code: function parseExpr(source) {
      if (!source) {
          return;
      }
  
      if (typeof source === 'object' && source.type) {
          return source;
      }
  
      var expr = readTertiaryExpr(new Walker(source));
      expr.raw = source;
      return expr;
  }
docs: 解析表达式
  
  @param {string} source 源码
  @return {Object}