Events: change, input, cut, copy, paste

Let’s discuss various events that accompany data updates.

Event: change

The change event triggers when the element has finished changing.

For text inputs that means that the event occurs when it looses focus.

For instance, while we are typing in the text field below – there’s no event. But when we move the focus somewhere else, for instance, click on a button – there will be a change event:

<input type="text" onchange="alert(this.value)">
<input type="button" value="Button">

For other elements: select, input type=checkbox/radio it triggers right after the selection changes.

Event: input

The input event triggers every time a value is modified.

For instance:

<input type="text" id="input"> oninput: <span id="result"></span>
<script>
  input.oninput = function() {
    result.innerHTML = input.value;
  };
</script>

If we want to handle every modification of an <input> then this event is the best choice.

Unlike keyboard events it works on any value change, even those that does not involve keyboard actions: pasting with a mouse or using speech recognition to dictate the text.

Can’t prevent anything in oninput

The input event occurs after the value is modified.

So we can’t use event.preventDefault() there – it’s just too late, there would be no effect.

Events: cut, copy, paste

These events occur on cutting/copying/pasting a value.

They belong to ClipboardEvent class and provide access to the data that is copied/pasted.

We also can use event.preventDefault() to abort the action.

For instance, the code below prevents all such events and shows what we are trying to cut/copy/paste:

<input type="text" id="input">
<script>
  input.oncut = input.oncopy = input.onpaste = function(event) {
    alert(event.type + ' - ' + event.clipboardData.getData('text/plain'));
    return false;
  };
</script>

Technically, we can copy/paste everything. For instance, we can copy and file in the OS file manager, and paste it.

There’s a list of methods in the specification to work with different data types, read/write to the clipboard.

But please note that clipboard is a “global” OS-level thing. Most browsers allow read/write access to the clipboard only in the scope of certain user actions for the safety. Also it is forbidden to create “custom” clipboard events in all browsers except Firefox.

Summary

Data change events:

Event Description Specials
change A value was changed. For text inputs triggers on focus loss.
input For text inputs on every change. Triggers immediately unlike change.
cut/copy/paste Cut/copy/paste actions. The action can be prevented. The event.clipbordData property gives read/write access to the clipboard.

Tasks

importance: 5

Create an interface that allows to enter a sum of bank deposit and percentage, then calculates how much it will be after given periods of time.

Here’s the demo:

Any input change should be processed immediately.

The formula is:

// initial: the initial money sum
// interest: e.g. 0.05 means 5% per year
// years: how many years to wait
let result = Math.round(initial * (1 + interest * years));

Open a sandbox for the task.

Tutorial map

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