for..in loops iterate over the entire prototype chain, which is virtually never what you want.

It means that using for..in will traverse the entire prototype chain, which is not a good way to implement it. Object.keys is recommended

formRules : {
  name : true,
  cardType: true,
  certificateNo: true
 },
 formData : {
  name :  '' ,
  certificateType:  '' ,
  certificateNo:  ''
 }

Original code

for ( const key in  this .formData) {
     if (! this .formData[key]) { this .formRules[key] = false ; valid = false }
     if ( this .formData[key]) this .formRules[key] = true
}

Modified code

Object.keys( this .formData).forEach(key => {
 if (! this .formData[key]) { this .formRules[key] = false ; valid = false }
 if ( this .formData[key]) this .formRules[key] = true
})