Compilers deprecation

If you’re here, you probably hit the deprecation notice. Sorry about that!

Will it break?

This is a soft deprecation, which means you get nagged about it, but it won’t break (yet).

Make it go away

To suppress this warning, execute mocha with the --no-deprecation flag (though you won’t get notice of any other deprecations you may encounter either).

… but why?

--compilers is redundant; we’ve yet to encounter a real-world situation in which the solution couldn’t be expressed using --require.

What should I use instead then?

Let’s say you want to compile using CoffeeScript. Ensure that you have the coffeescript package installed as a dev dependency:

npm install coffeescript --save-dev

Then update your package.json with the relevant require statement: --require coffeescript/register.

Here’s a list of popular compilers/transpilers:

• CoffeeScript: --compilers coffee:coffee-script/register becomes --require coffeescript/register
• Babel 6: --compilers js:babel-core/register becomes --require babel-core/register
• Babel 7: --require babel-core/register used if you are using Babel v6 becomes --require @babel/register with Babel v7.
• TypeScript: --compilers ts:ts-node/register becomes --require ts-node/register
• LiveScript: --compilers ls:livescript becomes --require livescript
• (feel free to add more examples!)

You’ll have to handle file extensions as well. Mocha, by default, only loads .js files when given a directory (and the default directory is test). Therefore, to use a different file extension (such as .coffee or .ts), you will need to supply a glob instead of simply a directory. If this was how you ran Mocha pre-v4:

$ mocha --compilers coffee:coffee-script/register --recursive ./test

Then this is how you’d accomplish the same thing (** roughly means “recursive”) in v4:

$ mocha --require coffeescript/register "test/**/*.js"

When you wrap a glob in quotes, file discovery is handed to the glob package. It’s recommended to wrap in double-quotes, because the result should be the same regardless of your shell or environment (Windows/Linux/macOS, etc.).

glob is powerful. For instance, if your test dir has tests written in both JS and CoffeeScript, you could do this:

$ mocha --require coffeescript/register "test/**/*.{js,coffee}"

How do I use this with --watch?

When using --watch, you will also need to specify the extension(s) to watch via --watch-extensions, e.g.:

$ mocha --require coffeescript/register --watch --watch-extensions js,coffee "test/**/*.{js,coffee}"