I work with FormAssembly and I have a form where I need to check if user is exist. So, the logic is: if text field with error is empty => checkbox should be checked. I have tried this code:

// tfa_808 is error message field
//tfa_817 is checkbox
function exist(){
  if (document.getElementById("tfa_808") == "") {
  document.getElementById("tfa_817").checked = true;
  }
}

Any help would be appreciated! Thank you!