Checkbox checked/unchecked when receiving 0/-1 from server

Question

I have a checkbox inside a modal that passes to the server by clicking a button the value 0 if it is not checked or the value -1 if it is checked. Now I am trying to do the reverse process, if it is recorded 0 when the page loads the checkbox should be unchecked, if it is recorded -1 it should be checked but I am not able to have this return, can someone give me a light? I’m learning javascript.

An example of JSON that is sent to the server:

{
Cdgrupo: 2,
Grupos: "grupo 2",
Dtalter: "2019-05-28T14:49:03.000Z",
Inativo: -1
},
{
Cdgrupo: 3,
Grupos: "grupo 3",
Dtalter: "2019-05-28T13:58:04.000Z",
Inativo: 0
},

This is the checkbox:

<input type="checkbox"  class="check" id="checkfor" style="position:absolute; left:330px; top:170px;" ></input>

And this is the javascript I tried to use to make the return:

$('#checkfor').ready(function() {
    if ($(this).attr('value', '-1')){
                 $(this).prop('checked', true);
             }
             else
              if ($(this).attr('value', '0')){
                 $(this).prop('checked', false);
             }
});

What I did wrong?

Follow a template of how I am making a request in ajax to get:

//Get de grupo
    $.ajax({
        url: "http://localhost:8081/datasnap/rest/TCadastros/GetListaGrupos//100/" + pag,       
        dataType: 'json',
        type: 'get',
        cache:false,
        success: function(data){

Answer 1

In my view you are using the wrong event... using the ready Javascript will execute the function when the HTML is mounted on the screen. The problem is that at this time you still do not have the data of your request. The time to update this checkbox would be on success of the AJAX function.

It would look like this:

$.ajax({
    url: "http://localhost:8081/datasnap/rest/TCadastros/GetListaGrupos//100/" + pag,       
    dataType: 'json',
    type: 'get',
    cache:false,
    success: function(data) {
        var element = $('#checkfor');
        element.val(data.Inativo); // Aqui você configura o valor

        // Com o valor já atribuído, você faz as modificações
        if (element.val() == -1) {
            element.prop('checked', true);
        }
        else if (element.val() == 0) {
            element.prop('checked', false);
        }
    }
});

Below is an example. In your case, you could call the function changeElement in the success ajax.

// Begin mock
var data = {
    Cdgrupo: 2,
    Grupos: "grupo 2",
    Dtalter: "2019-05-28T14:49:03.000Z",
    Inativo: -1
};

changeElement(data);
// End mock

function changeElement(data) {
  var element = $('#checkfor');
  element.val(data.Inativo); // Aqui você configura o valor

  // Com o valor já atribuído, você faz as modificações
  if (element.val() == -1) {
      element.prop('checked', true);
  }
  else if (element.val() == 0) {
      element.prop('checked', false);
  }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

<input type="checkbox"  class="check" id="checkfor" style="position:absolute; left:330px; top:170px;" ></input>