Function not modifying passed parameter value

Question

I have the following function:

function validateField(target, field, field_check) {
            if(target.val().trim().length > 0) {
                $.ajax({
                    url: '<?php echo $html->url('/fonykers/validate_',true); ?>'+ field + '/' + target.val(), 
                    dataType: 'json',
                    type: 'POST',
                    success: function(response) {
                        if(!response.ok) {
                            if(target.is('.ok')) {
                                target.removeClass('ok');
                            }
                            target.addClass('error');
                            error.html(response.msg);
                            error.fadeIn();
                            field_check = false;
                        } else {
                            if(target.is('.error')) {
                                target.removeClass('error');
                            }
                            target.addClass('ok');
                            alert('voy a asignar true');
                            field_check = true;
                        }
                    },
                        error:function (xhr, ajaxOptions, thrownError){
                            alert(xhr.statusText);
                            alert(thrownError);
                            field_check = false;
                        } 
                });        
            } else {
                error.html('You must specify a ' + field);
                error.fadeIn();
                target.addClass('error');
                field_check = false;
            } 

        }

I use it to validate my form input fields, the target variable es the field itself, for instance: $('#name_field') the 2nd variable es a string specifying the name of the field for instance name as to know what PHP function it's gonna call. The last is a boolean that I use to know wether or now the field is valid or not for instance name_check which I define at the beginning of the scripta and set to false.

My problem is that field_check gets changed in the local scope properly but it never modifies que external variable that has been passed in, what am I doing wrong?

Answer 1

This is because the variables are not passed by reference. You can either have that as a return value or use a workaround like this: http://sirdarckcat.blogspot.com/2007/07/passing-reference-to-javascript.html

Answer 2

Primitive variables (int / boolean / etc) are passed to functions by value - that is, when someone passes one of them to a function, the value of the variable is passed to your function, not the variable itself. What you're modifying is in fact a different variable altogether.

The easiest solution is to just return true or false when you've finished validating the field, and use the function's return value instead of checking field_check .

You can also do a couple different things with globals / properties. For instance...

var validator = function (target, field) {
  ...
  this.field_check = false;
}

validator(target, field);
if (validator.field_check) {
  ...
}

Note the use of this.field_check and the assignment of the function to a variable

Answer 3

In JavaScript simple types are always passed by value, never by reference. In other words you are passing a copy of the boolean value. The copy gets changes, but the original never will.

Your validate field function will need to either return the new value, or expect a callback function that will receive it. You have to explicitly set the new value to the original variable.