微信小程序目前是不支持动态修改tabbar的，如果想在不同的页面显示不同的底部导航，那只有自定义tabbar了，

首先创建tabbar.wxml的模板，代码如下：

<template name="tabBar">  
  <view class="tab-bar" style="color: {{tabBar.color}}; background: {{tarBar.backgroundColor}}; {{tabBar.position=='top'? 'top: 0' : 'bottom: 0'}}; {{tabBar.borderStyle? (tabBar.position=='top'? 'border-bottom: solid 1px '+tabBar.borderStyle + ';' : 'border-top: solid 1px '+tabBar.borderStyle + ';') : ''}}">  
  <block wx:for="{{tabBar.list}}" wx:key="pagePath">  
    <navigator url="{{item.pagePath}}" open-type="redirect" class="{{item.clas}}" style="{{item.active? 'color: '+(item.selectedColor? item.selectedColor : tabBar.selectedColor) : ''}}">  
      <image src="{{item.selectedIconPath}}" wx:if="{{item.active}}" class="img"></image>  
      <image src="{{item.iconPath}}" wx:if="{{!item.active}}" class="img"></image>
      <text>{{item.text}}</text>  
    </navigator>  
    </block>
    <view class="clear"></view>  
  </view>  
</template> 

在这个模板中，我们在list里面每个对象都增加了一个selectedColor和active属性，方便对每个tabBar当前页做样式
然后我们在app.js配置不同tabbar对象，代码如下：

//app.js
App({
  onLaunch: function () {
    //调用API从本地缓存中获取数据
    var logs = wx.getStorageSync('logs') || []
    logs.unshift(Date.now())
    wx.setStorageSync('logs', logs)
  },
  //第一种状态的底部
  editTabBar: function () {
    var _curPageArr = getCurrentPages();
    var _curPage = _curPageArr[_curPageArr.length - 1];  
    var _pagePath = _curPage.__route__;
    if (_pagePath.indexOf('/') != 0) {
      _pagePath = '/' + _pagePath;
    }
    var tabBar = this.globalData.tabBar;
    for (var i = 0; i < tabBar.list.length; i++) {
      tabBar.list[i].active = false;
      if (tabBar.list[i].pagePath == _pagePath) {
        tabBar.list[i].active = true;//根据页面地址设置当前页面状态  
      }
    }
    _curPage.setData({
      tabBar: tabBar
    });
  },
  //第二种状态的底部
  editTabBar2: function () {
    var _curPageArr = getCurrentPages();
    var _curPage = _curPageArr[_curPageArr.length - 1];
    var _pagePath = _curPage.__route__;
    if (_pagePath.indexOf('/') != 0) {
      _pagePath = '/' + _pagePath;
    }
    var tabBar = this.globalData.tabBar2;
    for (var i = 0; i < tabBar.list.length; i++) {
      tabBar.list[i].active = false;
      if (tabBar.list[i].pagePath == _pagePath) {
        tabBar.list[i].active = true;//根据页面地址设置当前页面状态  
      }
    }
    _curPage.setData({
      tabBar: tabBar
    });
  },
  getUserInfo:function(cb){
    var that = this
    if(this.globalData.userInfo){
      typeof cb == "function" && cb(this.globalData.userInfo)
    }else{
      //调用登录接口
      wx.login({
        success: function () {
          wx.getUserInfo({
            success: function (res) {
              that.globalData.userInfo = res.userInfo
              typeof cb == "function" && cb(that.globalData.userInfo)
            }
          })
        }
      })
    }
  },
  globalData:{
    userInfo:null,
    tabBar: {
      "color": "#9E9E9E",
      "selectedColor": "#f00",
      "backgroundColor": "#fff",
      "borderStyle": "#ccc",
      "list": [
        {
          "pagePath": "/pages/index/index",
          "text": "首页",
          "iconPath": "/img/home.png",
          "selectedIconPath": "/img/home.png",
          "clas":"menu-item",
          "selectedColor": "#4EDF80",  
          active: true
        },
        {
          "pagePath": "/pages/logs/logs",
          "text": "日志",
          "iconPath": "/img/note.png",
          "selectedIconPath": "/img/note.png",
          "selectedColor": "#4EDF80",
          "clas": "menu-item",
          active: false
        },
        {
          "pagePath": "/pages/test/test",
          "text": "指南",
          "iconPath": "/img/safari.png",
          "selectedColor": "#4EDF80",
          "clas": "menu-item",
          active: false
        }
      ],
      "position": "bottom"
    },
    tabBar2: {
      "color": "#9E9E9E",
      "selectedColor": "#f00",
      "backgroundColor": "#fff",
      "borderStyle": "#ccc",
      "list": [
        {
          "pagePath": "/pages/index/index",
          "text": "首页",
          "iconPath": "/img/home.png",
          "selectedIconPath": "/img/home.png",
          "clas": "menu-item2",
          "selectedColor": "#4EDF80",  
          active: true
        },
        {
          "pagePath": "/pages/logs/logs",
          "text": "日志",
          "iconPath": "/img/note.png",
          "selectedIconPath": "/img/note.png",
          "selectedColor": "#4EDF80",
          "clas": "menu-item2",
          active: false
        },
        {
          "pagePath": "/pages/cont/index",
          "text": "指南",
          "iconPath": "/img/note.png",
          "selectedIconPath": "/img/home.png",
          "selectedColor": "#4EDF80",
          "clas": "menu-item2",
          active: false
        },
        {
          "pagePath": "/pages/detail/index",
          "text": "内容",
          "iconPath": "/img/safari.png",
          "selectedColor": "#4EDF80",
          "clas": "menu-item2",
          active: false
        }
      ],
      "position": "bottom"
    }
  }
})

然后我们在相应的page.js中加载相应的tabbar对象

例如，我们在首页的index.js中加载tabBar，代码如下：

//index.js
var app = getApp();
Page({
  data: {
    
  },
  onLoad: function () {
    app.editTabBar();
  }
})

效果图

然后点击日志页面，如果在logs.js中加载tabBar2，代码如下：

//logs.js  
var app = getApp();  
var util = require('../../utils/util.js')  
Page({  
  data: {  
    logs: []  
  },  
  onLoad: function () {  
    app.editTabBar2();  
    this.setData({  
      logs: (wx.getStorageSync('logs') || []).map(function (log) {  
        return util.formatTime(new Date(log))  
      })  
    })  
  }  
})  

日志页的效果图

我们可以看到页面跳转到日志页后底部导航栏改变了

导航栏的样式随意写的，也贴出来吧，方便大家测试，我是写在app.wxss中

.menu-item{  
  width: 32%;  
  float: left;  
  text-align: center;  
  padding-top: 8px;  
}  
.menu-item2{  
  width: 24%;  
  float: left;  
  text-align: center;  
  padding-top: 8px;  
}  
.img{  
  width: 23px;  
  height: 23px;  
  display: block;  
  margin:auto;  
}  
.clear{  
  clear: both;  
}  
.tab-bar{  
  position: fixed;  
  width: 100%;  
  padding: 0px 2%;  
}