Babel Cannot read property error of undefined

This is more a reminder to myself about how to solve this error that I've stumbled across a couple of times with Babel 5 & 6.

If you downgrade Babel from 6 to 5 and leave a .babelrc file in your project with a Babel 6 specific setting like:

{
  "presets": ["es2015"]
}

then you might get an error like this:

if (!option) this.log.error("Unknown option: " + alias + "." + key, ReferenceError);

TypeError: Cannot read property 'error' of undefined

The solution is to remove the .babelrc file or remove the Babel 6 specific settings.

Also, if you're using Babel 5 and you require a module from another project that has a .babelrc file which is using Babel 6 and has a Babel 6 setting in it then this will cause the same problem in your Babel 5 project.