If your JavaScript specs contain ES6 syntax, Mocha,
by default, will not be able to interpret and run them. In order to run them
with Mocha, you will need to tell Mocha to use something like
Babel to compile them. The --compile flag can be
used to point Mocha to the babel-core/register package.
$ mocha --compilers js:babel-core/register path/to/specs/*.spec.js
If you already have a test command specified in your package.json file,
you can update it with the --compile portion of the above command.
This all assumes you've already setup your project with Babel and Babel presets.