We can classify events into three categories –
- W3 DOM Specification: W3C manages all the DOM event specifications, except those that deal with form elements.
- HTML5 Specification: These include all the events specifically used with HTML. These include submit, input, etc. New additions to this category include hashchange, beforeunload, etc.
- Browser Object Models: W3 specifications don’t yet cover these events. They deal with touchscreen devices and the events in this section include touchstart, touchend, etc.
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1. User Interface events
These occur as the result of any interaction with the browser window rather than the HTML page. In these events, we attach the event listener to the window object, not the document object. The various UI events are as follows.
The load event fires when the webpage finishes loading. It can also fire on nodes of elements like images, scripts, or objects.
This event fires before the users leave the page, i.e., the webpage is unloading. Page unloading usually happens because a new page has been requested.
It fires when we resize the browser window. But browsers repeatedly fire this event, so avoid using this event to trigger complicated code; it might make the page less responsive.
This event fires when the user scrolls up/down on the browser window. It can relate to the entire page or a specific element on the page.
2. Focus and blur events
These events fire when the HTML elements you can interact with gain/ lose focus. They are most commonly used in forms and especially helpful when you want to do the following tasks:
- To show tips or feedback to users as they interact with an element within a form. The tips are usually shown in the elements other than the one the user is interacting with.
- To trigger form validation as a user moves from one control to the next without waiting to submit the form.
The different focus and blur events are as follows:
This event fires, for a specific DOM node, when an element gains focus.
This fires, for a specific DOM node, when an element loses focus.
This event is the same as the focus event. But Firefox doesn’t yet support the focusin event.
The focus and blur events use the capture approach, while the focusin and focusout events use both capture and bubble approach of the event flow.
3. Mouse events
This event fires when the user clicks on the primary mouse button (usually the left button). This event also fires if the user presses the Enter key on the keyboard when an element has focus.
Touch-screen: A tap on the screen acts like a single left-click.
This event fires when the user clicks the primary mouse button, in quick succession, twice.
Touch-screen: A double-tap on the screen acts like a double left-click.
Accessibility: You can add the above two events to any element, but it’s better to apply it only on the items that are usually clicked, or it will not be accessible through keyboard navigation. All the mouse events discussed below cannot be triggered by the keyboard.
It fires when the user clicks down on any mouse button.
Touch-screen: You can use the touchstart event.
It fires when the user releases a mouse button.
Touch-screen: You can use the touchend event.
We have separate mousedown and mouseup events to add drag-and-drop functionality or controls in game development. Don’t forget a click event is the combination of mousedown and mouseup events.
It fires when the user moves the cursor, which was outside an element before, inside the element. We can say that it fires when we move the cursor over the element.
It fires when the user moves the cursor, which was inside an element before, outside the element. We can say that it fires when the cursor moves off the element.
The mouseover and mouseout events usually change the appearance of graphics on our webpage. A prefered alternative to this is to use the CSS: hover pseudo-class.
It fires when the user moves the cursor around the element. This event is frequently triggered.
4. Keyboard events
These events fire on any kind of device when a user interacts with a keyboard.
This event fires when the value of an <input> or a <textarea> changes (doesn’t fire for deleting in IE9). You can use keydown as a fallback in older browsers.
It fires when the user presses any key in the keyboard. If the user holds down the key, this event fires repeatedly.
It fires when the user presses a key that results in printing a character on the screen. This event fires repeatedly if the user holds down the key. This event will not fire for the enter, tab, or arrow keys; the keydown event would.
The keyup event fires when the user releases a key on the keyboard.
The keydown and keypress events fire before a character appears on the screen, the keyup fires after it shows.
To know the key pressed when you use the keydown and keypress events, the event object has a keyCode property. This property, instead of returning the letter for that key, returns the ASCII code of the lowercase for that key.
5. Form events
This event fires on the node representing the <form> element when a user submits a form.
It fires when the status of various form elements change. This is a better option than using the click event because clicking is not the only way users interact with the form.
The input event is very common with the <input> and the <textarea> elements.
We often use the focus and blur events with forms, but they are also available in conjunction with other elements like links.
6. Mutation events and observers
It fires when the script inserts a new node in the DOM tree using appendChild(), replaceChild(), insertBefore(), etc.
This event fires when the script removes an existing node from the tree using removeChild(), replaceChild(), etc.
It fires when the structure of the DOM tree changes i.e. the above two events occur.
This event fires when the script inserts a node in the DOM tree as the descendant of another node already in the document.
This event fires when the script removes a node from the DOM tree as the descendant of another node already in the document.
The problem with the mutation events is that lots of changes to your page can make your page feel slow or unresponsive. These can also trigger other event listeners, modifying DOM and leading to more mutation events firing. This is the reason for introducing mutation observers to the script.
Mutation observers wait until the script finishes its current task before reacting, then reports the changes in a batch (not one at a time). This reduces the number of events that fire when you change the DOM tree through your script. You can also specify which changes in the DOM you want them to react to.
7. HTML5 events
These are the page-level events included in the versions of the HTML5 specialization. New events support more recent devices like phones and tablets. They respond to events such as gestures and movements. You will understand them better after you master the above concepts, thus they are not discussed for now. Work with the events below for now and when you are a better developer, you can search for other events available. The three HTML5 events we will learn are as follows:
It fires when the URL hash changes without refreshing the entire window. Hashes (#) link specific parts (known as anchors) within a page. It works on the window object; the event object contains both the oldURL and the newURL properties holding the URLs before and after the hashchange.
This event fires on the window object just before the page unloads. This event should only be helpful for the user, not encouraging them to stay on the page. You can add a dialog box to your event, showing a message alerting the users like their changes are not saved.
8. CSS events
These events trigger when the script encounters a CSS element. As CSS is a crucial part of web development, the developers decided to add these events to js to make working with CSS easier. Some of the most common CSS events are as follows:
This event fires when a CSS transition ends in a program. It is useful to notify the script of the end of transition so that it can take further action.
These events fire when CSS animation starts in the program.
This event occurs when any CSS animation repeats itself. With this event, we can determine the number of times an animation iterates in the script.
It fires when the CSS animation comes to an end in the program. This is useful when we want to act just after the animation process finishes.
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