How do I implement onchange of <input type="text"> with jQuery?

You can use .change()

$('input[name=myInput]').change(function() { ... });

However, this event will only fire when the selector has lost focus, so you will need to click somewhere else to have this work.

If that's not quite right for you, you could use some of the other jQuery events like keyup, keydown or keypress - depending on the exact effect you want.

As @pimvdb said in his comment,

Note that change will only fire when the input element has lost focus. There is also the input event which fires whenever the textbox updates without it needing to lose focus. Unlike key events it also works for pasting/dragging text.

(See documentation.)

This is so useful, it is worth putting it in an answer. Currently (v1.8*?) there is no .input() convenience fn in jquery, so the way to do it is

$('input.myTextInput').on('input',function(e){
 alert('Changed!')
});

I would suggest using the keyup event something like below:

$('elementName').keyup(function() {
 alert("Key up detected");
});

There are a few ways of achieving the same result so I guess it's down to preference and depends on how you want it to work exactly.

Update: This only works for manual input not copy and paste.

For copy and paste I would recommend the following:

$('elementName').on('input',function(e){
 // Code here
});