moodle-local_ci and moodle-local_codechecker git clone https://github.com/moodlehq/moodle-local_ci local/ci git clone https://github.com/moodlehq/moodle-local_codechecker local/codechecker cd local/ci composer install cd - Install JS dependencies npm i Run eslint: npx grunt ignorefiles eslint Confirm no failures Edit a template and add a javascript snippet to the bottom, for example open lib/templates/welcome.mustache in your editor, and add {{#js}} const foo = 'bar'; window.console.log(foo); {{/js}} Run the mustachelint tool: php local/ci/mustache_lint/mustache_lint.php -f=`pwd`/lib/templates/welcome.mustache --basename=`pwd` -v=https://html5.validator.nu Confirm that there were no errors (Prior to this patch, Mustachelint would give confusing errors about const being a reserved word, or other weird errors) Create a file in the moodle root directory named test.js with content: const foo = 'bar'; Run eslint on it: npx eslint test.js Confirm that there were no errors (Prior to this patch, errors were reported about use of ES6 syntax) In a terminal window run grunt watch: npx grunt watch Edit a YUI file such as lib/yui/src/chooserdialogue/js/chooserdialogue.js and add an ES6-ism, at the top, such as: const foo = 'bar'; Save the file and look at the grunt output Confirm that it did complain about use of const