code: function goBackState(model) {
      return function goBack(step = 1, { method = actionTypes.pop } = {}) {
          let histories = model.histories,
              maxStep = histories.length - 1,
              argType = typeof step;
  
          // 按步数返回
          if (argType === 'number') {
  
              // 判断是否可以返回
              if (maxStep) {
  
                  // 获取返回步数
                  step = Math.min(maxStep, Math.abs(step));
  
                  // 判断步数不为【0
                  if (step) {
  
                      // 更新状态
                      model.state = {
                          method,
                          pathname: histories[maxStep - step],
                          action: actionTypes.pop
                      };
  
                      // 返回路径
                      return model.history.go(- step);
                  }
              }
  
              // 无需处理
              return;
          }
  
          // 返回到指定路径
          if (argType === 'string') {
              let idx = histories.lastIndexOf(step),
                  pathname = step;
  
              // 获取返回步数
              step = maxStep - idx;
  
              // 处理返回
              if (idx > -1) {
  
                  if (step && maxStep) {
  
                      // 更新状态
                      model.state = { method, pathname, action: actionTypes.pop };
                      model.history.go(- step);
                  }
  
              } else {
                  let next = () => {
                          model.state = { method, pathname, action: actionTypes.replace };
                          model.history.replace(pathname);
                      };
  
                  // 判断可返回数
                  if (maxStep) {
  
                      // 更新状态
                      model.state = {
                          pathname: histories[0],
                          action: actionTypes.pop,
                          next
                      };
  
                      // 返回路径
                      model.history.go(- maxStep);
                  } else {
                      next();
                  }
              }
          }
      };
  }
docs: ****************************************
  返回到指定路径
  ****************************************