Question

In a html page, if I use my mouse to select an item from a drop-down list(the select and option html elements), then the onchange event will be fired.
But if I use Javascript to change the selectedindex of a select element, then the onchange event won’t be fired.

How are those two actions different?

How can I trigger the onchange event (not by calling selectElement.onchange() explicitly)?

Answer 1

See this: http://jsfiddle.net/9xe9b/

$("#currency").change(function(){
    var val = $(this).val();

    $(this).val(formatCurrency(val));
});

$("#infinite").change(function(){
    var val = $(this).val();

    $(this).val(formatCurrency(val));
    $(this).change(); // simulate the change event. 
});

function formatCurrency(num) {
    num = isNaN(num) || num === '' || num === null ? 0.00 : num;
    return parseFloat(num).toFixed(2);
}

If you enter a value into the second textbox and pop open your console, you’ll see this error:
Uncaught RangeError: Maximum call stack size exceeded

This is what we would have to worry about if changing a value in javascript caused a related event to fire. Infinite loops!

So, if you need to repeat functionality when changing a value through the UI or through javascript, you should create a function that does what you want and call it in the appropriate instances.