javascript - Change the selected value of a drop-down list with jQuery

These solutions seem to assume that each item in your drop down lists has a val() value relating to their position in the drop down list.

Things are a little more complicated if this isn't the case.

To read the selected index of a drop down list, you would use this:

$("#dropDownList").prop("selectedIndex");

To set the selected index of a drop down list, you would use this:

$("#dropDownList").prop("selectedIndex", 1);

Note that the prop() feature requires JQuery v1.6 or later.

Let's see how you would use these two functions.

Supposing you had a drop down list of month names.

  January

  February
  March

You could add a "Previous Month" and "Next Month" button, which looks at the currently selected drop down list item, and changes it to the previous/next month:

And here's the JavaScript which these buttons would run:

function btnPrevMonth_Click() {
    var selectedIndex = $("#listOfMonths").prop("selectedIndex");
    if (selectedIndex > 0) {
        $("#listOfMonths").prop("selectedIndex", selectedIndex - 1);
    }
}
function btnNextMonth_Click() {

    //  Note:  the JQuery "prop" function requires JQuery v1.6 or later
    var selectedIndex = $("#listOfMonths").prop("selectedIndex");
    var itemsInDropDownList = $("#listOfMonths option").length;

    //  If we're not already selecting the last item in the drop down list, then increment the SelectedIndex
    if (selectedIndex < (itemsInDropDownList - 1)) {
        $("#listOfMonths").prop("selectedIndex", selectedIndex + 1);
    }
}

The following site is also useful, for showing how to populate a drop down list with JSON data:

http://mikesknowledgebase.com/pages/Services/WebServices-Page8.htm

Phew !!

Hope this helps.