Modify select field options using Javascript

Question

I am trying to create a Javascript function that will take the value from one select field and then use that to compare a JSON array . If any array elements match the user selected value then the function will add those values to another select field.

Logic:

• Activate function using "onchange" HTML attribute
• Get user selected value from select field
• Compare this value to JSON array
• If array value == selected value , append to other select field

Code:

//this function modifies a select field depending on what value was selected by its predecessor
        function tariffs()
        {   
            //variable selection equals the selected value chosen by the user in that field
            var selection = document.getElementById('tariff_net').options[document.getElementById('tariff_net').selectedIndex].value

            //for loop to go through JSON values to compare with the value chosen by the user
            for(var i = 0; i < values.preset_name.length ; i++)
                {
                    //if the JSON value equals the user selected value
                  if (values.preset_name[i] == selection)
                    {   
                        //append it to the "tariff_name" select field
                        var option = values.preset_name[i];
                        var selectfield = document.getElementById('tariff_name');
                        selectfield.appendChild(option);
                    }
                }


        }

Here is a screenshot showing the two select fields (the one on top will affect the bottom select fields options)

http://i.imgur.com/B0eHS.png

At the moment my code is not working and I am getting a Javascript Error:

Could not convert JavaScript argument arg 0 [nsIDOMHTMLInputElement.appendChild] [Break On This Error] selectfield.appendChild(option);

I am also wondering how I would be able to make the Javscript refresh the select field of all values and then repopulate it. This would prevent a large accumulation of duplicated options whenever the function is called.

Thanks

Answer 1

appendChild takes an element, not a string. Select options have a constructor to help you:

var option = new Option(values.preset_name[i]); //Dom element instead of a string
var selectfield = document.getElementById('tariff_name');
selectfield.appendChild(option);