As discussed in [exercism/xjavascript#101](https://github.com/exercism/xjavascript/issues/101) and [exercism/xjavascript#26](https://github.com/exercism/xjavascript/issues/26), ESLint seems to be a good tool to lint JavaScript and ECMAScript source code. Add it and configure it to lint source code at development time.