<view class="section section_gap">
 <text>{{text}}</text>
 <view class="btn-area">
  <button bindtap="add">add line</button>
  <button bindtap="remove">remove line</button>
 </view>
</view>

var initData = 'this is first line\nthis is second line'
Page({
 data: {
 text: initData
 },
 extraLine: [],
 add: function(e) {
 this.extraLine.push('other line')
 this.setData({
  text: initData + '\n' + this.extraLine.join('\n')
 })
 },
 remove: function(e) {
 if (this.extraLine.length > 0) {
  this.extraLine.pop()
  this.setData({
  text: initData + '\n' + this.extraLine.join('\n')
  })
 }
 }
})