var initData = 'this is first line\nthis is second line'

var extraLine = [];

Page({

  data: {

    text: initData

  },

  add: function(e) {

    extraLine.push('other line')

    this.setData({

      text: initData + '\n' + extraLine.join('\n')

    })

  },

  remove: function(e) {

    if (extraLine.length > 0) {

      extraLine.pop()

      this.setData({

        text: initData + '\n' + extraLine.join('\n')

      })

    }

  }

})

<view class="btn-area">

  <view class="body-view">

    <text>{{text}}</text>

    <button bindtap="add">add line</button>

    <button bindtap="remove">remove line</button>

  </view>

</view>

.btn-area{

    font-family:Verdana, Geneva, Tahoma, sans-serif;

    padding: 0 10px 0 10px;

}

.body-view button{

    margin-top :10px;

}