JQuery Validate dependency-expressions for custom rules

JQuery Validate has a nice feature that allows you to specify dependency-expression that must hold for rule to kick in. For example standard 'required' rule might accept expression like this:

$("#myForm").validate({
  rules: {
      myTextbox: {
          required: "#myCheckbox:checked"
      }
  }
});

This tells 'required' rule that it should be used only when myCheckbox is checked. However'required' is the only rule that comes out of box with an option to specify a condition under which it works. What should you do in case you need to make your own rule use dependency-expression?

To enable depedency behavior for a custom rule use the following code:

jQuery.validator.addMethod("greaterThanZero", function(value, element, param) {
 if ( !this.depend(param, element) )
         return "dependency-mismatch";
 return this.optional(element) || (parseFloat(value) > 0);
}, "* Amount must be greater than zero");

The if statement here does the trick. It checks if dependecy-expression is satisfied before proceeding to validation. Build in 'required' rule is actually using the same mechanism.