Using grunt-contrib-watch and grunt-contrib-coffee to only compile files as needed

Gruntfile.coffee

# It's pretty annoying to get single CoffeeScript file compilation with grunt-contrib-coffee 
# and grunt-contrib-watch because it doesn't work out of the box.
#
# This is an open issue and it depends on how your project is structured and how your tasks are configured. 
# See: https://github.com/gruntjs/grunt-contrib-watch/issues/149
#
# Below is my simple solution which in a sort of hacky way alters the grunt config object at runtime.

module.exports = (grunt) ->

  grunt.config.init

    coffee:
      default:
        expand: true
        cwd: 'source/scripts'
        src: ['**/*.coffee']
        dest: 'compiled/scripts'
        ext: '.js'
      tests:
        expand: true
        cwd: 'tests/source'
        src: ['**/*.coffee', '../*.coffee']
        dest: 'tests/compiled'
        ext: '.js'
  
    watch:
      coffee:
        files: ['source/scripts/**/*.coffee', 'tests/source/**/*.coffee']
        tasks: ['coffee']
        options:
          spawn: false  # spawn must be false for this to work
          
    grunt.loadNpmTasks('grunt-contrib-coffee')
    grunt.loadNpmTasks('grunt-contrib-watch')
    
    grunt.event.on 'watch', (action, filepath) ->
      # this is a bit of a hack so that we can compile coffescript files
      # only as they're needed.
      if filepath.substring(0, 6) is "source"
        relative = filepath.replace(grunt.config('coffee.default.cwd') + "/", "")
        grunt.config('coffee.default.src', [relative])
        grunt.config('coffee.tests.src', [])
      else if filepath.substring(0, 5) is "tests"
        relative = filepath.replace(grunt.config('coffee.tests.cwd') + "/", "")
        grunt.config('coffee.default.src', [])
        grunt.config('coffee.tests.src', [relative])