Gruntfile.js

module.exports = function(grunt) {

  grunt.loadNpmTasks('grunt-mocha-test');
  grunt.loadNpmTasks('grunt-contrib-watch');

  grunt.initConfig({
    mochaTest: {
      test: {
        options: {
          reporter: 'spec',
          clearRequireCache: true
        },
        src: ['test/*.js']
      },
    },

    watch: {
      js: {
        options: {
          spawn: false,
        },
        files: ['build.js', 'test/*.js'],
        tasks: ['default']
      }
    }
  });

  // On watch events, if the changed file is a test file then configure mochaTest to only
  // run the tests from that file. Otherwise run all the tests
  var defaultTestSrc = grunt.config('mochaTest.test.src');
  grunt.event.on('watch', function(action, filepath) {
    grunt.config('mochaTest.test.src', defaultTestSrc);
    if (filepath.match('test/')) {
      grunt.config('mochaTest.test.src', filepath);
    }
  });

  grunt.registerTask('default', 'mochaTest');
};