gruntfile.js

module.exports = function (grunt) {
  grunt.initConfig({
    less: {
      development: {
        options: {
          compress: true,
          yuicompress: true,
          optimization: 2,
        },
        files: {
          // target.css file: source.less file
          "src/index.css": "src/style/index.less",
        },
      },
    },
    watch: {
      styles: {
        // Which files to watch (all .less files recursively in the less directory)
        files: ["src/style/**/*.less"],
        tasks: ["less"],
      },
    },
  });

  grunt.loadNpmTasks("grunt-contrib-less");
  grunt.loadNpmTasks("grunt-contrib-watch");

  grunt.registerTask("default", ["watch"]);
};