How to invert prop 'checked'?

Question

JavaScript

There is a code that checks 3 parameters.

$('form').submit(function() { // проверка на пустоту заполненных полей. Атрибут html5 — required не подходит (не поддерживается Safari)
    if ($(this).find("input[name='name']").val() == '' || $(this).find("input[name='phone']").val() == '' || $('#checkbox4').prop('checked') ) {
      valid = false;
      return valid;
    }

By default, the checkbox is set to checked, so if you set the value for .prop('checked'), it turns out that submitting data from the form is prohibited if the checkbox had its checked state switched or was set by default. And it is necessary that when the checkbox in the html file is checked by default, sending data from the form is allowed. And vice versa, if the checkbox is deactivated, the checkbox is unchecked, then data transfer is prohibited.
If I set .prop('disabled') in the code, and it doesn't matter if the checkbox was enabled or not, the code somehow allows the data to go.
I think that the inversion of the code $('#checkbox4').prop('checked') with a boolean ! not.
But how to write this inversion correctly?

Answer 1

... || !$('#checkbox4').prop('checked')

Answer 2

$('.myCheckbox').prop('checked', true);
$('.myCheckbox').prop('checked', false);