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event delegation.… Delegation example: actions in markup.There are other uses for event delegation.… The “behavior” pattern.We can also use event delegation to add “behaviors” to elements declaratively,… Summary.Event delegation is really cool! It’s one of the most helpful patterns for DOM events.… The delegation has its limitations of course: First, the event must be bubbling.

Events mouseover/mouseout, relatedTarget.The mouseover event occurs when a mouse pointer comes over an… one, or maybe there will be no events at all.… The mouseover event on a descendant bubbles up.… Events mouseenter/mouseleave do not bubble. These events are extremely simple.… So we can’t use event delegation with them.

But native Drag Events also have limitations.… Also, mobile device support for such events is very weak.… So here we’ll see how to implement Drag’n’Drop using mouse events.… The ball is always on top over other elements, so events happen on it.… We can use event delegation for mousedown/up.

There are important peculiarities when working with focus events.… Events focus/blur.The focus event is called on focusing, and blur – when the element loses the focus.… ), and when the alert is dismissed, the focus comes back (focus event).… The best recipe is to be careful when using these events.… Delegation: focusin/focusout.Events focus and blur do not bubble.

Many events automatically lead to certain actions performed by the browser.… By the way, using event delegation here makes our menu very flexible.… Follow-up events Certain events flow one into another.… If we prevent the mousedown event, there’s no focus.… Sometimes we can use event.defaultPrevented instead, to signal other event handlers that the event was

The standard DOM Events describes 3 phases of event propagation: Capturing phase – the event goes down… Then the event bubbles up from event.target to the root, calling handlers assigned using on<event&… the event. event.currentTarget (=this) – the current element that handles the event (the one that has… The same for event handlers.… Bubbling and capturing lay the foundation for “event delegation” – an extremely powerful event handling

That’s pretty convenient, but the wrapper is not perfect, it can’t replace events for all cases.… The openRequest object will emit the blocked event instead of success.… We can use event delegation instead. IndexedDB events bubble: request → transaction → database.… All events are DOM events, with capturing and bubbling, but usually only bubbling stage is used.… Sometimes we can make our life easier by using event delegation, e.g. set handlers on the whole transactions

among other event types.… Pointer event types.Pointer events are named similarly to mouse events: Pointer event Similar mouse… ;event> events with pointer<event> in our code and expect things to continue working fine with… Pointer event properties.Pointer events have the same properties as mouse events, such as clientX/Y,… Pointer events extend mouse events.

Mouse event types.We’ve already seen some of these events: mousedown/mouseup Mouse button is clicked… …There are several other events too, we’ll cover them later.… Events order.As you can see from the list above, a user action may trigger multiple events.… Touch devices also generate similar events when one taps on them.… are true if the corresponding key was pressed during the event.

(event).… events.… Besides, the event class describes “what kind of event” it is, and if the event is custom, then we should… Events-in-events are synchronous.Usually events are processed in a queue.… Such events are processed immediately: the new event handlers are called, and then the current event

Event types.By default EventSource object generates three events: message – a message received, available… The server may specify another type of event with event: ... at the event start.… The server may set a custom event name in event:.… Such events should be handled using addEventListener, not on<event>.… There’s no way to set it from JavaScript. event: – event name, must precede data:.

Event handlers.To react on events we can assign a handler – a function that runs in case of an event.… Event name, e.g.… the event.… Many of them depend on the event type: keyboard events have one set of properties, pointer events – another… ) in case of an event.

There are few events that do not. This is governed by the composed event object property.… If you take a look at UI Events specification, most events have composed: true: blur, focus, focusin… All touch events and pointer events also have composed: true.… These events can be caught only on elements within the same DOM, where the event target resides.… Touch Events https://w3c.github.io/touch-events.

Browser JavaScript execution flow, as well as in Node.js, is based on an event loop.… Event Loop.The event loop concept is very simple.… until the event bubbled up and was handled on all levels.… In the chapter Dispatching custom events we saw an example: custom event menu-open is dispatched in setTimeout… Also, used in event handlers to schedule an action after the event is fully handled (bubbling done).

The submit event triggers when the form is submitted, it is usually used to validate the form before… Event: submit.There are two main ways to submit a form: The first – to click <input type="submit… Both actions lead to submit event on the form.… Relation between submit and click When a form is sent using Enter on an input field, a click event… Then the submit event is not generated.

Let’s cover various events that accompany data updates.… Event: change.The change event triggers when the element has finished changing.… Events: cut, copy, paste.These events occur on cutting/copying/pasting a value.… events must not provide access to the clipboard.… Summary.Data change events: Event Description Specials change A value was changed.

the functionality of EventEmitter to User, so that our users can emit events.… An important feature of many browser objects (for instance) is that they can generate events.… Events are a great way to “broadcast information” to anyone who wants it.… The name argument is a name of the event, optionally followed by additional arguments with event data… may assign handlers to react on that event.

Each event may be useful: DOMContentLoaded event – DOM is ready, so the handler can lookup DOM nodes… Let’s explore the details of these events.… That’s actually the delay until the DOMContentLoaded event. window.onload.The load event on the window… This event is available via the onload property.… Summary.Page load events: The DOMContentLoaded event triggers on document when the DOM is ready.

The scroll event allows reacting to a page or element scrolling.… In action: Current scroll = scroll the window The scroll event… But we can prevent scrolling by event.preventDefault() on an event that causes the scroll, for instance… keydown event for pageUp and pageDown.… If we add an event handler to these events and event.preventDefault() in it, then the scroll won’t start

It has a single method abort(), And a single property signal that allows to set event listeners on it… When abort() is called: controller.signal emits the "abort" event. controller.signal.aborted… We could implement the same kind of event listening in our code on our own, without the AbortController… It will listen to abort events on signal.… The “call abort()” → “listen to abort event” interaction is simple and universal.

Key codes described in the UI Events code specification.… For events triggered by auto-repeat, the event object has event.repeat property set to true.… Or we can use both event handlers together.… Legacy.In the past, there was a keypress event, and also keyCode, charCode, which properties of the event… We have input and change events to handle any input (covered later in the chapter Events: change, input

There are two events for it: onload – successful load, onerror – an error occurred.… script.onload.The main helper is the load event.… Important: Events onload/onerror track only the loading itself.… Errors that may occur during script processing and execution are out of scope for these events.… The readystatechange event also works for resources, but is rarely used, because load/error events are

Listen to xhr events for response.… Upload progress.The progress event triggers only on the downloading stage.… There’s another object, without methods, exclusively to track upload events: xhr.upload.… The error, abort, timeout, and load events are mutually exclusive. Only one of them may happen.… We’ve already seen another event: readystatechange.

Unhandled rejection.Remember the unhandledrejection event from the article Error handling with promises… But if we forget to add .catch, then, after the microtask queue is empty, the engine triggers the event… is complete: the engine examines promises and, if any of them is in the “rejected” state, then the event… Javascript engines, including browsers and Node.js, the concept of microtasks is closely tied with the “event… have no direct relation to promises, they are covered in another part of the tutorial, in the article Event

That event does not happen for object-like access. We’ll see that later in this chapter.… Storage event.When the data gets updated in localStorage or sessionStorage, storage event triggers, with… Also, event.storageArea contains the storage object – the event is the same for both sessionStorage and… We access keys as object properties, in that case storage event isn’t triggered.… Storage event: Triggers on setItem, removeItem, clear calls.

It delivers the data using events, as reading from disk may take time.… As the reading proceeds, there are events: loadstart – loading started. progress – occurs during reading… The most widely used events are for sure load and error.… Its reading methods read* do not generate events, but rather return a result, as regular functions do… We usually get File objects from user input, like <input> or Drag’n’Drop events (ondragend).

Then our component should render it properly, as a nice menu with given title and items, handle menu events… The browser renders it and uses for style inheritance, event propagation (more about that later).… But if the component code wants to know about slot changes, then slotchange event is available.… There are two slotchange events here: At initialization: slotchange: title triggers immediately, as… Please note: there’s no slotchange event after 2 seconds, when the content of slot="title"

In the browser we can catch such errors using the event unhandledrejection:… The event… If an error occurs, and there’s no .catch, the unhandledrejection handler triggers, and gets the event… In any case we should have the unhandledrejection event handler (for browsers, and analogs for other

Once the socket is created, we should listen to events on it.… There are totally 4 events: open – connection established, message – data received, error – websocket… So you’ll see events open → message → close. That’s actually it, we can talk WebSocket already.… For each accepted websocket, add it to the set clients.add(socket) and set message event listener to… Events: open, message, error, close.

Most browsers block popups if they are called outside of user-triggered event handlers like onclick.… There’s also window.onresize event.… There’s also window.onscroll event.… But please note that the blur event means that the visitor switched out from the window, but they still… Events focus and blur allow to track switching in and out of the window.

iframe.onload vs iframe.contentWindow.onload The iframe.onload event… If we set any event handlers on it, they will be ignored.… onmessage.To receive a message, the target window should have a handler on the message event.… If it is so, then targetWin triggers the message event with special properties: origin – the origin… We should use addEventListener to set the handler for this event inside the target window.

persistent connection with server, that doesn’t use any specific protocol like WebSocket or Server Sent Events… So, in this case, another method is preferred, such as Websocket or Server Sent Events.

API: events, class methods etc, to interact with other components.… Event retargeting and other minor stuff to make custom components better fit the development.

will be before the start (anchor): Selection events.There are events… direction, one of: “forward”, “backward” or “none” (if e.g. selected with a double mouse click), Events… Example: tracking selection.For example, this code uses onselect event to track selection:… triggers when something is selected, but not when the selection is removed. document.onselectionchange event… That’s convenient when there’s another event handler on the same action that triggers the select (e.g

Forms and control elements, such as <input> have a lot of special properties and events.… In the next chapter we’ll cover focus and blur events that may occur on any element, but are mostly handled

resources, because the JavaScript engine can do other jobs in the meantime: execute other scripts, handle events… We can catch such errors using a global unhandledrejection event handler as described in the chapter

Scripts with defer always execute when the DOM is ready (but before DOMContentLoaded event).… DOMContentLoaded event handler waits for the deferred script.

That’s why this event is only useful to show a nice progress bar.

Event: “transitionend”.When the CSS animation finishes, the transitionend event triggers.… finishes, and flips the direction: The event… And the transitionend event allows JavaScript to be run after the animation, so it integrates fine with

The yield* directive delegates

If we make more complex editing operations, e.g. remove the <b>edit</b>, the mutation event… Well, we can do it on DOMContentLoaded event, or put the script at the bottom of the page.

We can create custom HTML elements, described by our class, with its own methods and properties, events… For instance, inner elements can dispatch events like initialized, and outer ones can listen and react

Event Listeners – to see event listeners attached to DOM elements (we’ll cover them in the next part

about standards, we have: DOM specification Describes the document structure, manipulations, and events

That’s especially useful in browser environment, for event listeners.

when the script loads: The onload event

coordinates as clientX/clientY, the reasoning for such name will become clear later, when we study event

It serves as a base, so that all DOM nodes support so-called “events”, we’ll study them later.… element methods, Node – provides common DOM node properties, EventTarget – gives the support for events

Blocking top-navigation.We can block the transition caused by changing top.location in beforeunload event

are also advanced browser-related use cases of zero-delay timeout, that we’ll discuss in the chapter Event