How to use the source-map/util.join function in source-map

}
        else {
          mapping = {};
          mapping.generatedLine = generatedLine;

          // Generated column.
          temp = base64VLQ.decode(str);
          mapping.generatedColumn = previousGeneratedColumn + temp.value;
          previousGeneratedColumn = mapping.generatedColumn;
          str = temp.rest;

          if (str.length > 0 && !mappingSeparator.test(str.charAt(0))) {
            // Original source.
            temp = base64VLQ.decode(str);
            if (aSourceRoot) {
              mapping.source = util.join(aSourceRoot, this._sources.at(previousSource + temp.value));
            }
            else {
              mapping.source = this._sources.at(previousSource + temp.value);
            }
            previousSource += temp.value;
            str = temp.rest;
            if (str.length === 0 || mappingSeparator.test(str.charAt(0))) {
              throw new Error('Found a source, but no line and column');
            }

            // Original line.
            temp = base64VLQ.decode(str);
            mapping.originalLine = previousOriginalLine + temp.value;
            previousOriginalLine = mapping.originalLine;
            // Lines are stored 0-based
            mapping.originalLine += 1;

return this._sources.toArray().map(function (s) {
        return this._sourceRoot ? util.join(this._sourceRoot, s) : s;
      }, this);
    }