code: function assignStrProps() {
    var ret = '',
        retObj = assign.apply(tools, arguments);
  
    for (var k in retObj) {
      var v = retObj[k];
      ret += ' ' + k + (k !== v ? '="' + v + '"' : ' ');
    }
  
    return ret;
  }
docs: 合并字符串属性