code: function extend(prototype, utils) {
    const { debug, strong, warn } = utils.cnsl;
    const { sourceMapCommentStrip } = utils.string;
  
    /**
     * Compress file contents
     * @param {Object} buildOptions
     *  - {Boolean} bootstrap
     *  - {Boolean} boilerplate
     *  - {Boolean} browser
     *  - {Boolean} bundle
     *  - {Boolean} compress
     *  - {Array} ignoredFiles
     *  - {Boolean} helpers
     *  - {Boolean} watchOnly
     * @param {Function} fn(err)
     */
    prototype.compress = function compress(buildOptions, fn) {
      const options = Object.assign(
        this.hasMaps
          ? {
              sourceMap: {
                content: this.map.toJSON(),
                url: this.mapUrl
              }
            }
          : {},
        DEFAULT_OPTIONS,
        this.options.pluginOptions.uglify || {}
      );
      const minified = minify(this.content, options);
  
      if ('error' in minified) {
        fn(minified.error);
        return;
      }
  
      debug(`compress: ${strong(this.relpath)}`, 4);
      this.setContent(sourceMapCommentStrip(minified.code));
      if (minified.map) {
        this.setMap(minified.map);
      }
      fn();
    };
  }
docs: Extend 'prototype' with new behaviour
  @param {Object} prototype
  @param {Object} utils