Stop scope until change input value

Question

I have a form, where a captcha is valid via json:

$.getJSON('?verify&string=' + captcha, function( content ){
    if(content.comparation){
       $("#tokenCaptcha").val(content.token);
    }else{
        complete = false;
    }
});

After a few lines, the function returns true or false to send the form or not. The fact is that it takes a long time to change the value. So much so that the form is sent before the value is modified. I did a test with a value Alert, it took about 1 second to give the token Alert.