调用vant中的button组件，点击按钮，数据无法提交？

<form bingsubmit='onSubmit'>
<input name="title"></input>
<button form-type="submit">提交</button>
</form>

js部分：
const db = wx.cloud.database();
const todos = db.collection('todos')
Page({
 onSubmit:function(event){
   console.log(event.detail.value.title)
   todos:add({
     data:{
       title:event.detail.value.title
     }
   }).then(res =>{
     wx.showToast({
       title:'Success',
       icon:'success'
     })
   }
   )
 }
})

app.json部分：
 "usingComponents":{
     "van-button":"@vant/weapp/button",
      "van-cell": "@vant/weapp/cell",
      "van-cell-group": "@vant/weapp/cell-group"
  },