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property change listener javascript

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Watch for Object Changes with JavaScript

Watching for changes to an object's property has always been a much sought after task; many shims have been used over the years to listen to object changes.  These days we have better methods for listening to object changes:  the Proxy API.  Sindre Sorhus has created  on-change , a tiny JavaScript tool that allows you to listen for changes on JavaScript objects and arrays!

on-change JavaScript

Since the on-change code is so tiny, here it is in its full glory:

The onChange  function returns a proxy which you'll use to make any object changes.

Using on-change

Pass onChange the object you'd like to spy on and a change handler function:

Then use that Proxy to change properties and get notified of changes:

Note that the original object's values also change -- that's the beauty of Proxy!  Also note that on-change doesn't tell you which property changed; the use case, as Sindre describes, is saving an object (to server, etc.) when any part of an object changes.  You could also use ti for a small library with a render function, re-rendering the content whenever a property changed!

This on-change library is really nice if you don't need to know which property changed, just that  something changed.

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If someone wonders what this Reflect object it. I stumbled across on-change a few days ago and wrote about it. :)

https://www.stefanjudis.com/today-i-learned/the-global-reflect-object-its-use-cases-and-things-to-watch-out-for/

Just a note that watching an object really bogs down all accesses to the object. It’s great for debugging but don’t build a lot of functionality around it in your production code.

It is worth mentioning that arrays are not handled by this function and push/pop/delete to an array needs to be handled by overwriting those methods.

Thanks for this David, I’m relatively novice to programming (used mostly in webobjects within Storyline) so forgive me if my question seems idiotic. I checked out Proxy Api and it’s browser compatibility excludes IE. Is there an alternative to onChange for IE?

Nice solution,

But to create a proxy for change notification purposes is not the same that observe changes on an object. In your solution, you create a different object – the proxy – to operate with and force the developer to abandon the use of the original object. The best explanation is an example:

Why this detail is, sometimes, important. Because, sometimes, you are not the owner of the object j and the owner does not want to change the j reference for your proxy. Imagine a situation where you are a framework developer and want to observe changes in some code of your user by maintaining transparency from her perspective.

If you want another example I can detail the corner case that brought me here. In my case, my j object is a DOM element and I cannot replace it with a proxy.

Wrap your code in <pre class="{language}"></pre> tags, link to a GitHub gist, JSFiddle fiddle, or CodePen pen to embed!

Home » JavaScript DOM » JavaScript change Event

JavaScript change Event

Summary : in this tutorial, you’ll learn about the JavaScript change event of the input text, radio button, checkbox, and select elements.

The change event occurs when the element has completed changing.

To attach an event handler to the change event of an element, you can either call the addEventListener() method:

or use the onchange attribute of the element. For example:

However, it is a good practice to use the addEventListener() method.

Using JavaScript change event for input elements

The change event of an <input> element fires when the <input> element loses focus. The change event does not fire when you’re tying.

The following example shows the value of the input text when it loses focus.

In this example, if you type some text on the <input> element and move focus to the button, the change event fires to show the entered text.

Note that if you want to handle every change of the value, you use the input event instead.

Using JavaScript change event for radio buttons

A radio button fires the change event after you select it.

The following example shows how to handle the change event of the radio buttons:

How it works:

  • First, register an event handler to the change event of the body . When a radio button is clicked, its change event is bubbled to the body. This technique is called event delegation .
  • Then, show a corresponding message based on which radio button is selected.

Using JavaScript change event for checkboxes

Similar to radio buttons, checkboxes fire the change event after selection, whether checked or unchecked. For example:

Using JavaScript change event for the select element

The <select> element fires the change event once the selection has completed.

The following example shows how to handle the change event of the <select> element. The <p> element with the id result will display the selected item:

  • First, select the <select> element by its id ( lang );
  • Then, show the selected value in the <p> element.
  • The <input> element fires the change event once it loses focus.
  • The radio button, checkbox, and select elements fire the change event after they have been selected.
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EventTarget: addEventListener() method

The addEventListener() method of the EventTarget interface sets up a function that will be called whenever the specified event is delivered to the target.

Common targets are Element , or its children, Document , and Window , but the target may be any object that supports events (such as IDBRequest ).

Note: The addEventListener() method is the recommended way to register an event listener. The benefits are as follows:

  • It allows adding more than one handler for an event. This is particularly useful for libraries, JavaScript modules, or any other kind of code that needs to work well with other libraries or extensions.
  • In contrast to using an onXYZ property, it gives you finer-grained control of the phase when the listener is activated (capturing vs. bubbling).
  • It works on any event target, not just HTML or SVG elements.

The method addEventListener() works by adding a function, or an object that implements a handleEvent() function, to the list of event listeners for the specified event type on the EventTarget on which it's called. If the function or object is already in the list of event listeners for this target, the function or object is not added a second time.

Note: If a particular anonymous function is in the list of event listeners registered for a certain target, and then later in the code, an identical anonymous function is given in an addEventListener call, the second function will also be added to the list of event listeners for that target.

Indeed, anonymous functions are not identical even if defined using the same unchanging source-code called repeatedly, even if in a loop .

Repeatedly defining the same unnamed function in such cases can be problematic. (See Memory issues , below.)

If an event listener is added to an EventTarget from inside another listener — that is, during the processing of the event — that event will not trigger the new listener. However, the new listener may be triggered during a later stage of event flow, such as during the bubbling phase.

A case-sensitive string representing the event type to listen for.

The object that receives a notification (an object that implements the Event interface) when an event of the specified type occurs. This must be null , an object with a handleEvent() method, or a JavaScript function . See The event listener callback for details on the callback itself.

An object that specifies characteristics about the event listener. The available options are:

A boolean value indicating that events of this type will be dispatched to the registered listener before being dispatched to any EventTarget beneath it in the DOM tree. If not specified, defaults to false .

A boolean value indicating that the listener should be invoked at most once after being added. If true , the listener would be automatically removed when invoked. If not specified, defaults to false .

A boolean value that, if true , indicates that the function specified by listener will never call preventDefault() . If a passive listener does call preventDefault() , the user agent will do nothing other than generate a console warning.

If this option is not specified it defaults to false – except that in browsers other than Safari, it defaults to true for wheel , mousewheel , touchstart and touchmove events. See Using passive listeners to learn more.

An AbortSignal . The listener will be removed when the given AbortSignal object's abort() method is called. If not specified, no AbortSignal is associated with the listener.

A boolean value indicating whether events of this type will be dispatched to the registered listener before being dispatched to any EventTarget beneath it in the DOM tree. Events that are bubbling upward through the tree will not trigger a listener designated to use capture. Event bubbling and capturing are two ways of propagating events that occur in an element that is nested within another element, when both elements have registered a handle for that event. The event propagation mode determines the order in which elements receive the event. See DOM Level 3 Events and JavaScript Event order for a detailed explanation. If not specified, useCapture defaults to false .

Note: For event listeners attached to the event target, the event is in the target phase, rather than the capturing and bubbling phases. Event listeners in the capturing phase are called before event listeners in any non-capturing phases.

A Firefox (Gecko)-specific parameter. If true , the listener receives synthetic events dispatched by web content (the default is false for browser chrome and true for regular web pages). This parameter is useful for code found in add-ons, as well as the browser itself.

Return value

None ( undefined ).

Usage notes

The event listener callback.

The event listener can be specified as either a callback function or an object whose handleEvent() method serves as the callback function.

The callback function itself has the same parameters and return value as the handleEvent() method; that is, the callback accepts a single parameter: an object based on Event describing the event that has occurred, and it returns nothing.

For example, an event handler callback that can be used to handle both fullscreenchange and fullscreenerror might look like this:

Safely detecting option support

In older versions of the DOM specification, the third parameter of addEventListener() was a Boolean value indicating whether or not to use capture. Over time, it became clear that more options were needed. Rather than adding more parameters to the function (complicating things enormously when dealing with optional values), the third parameter was changed to an object that can contain various properties defining the values of options to configure the process of removing the event listener.

Because older browsers (as well as some not-too-old browsers) still assume the third parameter is a Boolean, you need to build your code to handle this scenario intelligently. You can do this by using feature detection for each of the options you're interested in.

For example, if you want to check for the passive option:

This creates an options object with a getter function for the passive property; the getter sets a flag, passiveSupported , to true if it gets called. That means that if the browser checks the value of the passive property on the options object, passiveSupported will be set to true ; otherwise, it will remain false . We then call addEventListener() to set up a fake event handler, specifying those options, so that the options will be checked if the browser recognizes an object as the third parameter. Then, we call removeEventListener() to clean up after ourselves. (Note that handleEvent() is ignored on event listeners that aren't called.)

You can check whether any option is supported this way. Just add a getter for that option using code similar to what is shown above.

Then, when you want to create an actual event listener that uses the options in question, you can do something like this:

Here we're adding a listener for the mouseup event on the element someElement . For the third parameter, if passiveSupported is true , we're specifying an options object with passive set to true ; otherwise, we know that we need to pass a Boolean, and we pass false as the value of the useCapture parameter.

You can learn more in the Implementing feature detection documentation and the explainer about EventListenerOptions from the Web Incubator Community Group .

The value of "this" within the handler

It is often desirable to reference the element on which the event handler was fired, such as when using a generic handler for a set of similar elements.

When attaching a handler function to an element using addEventListener() , the value of this inside the handler will be a reference to the element. It will be the same as the value of the currentTarget property of the event argument that is passed to the handler.

As a reminder, arrow functions do not have their own this context .

If an event handler (for example, onclick ) is specified on an element in the HTML source, the JavaScript code in the attribute value is effectively wrapped in a handler function that binds the value of this in a manner consistent with the addEventListener() ; an occurrence of this within the code represents a reference to the element.

Note that the value of this inside a function, called by the code in the attribute value, behaves as per standard rules . This is shown in the following example:

The value of this within logID() is a reference to the global object Window (or undefined in the case of strict mode .

Specifying "this" using bind()

The Function.prototype.bind() method lets you establish a fixed this context for all subsequent calls — bypassing problems where it's unclear what this will be, depending on the context from which your function was called. Note, however, that you'll need to keep a reference to the listener around so you can remove it later.

This is an example with and without bind() :

Another solution is using a special function called handleEvent() to catch any events:

Another way of handling the reference to this is to use an arrow function, which doesn't create a separate this context.

Getting data into and out of an event listener

It may seem that event listeners are like islands, and that it is extremely difficult to pass them any data, much less to get any data back from them after they execute. Event listeners only take one argument, the Event Object , which is automatically passed to the listener, and the return value is ignored. So how can we get data in and back out of them? There are a number of good methods for doing this.

Getting data into an event listener using "this"

As mentioned above , you can use Function.prototype.bind() to pass a value to an event listener via the this reference variable.

This method is suitable when you don't need to know which HTML element the event listener fired on programmatically from within the event listener. The primary benefit to doing this is that the event listener receives the data in much the same way that it would if you were to actually pass it through its argument list.

Getting data into an event listener using the outer scope property

When an outer scope contains a variable declaration (with const , let ), all the inner functions declared in that scope have access to that variable (look here for information on outer/inner functions, and here for information on variable scope). Therefore, one of the simplest ways to access data from outside of an event listener is to make it accessible to the scope in which the event listener is declared.

Note: Although inner scopes have access to const , let variables from outer scopes, you cannot expect any changes to these variables to be accessible after the event listener definition, within the same outer scope. Why? Because by the time the event listener would execute, the scope in which it was defined would have already finished executing.

Getting data into and out of an event listener using objects

Unlike most functions in JavaScript, objects are retained in memory as long as a variable referencing them exists in memory. This, and the fact that objects can have properties, and that they can be passed around by reference, makes them likely candidates for sharing data among scopes. Let's explore this.

Note: Functions in JavaScript are actually objects. (Hence they too can have properties, and will be retained in memory even after they finish executing if assigned to a variable that persists in memory.)

Because object properties can be used to store data in memory as long as a variable referencing the object exists in memory, you can actually use them to get data into an event listener, and any changes to the data back out after an event handler executes. Consider this example.

In this example, even though the scope in which both the event listener and the interval function are defined would have finished executing before the original value of someObject.aProperty would have changed, because someObject persists in memory (by reference ) in both the event listener and interval function, both have access to the same data (i.e. when one changes the data, the other can respond to the change).

Note: Objects are stored in variables by reference, meaning only the memory location of the actual data is stored in the variable. Among other things, this means variables that "store" objects can actually affect other variables that get assigned ("store") the same object reference. When two variables reference the same object (e.g., let a = b = {aProperty: 'Yeah'}; ), changing the data in either variable will affect the other.

Note: Because objects are stored in variables by reference, you can return an object from a function to keep it alive (preserve it in memory so you don't lose the data) after that function stops executing.

Memory issues

In the first case above, a new (anonymous) handler function is created with each iteration of the loop. In the second case, the same previously declared function is used as an event handler, which results in smaller memory consumption because there is only one handler function created. Moreover, in the first case, it is not possible to call removeEventListener() because no reference to the anonymous function is kept (or here, not kept to any of the multiple anonymous functions the loop might create.) In the second case, it's possible to do myElement.removeEventListener("click", processEvent, false) because processEvent is the function reference.

Actually, regarding memory consumption, the lack of keeping a function reference is not the real issue; rather it is the lack of keeping a static function reference.

Using passive listeners

If an event has a default action — for example, a wheel event that scrolls the container by default — the browser is in general unable to start the default action until the event listener has finished, because it doesn't know in advance whether the event listener might cancel the default action by calling Event.preventDefault() . If the event listener takes too long to execute, this can cause a noticeable delay, also known as jank , before the default action can be executed.

By setting the passive option to true , an event listener declares that it will not cancel the default action, so the browser can start the default action immediately, without waiting for the listener to finish. If the listener does then call Event.preventDefault() , this will have no effect.

The specification for addEventListener() defines the default value for the passive option as always being false . However, to realize the scroll performance benefits of passive listeners in legacy code, browsers other than Safari have changed the default value of the passive option to true for the wheel , mousewheel , touchstart and touchmove events on the document-level nodes Window , Document , and Document.body . That prevents the event listener from canceling the event , so it can't block page rendering while the user is scrolling.

Note: See the compatibility table below if you need to know which browsers (and/or which versions of those browsers) implement this altered behavior.

Because of that, when you want to override that behavior and ensure the passive option is false in all browsers, you must explicitly set the option to false (rather than relying on the default).

You don't need to worry about the value of passive for the basic scroll event. Since it can't be canceled, event listeners can't block page rendering anyway.

See Improving scroll performance using passive listeners for an example showing the effect of passive listeners.

Older browsers

In older browsers that don't support the options parameter to addEventListener() , attempting to use it prevents the use of the useCapture argument without proper use of feature detection .

Add a simple listener

This example demonstrates how to use addEventListener() to watch for mouse clicks on an element.

In this code, modifyText() is a listener for click events registered using addEventListener() . A click anywhere in the table bubbles up to the handler and runs modifyText() .

Add an abortable listener

This example demonstrates how to add an addEventListener() that can be aborted with an AbortSignal .

In the example above, we modify the code in the previous example such that after the second row's content changes to "three", we call abort() from the AbortController we passed to the addEventListener() call. That results in the value remaining as "three" forever because we no longer have any code listening for a click event.

Event listener with anonymous function

Here, we'll take a look at how to use an anonymous function to pass parameters into the event listener.

Notice that the listener is an anonymous function that encapsulates code that is then, in turn, able to send parameters to the modifyText() function, which is responsible for actually responding to the event.

Event listener with an arrow function

This example demonstrates a simple event listener implemented using arrow function notation.

Please note that while anonymous and arrow functions are similar, they have different this bindings. While anonymous (and all traditional JavaScript functions) create their own this bindings, arrow functions inherit the this binding of the containing function.

That means that the variables and constants available to the containing function are also available to the event handler when using an arrow function.

Example of options usage

Click the outer, middle, inner containers respectively to see how the options work.

Before using a particular value in the options object, it's a good idea to ensure that the user's browser supports it, since these are an addition that not all browsers have supported historically. See Safely detecting option support for details.

Event listener with multiple options

You can set more than one of the options in the options parameter. In the following example we are setting two options:

  • passive , to assert that the handler will not call preventDefault()
  • once , to ensure that the event handler will only be called once.

Improving scroll performance using passive listeners

The following example shows the effect of setting passive . It includes a <div> that contains some text, and a check box.

The code adds a listener to the container's wheel event, which by default scrolls the container. The listener runs a long-running operation. Initially the listener is added with the passive option, and whenever the checkbox is toggled, the code toggles the passive option.

The effect is that:

  • Initially, the listener is passive, so trying to scroll the container with the wheel is immediate.
  • If you uncheck "passive" and try to scroll the container using the wheel, then there is a noticeable delay before the container scrolls, because the browser has to wait for the long-running listener to finish.

Specifications

Browser compatibility.

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

  • EventTarget.removeEventListener()
  • Creating and triggering custom events
  • More details on the use of this in event handlers

How to Listen for Changes to HTML Elements with the contenteditable Attribute with JavaScript?

  • Post author By John Au-Yeung
  • Post date March 3, 2021
  • No Comments on How to Listen for Changes to HTML Elements with the contenteditable Attribute with JavaScript?

white and brown long coated small dog lying on white textile

Elements with the contenteditable attribute added to it lets users edit the content inside the element.

Sometimes, we may want to listen for change events that are made to the element.

In this article, we’ll look at how to listen for changes triggered in elements that have the contenteditable attribute applied.

Listen to the input Event Emitted by the Element

When we change the content of an element that has the contenteditable attribute added, then input event will be emitted.

Therefore, we can listen to the input event of the element to listen for changes in its content.

For instance, we can write the following HTML:

And then we can add an event listener with the addEventListener with:

When we change the content in the div, we’ll see the input event listener run.

We can get the element’s attributes and styles with the e.target property.

The e.timestamp property is the time in milliseconds at which the event is created.

MutationObserver

We can also use the MutationObserver to watch for changes to the content of an element.

This is because it can listen for changes in the DOM, including any content changes in an element.

For instance, we can write:

and keep the HTML the same.

We pass in a callback with the mutationRecords object to get the mutation records.

chartacterData set to true means we watch for text content changes.

And subtree set to true means we watch for the element’s DOM subtree changes.

We get the latest content of the div with the mutationRecords[0].target.data property.

We can watch for changes of the content of an HTML with the contenteditable attribute applied with the MutationObserver or listen to the input event of the element.

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Web developer specializing in React, Vue, and front end development.

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Listen for a class change in JavaScript

JavaScript possesses the ability to listen for a class change right out of the box. Without any additional libraries or hacky workarounds. It can be done with the MutationObserver interface .

As a matter of fact, the MutationObserver allows listening to any change being made to the DOM tree, not only to the class name. MutationObserver is supported in all major browsers .

For the sake of a demonstration let’s look at how it works with changing the class names.

Say we have a button:

And this button might have additional classes added to it like btn--active , btn--disabled , btn--loading , etc.

To listen to class change we need to instantiate the MutationObserver object, pass the callback function, and call the observe() method.

The observe() method accepts two arguments the target node and options object which should contain attributes property that is set to true.

Now every time the class is changed on the button the callback function will run. From that point, you can handle the event as you need to.

Quite often the case of the class name change (or any other HTML attribute for that matter) could be outside the scope of your program (e.g. WordPress environment or 3rd party library). That’s where the MutationObserver comes into play and listens for such events.

Video of MutationObserver in action:

Understanding DOM Events and JavaScript Event Listeners

JavaScript code in the browser uses an event-driven programming pattern. What this means is that when a specific DOM event happens in the browser, a piece of code will be executed as a response to that action.

In this article, I will help you see and understand how to listen and respond to DOM events using JavaScript.

If you need a refresher on the DOM, I have written an article that explains what the DOM is and how JavaScript interacts with it .

What Are DOM Events and Why Are They Useful?

DOM events are signals exposed by the browser that you can use to run a piece of JavaScript code.

These DOM events occur when the user interacts with the application we've created, such as clicking a button or typing letters into an input field.

As a web developer, you can instruct JavaScript to listen for a specific event and do something in response to that event.

For example:

  • When a button is clicked , do change the text of a paragraph.
  • When a form is submitted , do a POST request using the Fetch API.

How to Listen to DOM Events

To listen for an event, you need to attach an event listener to an element by using the addEventListener() method.

The addEventListener() method accepts two parameters:

  • The event type to listen to
  • A function to run when the event is triggered

Back to the example, suppose you want to change the text of a paragraph when a button element is clicked. Here’s how you do it:

To insert JavaScript code into the HTML document, we need to use the script tag as shown above.

The button element is selected using document.querySelector() method, then the method addEventListener() is called on the element. This means you attach an event listener to the button.

First, you specify the type of event to listen to, which is a click event in this case. Next, you specify the function to run when that event happens.

In the code above, the newText function will be executed when the click event is triggered.

The event listener will also send an event object, which carries information about the event that was triggered. That’s why there’s an event parameter in the newText function above.

You can log the event to the console to see its details:

If you click on the button again, you will have the following output:

Event object log

Depending on what you want to do when an event is triggered, you may need to use the information contained inside the event object.

Here, all we want to do is to change the text of the paragraph, so the event object is not needed. We’ll see an example of using the event object later, when handling the keyboard events.

There are many events you can listen to in the browser. Here are some of the most common events you may need when developing a web application:

If you want to read the full list of DOM event types, you can visit this page .

The DOM Events are broken into several categories. Here we will just look at two of the most common events you might use in your project: keyboard and mouse events.

Keyboard Events

For the keyboard, you can track the keydown and keyup events, which run when you press and release a key, respectively.

To show you an example, run the following code from the console:

Once you run the code above, press a key on your keyboard slowly, then release it slowly.

You should see a log output as follows:

Keyboard events log

Notice how the 'keydown' log appears as soon as you press a key, and the 'keyup' log appears only when you release the key.

The keyboard events are usually attached to the document object instead of a specific element, because the whole website should be able to listen to that event.

Mouse Events

Aside from keyboard events, the DOM also provides a way to track any mouse events.

The most common mouse events that you can track are:

  • mousedown – the mouse button was pressed
  • mouseup – the mouse button was released
  • click – a click event
  • dblclick – a double click event
  • mousemove – when the mouse is moved over the element
  • contextmenu – when the context menu is opened, for example on a right mouse button click

Again, you can test these events by adding an event listener directly to the document object:

Run the code above, then click anywhere inside the browser. You should see the mousedown and mouseup events logged, respectively.

How to Remove Event Listeners

To remove an event listener attached to an element, you need to call the removeEventListener() method, passing the type of the event and the function you passed to the addEventListener() method as follows:

The above code is enough to remove the 'click' event listener from the button element. Notice how you need to call the removeEventListener() method on the element while also passing the function newText to the method.

To correctly remove an event listener, you need to have a reference to the function attached to the event. If you pass a nameless function to the addEventListener() method, then that event can’t be removed:

Without the function name as in the example above, you won’t be able to remove the event listener.

How to Listen to Events using HTML Attributes

Aside from using the addEventListener() method, you can also listen to events by adding the on[eventname] attribute to your HTML elements.

For example, suppose you want to listen to a button click. You can add the onclick attribute to your button as follows:

In the button element above, we add the onclick property and pass the handleClick() function to it.

When we click on the button, the handleClick() function will be executed.

You can also add the onclick attribute using JavaScript as follows:

Here, we assign a reference to the handleClick function to the onclick property using JavaScript.

To remove the onclick attribute, you can assign the property to null:

Which One Should You Use?

As you can see, there are two ways you can listen to DOM events: the addEventListener() method and the on[eventname] HTML attribute. Which one should you use?

The answer is that the addEventListener() method can be used when you need more extensibility, and the on[eventname] can be used when you prefer things to be simple.

When developing web applications, the .html file should only serve as the structure of the page, while the .js file should define any behavior the web application can have.

To make your application easier to maintain and extend, JavaScript should have access to HTML elements, but no HTML elements should be able to execute JavaScript functions. This is why addEventListener() should be the recommended method.

But addEventListener() doesn't come without a cost: you trade extensibility with verbosity, making your code quite cumbersome to read.

When using the on[eventname] attribute, you only need to specify the function name in your HTML element:

But when you use the addEventListener() method, you need to query the element you need, call the method, then specify the event and the callback function to run:

As you can see above, there are two additional lines that you don't need to write when you use the on[eventname] attribute.

While it might look insignificant, it will be a serious issue when you work on a large scale application with many HTML and JS files.

In addition, the addEventListener() method also allows you to attach multiple listeners to the same element as follows:

When you click on the button above, JavaScript will execute both event listeners.

This is not possible with the onclick property because you can only assign one function as a reference at a time:

But I never encountered a situation where I needed to listen to the same event twice, so this advantage might not be useful at all.

The DOM events exposed by the browser enable you to respond to user actions in the appropriate way.

This pattern of using event listeners to do specific tasks is known as event-driven programming, and you'll use this pattern a lot when developing a web application using JavaScript.

There are two ways you can listen to events: using the addEventListener() JavaScript method and the on[eventname] HTML attributes. Each has its advantages and disadvantages, so it's good to be familiar with both.

If you enjoyed this article and want to take your JavaScript skills to the next level, I recommend you check out my new book Beginning Modern JavaScript here .

beginning-js-cover

The book is designed to be easy for beginners and accessible to anyone looking to learn JavaScript. It provides a step-by-step gentle guide that will help you understand how to use JavaScript to create a dynamic web application.

Here's my promise: You will actually feel like you understand what you're doing with JavaScript.

Until next time!

JavaScript Full Stack Developer currently working with fullstack JS using React and Express. Nathan loves to write about his experience in programming to help other people.

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Js versions, js functions, js html dom, js browser bom, js web apis, js vs jquery, js graphics, js examples, js references, javascript html dom eventlistener, the addeventlistener() method.

Add an event listener that fires when a user clicks a button:

The addEventListener() method attaches an event handler to the specified element.

The addEventListener() method attaches an event handler to an element without overwriting existing event handlers.

You can add many event handlers to one element.

You can add many event handlers of the same type to one element, i.e two "click" events.

You can add event listeners to any DOM object not only HTML elements. i.e the window object.

The addEventListener() method makes it easier to control how the event reacts to bubbling.

When using the addEventListener() method, the JavaScript is separated from the HTML markup, for better readability and allows you to add event listeners even when you do not control the HTML markup.

You can easily remove an event listener by using the removeEventListener() method.

The first parameter is the type of the event (like " click " or " mousedown " or any other HTML DOM Event .)

The second parameter is the function we want to call when the event occurs.

The third parameter is a boolean value specifying whether to use event bubbling or event capturing. This parameter is optional.

Note that you don't use the "on" prefix for the event; use " click " instead of " onclick ".

Add an Event Handler to an Element

Alert "Hello World!" when the user clicks on an element:

You can also refer to an external "named" function:

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Add Many Event Handlers to the Same Element

The addEventListener() method allows you to add many events to the same element, without overwriting existing events:

You can add events of different types to the same element:

Add an Event Handler to the window Object

The addEventListener() method allows you to add event listeners on any HTML DOM object such as HTML elements, the HTML document, the window object, or other objects that support events, like the xmlHttpRequest object.

Add an event listener that fires when a user resizes the window:

Passing Parameters

When passing parameter values, use an "anonymous function" that calls the specified function with the parameters:

Event Bubbling or Event Capturing?

There are two ways of event propagation in the HTML DOM, bubbling and capturing.

Event propagation is a way of defining the element order when an event occurs. If you have a <p> element inside a <div> element, and the user clicks on the <p> element, which element's "click" event should be handled first?

In bubbling the inner most element's event is handled first and then the outer: the <p> element's click event is handled first, then the <div> element's click event.

In capturing the outer most element's event is handled first and then the inner: the <div> element's click event will be handled first, then the <p> element's click event.

With the addEventListener() method you can specify the propagation type by using the "useCapture" parameter:

The default value is false, which will use the bubbling propagation, when the value is set to true, the event uses the capturing propagation.

The removeEventListener() method

The removeEventListener() method removes event handlers that have been attached with the addEventListener() method:

HTML DOM Event Object Reference

For a list of all HTML DOM events, look at our complete HTML DOM Event Object Reference .

Test Yourself With Exercises

Use the eventListener to assign an onclick event to the <button> element.

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addOwnPropertyChangeListener(key, listener, beforeChange?)

Adds a listener for an owned property with the given name.

The listener itself can be a function or a “handler” object. The function receives the arguments, (value, key, object) , familiar from iterator callbacks. A handler object must implement a method that receives the same arguments. The dispatcher gives precedence to the most specific handler method name. If the name is Value , the preferred handler method name is handleValueChange , or handleValueWillChange if beforeChange is true. Otherwise, the dispatcher falls back to the generic handlePropertyChange or handlePropertyWillChange .

The formula for handler method names gives precedence to the most specific name implemented by the handler:

  • handle + key + Change or WillChange
  • handleProperty + Change or WillChange

To register a will-change listener, use addBeforeOwnPropertyChangeListener and avoid using the beforeChange boolean argument unless it does in fact depend on a variable with that name, for the sake of readability.

On collections

  • SortedArray
  • SortedArraySet
  • SortedArrayMap
  • PropertyChanges
  • addOwnPropertyChangeListener(key, listener)
  • addOwnPropertyChangeListener(key, listener, beforeChange)

Adds a listener for before a property changes.

Unregisters a property change listener provided by addOwnPropertyChangeListener .

Informs property change listeners that the value for a property name has changed.

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Interface PropertyChangeListener

  • All Superinterfaces: EventListener All Known Implementing Classes: BasicButtonListener , BasicColorChooserUI.PropertyHandler , BasicComboBoxUI.PropertyChangeHandler , BasicComboPopup.PropertyChangeHandler , BasicDirectoryModel , BasicInternalFrameTitlePane.PropertyChangeHandler , BasicInternalFrameUI.InternalFramePropertyChangeListener , BasicLabelUI , BasicListUI.PropertyChangeHandler , BasicOptionPaneUI.PropertyChangeHandler , BasicRootPaneUI , BasicScrollBarUI.PropertyChangeHandler , BasicScrollPaneUI.PropertyChangeHandler , BasicSliderUI.PropertyChangeHandler , BasicSplitPaneDivider , BasicSplitPaneUI.PropertyHandler , BasicTabbedPaneUI.PropertyChangeHandler , BasicToolBarUI.PropertyListener , BasicTreeUI.PropertyChangeHandler , BasicTreeUI.SelectionModelPropertyChangeHandler , BeanContextServicesSupport , BeanContextSupport , DefaultTableColumnModel , JLayer , JList.AccessibleJList , JPopupMenu.AccessibleJPopupMenu , JScrollPane.AccessibleJScrollPane , JSpinner.DateEditor , JSpinner.DefaultEditor , JSpinner.ListEditor , JSpinner.NumberEditor , JTable.AccessibleJTable , MetalComboBoxUI.MetalPropertyChangeListener , MetalFileChooserUI.FilterComboBoxModel , MetalLabelUI , MetalRootPaneUI , MetalSliderUI.MetalPropertyListener , MetalToolBarUI.MetalRolloverListener , ProgressMonitor.AccessibleProgressMonitor , PropertyChangeListenerProxy , SynthButtonUI , SynthCheckBoxMenuItemUI , SynthCheckBoxUI , SynthColorChooserUI , SynthComboBoxUI , SynthDesktopIconUI , SynthDesktopPaneUI , SynthInternalFrameUI , SynthLabelUI , SynthListUI , SynthMenuBarUI , SynthMenuItemUI , SynthMenuUI , SynthOptionPaneUI , SynthPanelUI , SynthPopupMenuUI , SynthProgressBarUI , SynthRadioButtonMenuItemUI , SynthRadioButtonUI , SynthRootPaneUI , SynthScrollBarUI , SynthScrollPaneUI , SynthSeparatorUI , SynthSliderUI , SynthSpinnerUI , SynthSplitPaneUI , SynthTabbedPaneUI , SynthTableHeaderUI , SynthTableUI , SynthToggleButtonUI , SynthToolBarUI , SynthToolTipUI , SynthTreeUI , SynthViewportUI public interface PropertyChangeListener extends EventListener A "PropertyChange" event gets fired whenever a bean changes a "bound" property. You can register a PropertyChangeListener with a source bean so as to be notified of any bound property updates.

Method Summary

Method detail, propertychange.

Submit a bug or feature For further API reference and developer documentation, see Java SE Documentation . That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples. Copyright © 1993, 2024, Oracle and/or its affiliates. All rights reserved. Use is subject to license terms . Also see the documentation redistribution policy .

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Trump ordered to pay over $355M for fraudulent business practices in New York

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Former U.S. President Donald Trump and his lawyers Christopher Kise and Alina Habba attend the closing arguments in the Trump Organization civil fraud trial on Jan. 11 in New York City. Shannon Stapleton/Getty Images hide caption

Former U.S. President Donald Trump and his lawyers Christopher Kise and Alina Habba attend the closing arguments in the Trump Organization civil fraud trial on Jan. 11 in New York City.

A New York judge has ordered former President Donald Trump and executives at the Trump Organization to pay over $364 million in a civil fraud case, handing a win to New York Attorney General Letitia James, who sued Trump and his associates after a three-year investigation.

The Friday decision from Judge Arthur Engoron orders Trump and his flagship organization to pay the bulk of that amount: almost $355 million. Trump's two sons and co-defendants, Eric Trump and Donald Trump Jr., are each liable for $4 million. Allen Weisselberg, a former Trump Organization executive, is liable for $1 million. The total is even higher with interest — more than $450 million overall, according to the attorney general's office.

"Their complete lack of contrition and remorse borders on pathological. They are accused only of inflating asset values to make more money. The documents prove this over and over again. This is a venial sin, not a mortal sin," Engoron wrote in the court filing. "Yet, defendants are incapable of admitting the error of their ways."

Trump himself called the decision a "Complete and Total SHAM" in an emailed statement and repeated his accusation that the justice system overall is politically biased against him.

James, however, declared that "justice has been served."

"This is a tremendous victory for this state, this nation, and for everyone who believes that we all must play by the same rules — even former presidents," the state attorney general said in a statement.

Additional consequences

The judge also decided to limit Trump and his co-defendants' ability to do business in the Empire State. Trump and his companies are prohibited from serving as an officer or director of any New York business or applying for loans for three years. His sons are limited from similar leadership roles for two years.

Jeffrey McConney, ex-controller of the Trump Organization and also a defendant, was not ordered to pay any amount, but he and Weisselberg are permanently barred from serving in the financial control function of any New York corporation or similar business entity registered or licensed in New York state.

"This Court is not constituted to judge morality; it is constituted to find facts and apply the law. In this particular case, in applying the law to the facts, the Court intends to protect the integrity of the financial marketplace and, thus, the public as a whole," Engoron wrote.

The ruling comes at a crucial time for Trump, the front-runner for the Republican presidential nomination. Engoron's decision comes a day after another judge set the date for what could be Trump's first criminal trial, related to hush money payments issued during the 2016 election .

Trump's New York hush money trial will start March 25

Trump's New York hush money trial will start March 25

He is facing a combined 91 state and federal charges, including several related to his role to stay in office after he lost the 2020 presidential election to Joe Biden. But the charges have done little to dent Trump's popularity among his base. Instead, the charges appear to have bolstered his credentials, potentially setting up a rematch with Biden.

The facts of the case

Trump and his two older sons are accused of knowingly committing fraud by submitting financial statements that inflated the value of their properties and other assets. The lawsuit alleges that from 2011 to 2021, Donald Trump and his organization created more than 200 false valuations to inflate his net worth by billions of dollars with the goal of getting better business, insurance and banking deals.

Engoron had already determined that there was fraud and that the former president, his sons and other executives were liable.

Throughout the trial, legal teams argued whether the value of notable Trump properties, such as Manhattan's Trump Tower and 40 Wall Street, were inflated deliberately.

Documents shown during trial ranged from spreadsheets to signed financial statements. In one example, the attorney general's legal team showed that Trump's triplex in his eponymously named Manhattan building was marked as being almost 11,000 square feet in 1994 and later as 30,000 square feet. A Forbes magazine article in 2017 originally shed light on the discrepancy.

The former president and three of his children, Donald Jr., Eric and Ivanka, who is not a defendant, all took the stand to testify about the valuation process and their involvement in the Trump Organization. Testifying in November , Trump argued that the estimated property values were actually conservative, and he said that he relied on others to compile the statements. His sons also testified that they relied on others , including their accounting firm, to come up with the numbers — even as emails and documents showed the Trumps ultimately approved them.

In closing briefs, Trump's team doubled down on the argument that the three members of the Trump family did not have knowledge or involvement in the creation, preparation or use of the fraudulent financial statements.

Closing arguments concludes in Trump civil fraud trial in New York

Closing arguments concludes in Trump civil fraud trial in New York

Trump says he won't testify as planned in his civil fraud trial

Trump says he won't testify as planned in his civil fraud trial

Who else testified.

Witnesses included former Trump allies such as Michael Cohen and Weisselberg , who was also a defendant.

Cohen testified that it was his responsibility, along with that of Weisselberg , "to reverse-engineer the very different asset classes, increase those assets in order to achieve the numbers" Trump had asked for.

Weisselberg, however, testified that he couldn't remember whether he discussed the financial statements with Trump as they were finalized.

The decision on Friday comes as Trump continues to campaign for the presidency. He will likely appeal this ruling, as he has in the other cases where he has suffered legal setbacks. It may take years before he parts with any money in the case.

New York judge brings back gag order on Donald Trump in civil fraud trial

New York judge brings back gag order on Donald Trump in civil fraud trial

  • trump corp.
  • letetia james
  • judge arthur engoron
  • Former President Trump
  • new york state

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Trump’s Harsh Punishment Was Made Possible by This New York Law

The little-known measure meant hundreds of millions in penalties in the civil fraud case brought by Attorney General Letitia James.

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Letitia James sits in court behind Donald Trump, who is blurred and out of focus.

By Ben Protess and Jonah E. Bromwich

The $355 million penalty that a New York judge ordered Donald J. Trump to pay in his civil fraud trial might seem steep in a case with no victim calling for redress and no star witness pointing the finger at Mr. Trump. But a little-known 70-year-old state law made the punishment possible.

The law, often referred to by its shorthand, 63(12), which stems from its place in New York’s rule book, is a regulatory bazooka for the state’s attorney general, Letitia James. Her office has used it to aim at a wide range of corporate giants: the oil company Exxon Mobil, the tobacco brand Juul and the pharma executive Martin Shkreli.

On Friday, the law enabled Ms. James to win an enormous victory against Mr. Trump. Along with the financial penalty , the judge barred Mr. Trump from running a business in New York for three years. His adult sons were barred for two years.

The judge also ordered a monitor, Barbara Jones, to assume more power over Mr. Trump’s company, and asked her to appoint an independent executive to report to her from within the company.

A lawyer for Mr. Trump, Christopher M. Kise, reacted with fury, saying “the sobering future consequences of this tyrannical abuse of power do not just impact President Trump.”

“When a court willingly allows a reckless government official to meddle in the lawful, private and profitable affairs of any citizen based on political bias, America’s economic prosperity and way of life are at extreme risk of extinction,” he said.

In the Trump case, Ms. James accused the former president of inflating his net worth to obtain favorable loans and other financial benefits. Mr. Trump, she argued, defrauded his lenders and in doing so, undermined the integrity of New York’s business world.

Mr. Trump’s conduct “distorts the market,” Kevin Wallace, a lawyer for Ms. James’s office, said during closing arguments in the civil fraud trial.

“It prices out honest borrowers and can lead to more catastrophic results,” Mr. Wallace said, adding, “That’s why it’s important for the court to take the steps to protect the marketplace to prevent this from happening again.”

Yet the victims — the bankers who lent to Mr. Trump — testified that they were thrilled to have him as a client. And while a parade of witnesses echoed Ms. James’s claim that the former president’s annual financial statements were works of fiction, none offered evidence showing that Mr. Trump explicitly intended to fool the banks.

That might seem unusual, but under 63(12), such evidence was not necessary to find fraud.

The law did not require the attorney general to show that Mr. Trump had intended to defraud anyone or that his actions resulted in financial loss.

“This law packs a wallop,” said Steven M. Cohen, a former federal prosecutor and top official in the attorney general’s office, noting that it did not require the attorney general to show that anyone had been harmed.

With that low bar, Justice Arthur F. Engoron, the judge presiding over the case, sided with Ms. James on her core claim before the trial began, finding that Mr. Trump had engaged in a pattern of fraud by exaggerating the value of his assets in statements filed to his lenders.

Ms. James’s burden of proof at the trial was higher: To persuade the judge that Mr. Trump had violated other state laws, she had to convince him that the former president acted with intent. And some of the evidence helped her cause: Two of Mr. Trump’s former employees testified that he had final sign-off on the financial statements, and Mr. Trump admitted on the witness stand that he had a role in drafting them.

Still, her ability to extract further punishments based on those other violations is also a product of 63(12), which grants the attorney general the right to pursue those who engage in “repeated fraudulent or illegal acts.”

In other fraud cases, authorities must persuade a judge or jury that someone was in fact defrauded. But 63(12) required Ms. James only to show that conduct was deceptive or created “an atmosphere conducive to fraud.” Past cases suggest that the word “fraud” itself is effectively a synonym for dishonest conduct, the attorney general argued in her lawsuit.

Once the attorney general has convinced a judge or jury that a defendant has acted deceptively, the punishment can be severe. The law allows Ms. James to seek the forfeit of money obtained through fraud.

Of the roughly $355 million that Mr. Trump was ordered to pay, $168 million represents the sum that Mr. Trump saved on loans by inflating his worth, she argued. In other words, the extra interest the lenders missed.

The penalty was in the judge’s hands — there was no jury — and 63(12) gave him wide discretion.

The law also empowered Justice Engoron to set new restrictions on Mr. Trump and his family business, all of which Mr. Trump is expected to appeal.

The judge also ordered a monitor to assume more power over Mr. Trump’s company, who will appoint an independent executive who will report to the monitor from within the company.

Even before she filed her lawsuit against the Trumps in 2022, Ms. James used 63(12) as a cudgel to aid her investigation.

The law grants the attorney general’s office something akin to prosecutorial investigative power. In most civil cases, a person or entity planning to sue cannot collect documents or conduct interviews until after the lawsuit is filed. But 63(12) allows the attorney general to do a substantive investigation before deciding whether to sue, settle or abandon a case. In the case against Mr. Trump, the investigation proceeded for nearly three years before a lawsuit was filed.

The case is not Mr. Trump’s first brush with 63(12). Ms. James’s predecessors used it in actions against Trump University, his for-profit education venture, which paid millions of dollars to resolve the case.

The law became so important to Ms. James’s civil fraud case that it caught the attention of Mr. Trump, who lamented the sweeping authority it afforded the attorney general and falsely claimed that her office rarely used it.

He wrote on social media last year that 63(12) was “VERY UNFAIR.”

William K. Rashbaum contributed reporting.

Ben Protess is an investigative reporter at The Times, writing about public corruption. He has been covering the various criminal investigations into former President Trump and his allies. More about Ben Protess

Jonah E. Bromwich covers criminal justice in New York, with a focus on the Manhattan district attorney's office, state criminal courts in Manhattan and New York City's jails. More about Jonah E. Bromwich

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  4. Exemple d'observateur de propriétés avec PropertyChangeListener en Java

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  5. Java Property Change Listener: How To Implement And Use

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COMMENTS

  1. Listener for property value changes in a Javascript object

    41 Going through Javascript documentation, I found the following two functions on a Javascript object looks interesting: .watch - Watches for a property to be assigned a value and runs a function when that occurs. .unwatch - Removes a watchpoint set with the watch method. UPDATE: Deprecation warning Do not use watch () and unwatch ()!

  2. HTMLElement: change event

    Syntax Use the event name in methods like addEventListener (), or set an event handler property. js addEventListener("change", (event) => {}); onchange = (event) => {}; Event type A generic Event. Examples <select> element HTML html <label> Choose an ice cream flavor:

  3. PropertyChanges

    The property change listener system installs property descriptors on arbitrary objects. Arrays are special. You cannot add a change listener on length, and it would be impractical to install change listeners on every index of an array.

  4. Watch for Object Changes with JavaScript

    Watching for changes to an object's property has always been a much sought after task; many shims have been used over the years to listen to object ... Sindre Sorhus has created on-change, a tiny JavaScript tool that allows you to listen for changes on JavaScript objects and arrays! on-change JavaScript. Since the on-change code is so tiny ...

  5. JavaScript change Event

    JavaScript change Event Summary: in this tutorial, you'll learn about the JavaScript change event of the input text, radio button, checkbox, and select elements. The change event occurs when the element has completed changing. To attach an event handler to the change event of an element, you can either call the addEventListener () method:

  6. EventTarget: addEventListener() method

    The addEventListener () method of the EventTarget interface sets up a function that will be called whenever the specified event is delivered to the target. Common targets are Element, or its children, Document, and Window , but the target may be any object that supports events (such as IDBRequest ).

  7. onchange Event

    Description. The onchange event occurs when the value of an HTML element is changed. Tip: This event is similar to the oninput event. The difference is that the oninput event occurs immediately after the value of an element has changed, while onchange occurs when the element loses focus, after the content has been changed.

  8. Watch an object for changes in Vanilla JavaScript

    Method 2: Object.prototype.watch () The watch function watches any property of an object for changes. This also includes the first assignment of value to the property. The watch function includes ...

  9. Listener for property value changes in a Javascript object

    Listener for property value changes in a Javascript object Asked 3 months ago Modified 3 months ago Viewed 100 times 0 The purpose of this code is add proxy on nested object for log changes on property. This is a debug utility! // check if value is object or array function isObject(value) {

  10. How to Listen for Changes to HTML Elements with the contenteditable

    And then we can add an event listener with the addEventListener with: const div = document.querySelector("div") div.addEventListener("input", (e) => { console.log(e); }, false); When we change the content in the div, we'll see the input event listener run. We can get the element's attributes and styles with the e.target property.

  11. Listen for a class change in JavaScript

    To listen to class change we need to instantiate the MutationObserver object, pass the callback function, and call the observe () method. The observe () method accepts two arguments the target node and options object which should contain attributes property that is set to true.

  12. Understanding DOM Events and JavaScript Event Listeners

    How to Listen to DOM Events. To listen for an event, you need to attach an event listener to an element by using the addEventListener () method. The addEventListener () method accepts two parameters: The event type to listen to. A function to run when the event is triggered. Element.addEventListener(type, function);

  13. JavaScript DOM EventListener

    Syntax element .addEventListener ( event, function, useCapture ); The first parameter is the type of the event (like " click " or " mousedown " or any other HTML DOM Event .) The second parameter is the function we want to call when the event occurs. The third parameter is a boolean value specifying whether to use event bubbling or event capturing.

  14. addOwnPropertyChangeListener (key, listener, beforeChange?)

    Adds a listener for an owned property with the given name. The listener itself can be a function or a "handler" object. The function receives the arguments, (value, key, object), familiar from iterator callbacks. A handler object must implement a method that receives the same arguments.

  15. jquery

    JavaScript doesn't use a function to set properties. They're just variables, and setting them doesn't require any elaborate wrappers. You could use a function to set the property, though — the same sort of a getter/setter arrangement you might use in a language that supported private data in classes.

  16. Codereview: Listener for property value changes in a Javascript object

    Listener for property value changes in a Javascript object Hey guys! Hopefully you found a solution that helped you! The Content is licensed under (https://m...

  17. How to Write a Property Change Listener

    You can register a property change listener in two ways. The first uses the method addPropertyChangeListener (PropertyChangeListener). When you register a listener this way, you are notified of every change to every bound property for that object.

  18. PropertyChangeListener (Java Platform SE 8 )

    Method Detail propertyChange void propertyChange ( PropertyChangeEvent evt) This method gets called when a bound property is changed. Parameters: evt - A PropertyChangeEvent object describing the event source and the property that has changed.

  19. javascript event listener for changes in an object variable

    Listener for property value changes in a Javascript object (8 answers) Closed 8 years ago. I have an object variable that has several properties. MyVar = {"prop1" : 0, "prop2": 0....}; How do I write an event listener that listens for a change in any of the properties. Thanks for your suggestions. javascript jquery Share Follow

  20. N.Y. judge orders Trump and executives to pay over $364 million in

    A New York judge has ordered former President Donald Trump and executives at the Trump Organization to pay over $364 million in a civil fraud case, handing a win to New York Attorney General ...

  21. javascript

    2,623 5 37 55 Add a comment 2 Answers Sorted by: 5 Mutation events like DOMAttrModified are deprecated. Consider using a MutationObserver instead. Example: <div>use devtools to change the <code>background-color</code> property of this node to <code>red</code></div> <p>status...</p> JS:

  22. Trump's Harsh Punishment Was Made Possible by This New York Law

    Yet the victims — the bankers who lent to Mr. Trump — testified that they were thrilled to have him as a client. And while a parade of witnesses echoed Ms. James's claim that the former ...