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In this chapter we’ll get into more details about mouse events and their properties.… Please note: such events may come not only from “mouse devices”, but are also from other devices, such… Mouse event types.We’ve already seen some of these events: mousedown/mouseup Mouse button is clicked… Every mouse move over an element triggers that event. click Triggers after mousedown and then mouseup… Mouse button.Click-related events always have the button property, which allows to get the exact mouse

But native Drag Events also have limitations.… So here we’ll see how to implement Drag’n’Drop using mouse events.… And mouse events only happen on the top element, not on those below it.… We can use it in any of our mouse event handlers to detect the potential droppable under the pointer,… We can use event delegation for mousedown/up.

Long ago, in the past, there were only mouse events.… Pointer event types.Pointer events are named similarly to mouse events: Pointer event Similar mouse… Replacing mouse<event> with pointer<event> in our code We can replace mouse<… That works well for mouse events.… Pointer events extend mouse events.

Let’s dive into more details about events that happen when the mouse moves between elements.… Events mouseover/mouseout, relatedTarget.The mouseover event occurs when a mouse pointer comes over an… When you move the mouse, you can see mouse events in the text area.… Skipping elements.The mousemove event triggers when the mouse moves.… If you move the mouse fast over them, then maybe only the child div triggers events, or maybe the parent

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.… Event name, e.g.… the event.… Many of them depend on the event type: keyboard events have one set of properties, pointer events – another

(event).… Like clientX/clientY for a mouse event:… Events-in-events are synchronous.Usually events are processed in a queue.… That is: if the browser is processing onclick and a new event occurs, e.g. mouse moved, then its handling… For instance, clientX for mouse events. For custom events we should use CustomEvent constructor.

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.… When a user moves their mouse, the task is to dispatch mousemove event and execute handlers.… 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).

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.

Many events automatically lead to certain actions performed by the browser.… Follow-up events Certain events flow one into another.… But not with the mouse click any more.… Sometimes we can use event.defaultPrevented instead, to signal other event handlers that the event was… By default the browser on contextmenu event (right mouse click) shows a context menu with standard options

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

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.

Selecting something with a mouse can be done in both directions: either “left-to-right” or “right-to-left… In other words, when the mouse button is pressed, and then it moves forward in the document, then its… For instance, when the user presses the mouse button on it and starts to move the pointer.… click), Events: input.onselect – triggers when something is selected.… That’s convenient when there’s another event handler on the same action that triggers the select (e.g

coordinates as clientX/clientY, the reasoning for such name will become clear later, when we study event… possible for width/height to be negative, that allows for “directed” rectangle, e.g. to represent mouse

Clicking the button in the left-upper corner allows us to choose a node from the webpage using a mouse… Event Listeners – to see event listeners attached to DOM elements (we’ll cover them in the next part

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.

Clickjacking is for clicks, not for keyboard The attack only affects mouse actions (or similar… Blocking top-navigation.We can block the transition caused by changing top.location in beforeunload 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:.

For example, we gather statistics on how the current visitor uses our page (mouse clicks, page fragments… 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.

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.… element methods, Node – provides common DOM node properties, EventTarget – gives the support for events… For instance, if the existing text was selected with the mouse, then most browsers will remove the selection

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… Any event handler can stop the event by calling event.stopPropagation(), but that’s not recommended,… The same for event handlers.

Try to move control points using a mouse in the example below: As you can notice, the curve… Control points (1,2 and 3) can be moved by the mouse.… The demo for 4 points (points can be moved by a mouse): The algorithm for 4 points: Connect… saw – there’s actually no need to know it, most people just draw the curve by moving points with a mouse… Good properties of Bezier curves: We can draw smooth lines with a mouse by moving control points.

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.

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.

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

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.… Note that they must be assigned using elem.addEventListener, not on<event>.

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"

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.… An uncaught error becomes an “unhandled promise rejection” event on window object.

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.

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

In this example, there’s a <canvas> where we can draw by moving a mouse over it.

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

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

React to user actions, run on mouse clicks, pointer movements, key presses.

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