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Another code may listen for the events and observe what’s happening with the menu.… (event).… MouseEvent, KeyboardEvent and others.Here’s a short list of classes for UI Events from the UI Event specification… The full list of properties for different UI events is in the specification, for instance, MouseEvent… Any handler can listen for that event with rabbit.addEventListener('hide',...) and, if needed, cancel

Here’s a list of the most useful DOM events, just to take a look at: Mouse events: click – when the… Event handlers.To react on events we can assign a handler – a function that runs in case of an event.… handler function. options An additional optional object with properties: once: if true, then the listener… the event.… Please note that we need to explicitly setup the events to listen using addEventListener.

Also, writing code that listens for both touch and mouse events was cumbersome.… 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.

It has a single method abort(), And a single property signal that allows to set event listeners on it… 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.… We just need to listen to its abort event in our tasks:… The “call abort()” → “listen to abort event” interaction is simple and universal.

In this chapter we’ll get into more details about mouse events and their properties.… 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.

The name argument is a name of the event, optionally followed by additional arguments with event data… Also the method .on(name, handler) that adds handler function as the listener to events with the given… …And the method .off(name, handler) that removes the handler listener.… And another object, say, calendar may want to listen for such events to load the calendar for the logged-in… Now, if we’d like any code to react to a menu selection, we can listen

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.… So we should listen on document to catch it.… We can use event delegation for mousedown/up.

The standard DOM Events describes 3 phases of event propagation: Capturing phase – the event goes down… on <p>, then the sequence is: HTML → BODY → FORM → DIV -> P (capturing phase, the first listener… ): P → DIV → FORM → BODY → HTML (bubbling phase, the second listener).… Please note, the P shows up twice, because we’ve set two listeners: capturing and bubbling.… Listeners on the same element and same phase run in their set order If we have multiple event

The scroll event allows reacting to a page or element scrolling.… We can’t prevent scrolling by using event.preventDefault() in onscroll listener, because it triggers… 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

Listen to xhr events for response.… You can find readystatechange listeners in really old code, it’s there for historical reasons… The full list is in the specification.… We’ve already seen another event: readystatechange.… If we need to track uploading specifically, then we should listen to same events on xhr.upload object

We can set a listener on keydown and check which key is pressed.… There’s a dilemma here: in such a listener, should we check the value of event.key or event.code?… For events triggered by auto-repeat, the event object has event.repeat property set to true.… 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

The call returns openRequest object, we should listen to events on it: success: database is ready, there… We should listen for it and close the old database connection (and probably suggest a page reload, to… If we don’t listen for the versionchange event and don’t close the old connection, then the second, new… …In other words, here we do two things: The db.onversionchange listener informs us about a parallel… To detect the moment of successful completion, we can listen to transaction.oncomplete event:

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.… There are many services (and we can build our own) that listen for global errors using window.onerror… The readystatechange event also works for resources, but is rarely used, because load/error events are

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.… For instance, here’s a list.

Once the socket is created, we should listen to events on it.… So you’ll see events open → message → close. That’s actually it, we can talk WebSocket already.… The full list can be found in RFC6455, §7.4.1.… For each accepted websocket, add it to the set clients.add(socket) and set message event listener to… Events: open, message, error, close.

That event does not happen for object-like access. We’ll see that later in this chapter.… If both windows are listening for window.onstorage, then each one will react on updates that happened… 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.

Many events automatically lead to certain actions performed by the browser.… Follow-up events Certain events flow one into another.… If we prevent the first event, there will be no second.… 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

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"

If we set any event handlers on it, they will be ignored.… But it can be relaxed if we provide a space-separated list of restrictions that should not be applied… Here’s a list of limitations: allow-same-origin By default "sandbox" forces the “different… onmessage.To receive a message, the target window should have a handler on the message event.… We should use addEventListener to set the handler for this event inside the target window.

We can create custom HTML elements, described by our class, with its own methods and properties, events… Here’s a sketch with the full list:… We can observe attributes by providing their list in observedAttributes() static getter.… For a change of an attribute, listed in observedAttributes(), attributeChangedCallback triggers.… For instance, inner elements can dispatch events like initialized, and outer ones can listen and react

Components may have subcomponents, e.g. messages may be parts of a higher-level “message list” component… API: events, class methods etc, to interact with other components.… Event retargeting and other minor stuff to make custom components better fit the development.

Event Listeners – to see event listeners attached to DOM elements (we’ll cover them in the next part… For instance, $0.style.background = 'red' makes the selected list item red, like this:

Then use new MyClass() to create a new object with all the listed… That’s especially useful in browser environment, for event listeners.

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

Then after any changes, the callback is executed: changes are passed in the first argument as a list… 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.… In such cases, we use observer.takeRecords() – gets a list of unprocessed mutation records – those that

let’s cover animation properties one by one. transition-property.In transition-property, we write a list… Here’s a list of digits, without any animations, just as a source: Resultstyle.cssindex.htmlIn the HTML… Event: “transitionend”.When the CSS animation finishes, the transitionend event triggers.… You can find a longer list of CSS properties and which stages they trigger at https://csstriggers.com… And the transitionend event allows JavaScript to be run after the animation, so it integrates fine with

You give your fans a list.… In terms of our analogy: this is the “subscription list”.… analogy isn’t terribly accurate, because JavaScript promises are more complex than a simple subscription list… Subscriptions in real life must be done prior to the event. Promises are more flexible.… Each time, we’re adding a new “fan”, a new subscribing function, to the “subscription list”.

event delegation.… Delegation example: actions in markup.There are other uses for event delegation.… A document-wide handler tracks events, and if an event happens on an attributed element – performs the… It’s one of the most helpful patterns for DOM events.… If the event happened inside an element that interests us, then handle the event.

Also, some parts have additional specs listed at https://spec.whatwg.org.… about standards, we have: DOM specification Describes the document structure, manipulations, and events

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:.

Here’s the full list of all possible fetch options with their default values (alternatives in comments… ): An impressive list, right?… We can use the window.onunload event for that:

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.

Add an event listener on the “Start sending messages” button, responsible for starting the logger when… Add an event listener on the “Close” button, responsible for closing the logs display window when clicked

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.

It serves as a base, so that all DOM nodes support so-called “events”, we’ll study them later.… It gets properties and methods as a superposition of (listed in inheritance order): HTMLInputElement… element methods, Node – provides common DOM node properties, EventTarget – gives the support for events… If we want to know the full list of supported properties for a given class, we can find them in the specification

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.

That’s good for functionality that doesn’t depend on anything, like counters, ads, document-level event… listeners.

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.

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.

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

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).

Spread syntax.We’ve just seen how to get an array from the list of parameters.… Passing it “as is” won’t work, because Math.max expects a list of numeric arguments, not a single array… The list of characters is passed to array initializer [...str].… The spread syntax is used to pass an array to functions that normally require a list of many arguments… Together they help to travel between a list and an array of parameters with ease.

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

In the HTML document, an HTML-tag may contain a list of: Text pieces. HTML-comments.… Linked list.Imagine, we want to store an ordered list of objects.… next linked list element or null if that’s the end.… Naturally, lists are not always better than arrays. Otherwise everyone would use only lists.… a list (or null).

Unicode supports many different properties, their full list would require a lot of space, so here are… the references: List all properties by a character: https://unicode.org/cldr/utility/character.jsp.… List all characters by a property: https://unicode.org/cldr/utility/list-unicodeset.jsp.… system), that may have a value: Cyrillic, Greek, Arabic, Han (Chinese) and so on, here’s the full list

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.

It contains a comma-separated list of unsafe header names that should be made accessible.… Access-Control-Request-Headers header provides a comma-separated list of its unsafe HTTP-headers.… Access-Control-Allow-Headers must have a list of allowed headers.… Access-Control-Request-Headers – a comma-separated list of “unsafe” headers.… Access-Control-Request-Headers lists unsafe requested headers.

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.

Arguments of these methods are an arbitrary list of DOM nodes to insert, or text strings (that become… Here’s an example of using these methods to add items to a list and the text before/after it:… Here’s a visual picture of what the methods do: So the final list… DocumentFragment.DocumentFragment is a special DOM node that serves as a wrapper to pass around lists… The following code inserts a new list item before the second <li>: