For a simple example of the onchange event hander here is an example that uses the document.querySelector method to get references to an input tag, as well as a paragraph tag. When the text of the text input element changes the event fires, and the value of the input element can be used to update the output that is set in the paragraph element.
When adding an event listener for the onchange event with the addEventListener element object method the first argument is the event that I want to attach for, in this case it is onchange. The second argument is a call back method that will fire each time this event occurs for the input element. The first argument that is suppled to this callback method is an event object that can be used to gain the value of the element from which the event fired via the target property of the event object which is a reference to the input element where this onchange event happened.
With that said in this event object I am using the target property of the event object as a way to gain a reference to the input element completely. In this example it is not a big deal to just use the inputPow global, however in a more complex project that involves a lot of elements it is often better to use the target property inside the body of a callback method as a way to gain access to the element from which an event like onchange as fired in a situation in which there might be many such elements.
There are other ways of going about attaching events, although I would go with add event listener for the most part. In addition when making a real interface the onchnage event is just one such event that comes to mind. So lets look at some more examples that make use of other ways of attaching, as well as additional events.
For input elements there is an onchange attribute that can be used to reference a single change event handler for the element. This attribute may not be depreciated, but generally most developers might prefer the use of addEventListener as it allows for the attachment of more than one hander for an element.
The one thing that comes to mind about this is that it would result in greater backward support for old browsers. However these days we are talking very out dated platforms that most people are just not using any more, at least that is the case when I look at my stats when it comes to that.
For a more advanced example of the onchange event, I made a quick little app that can be used to estimate the amount of money that a blog can make if the revenue per mille, and page views counts are known. In this example I am attaching the same event handler for more than one input element.
Here I have the code in my onchange.js file that makes use of the onchange event to update the text each time a change event fires. I have a state object that holds the current values for money, page views and rpm.
Here I am using Function.call to use the Array.forEach method as a way to loop over the children property of my controls div. This is necessary if I want to use an Array method like Array.forEach with the children property because it is not an Array but rather an HTMLCollection.
For this section I wanted to make a quick little project that is more of an actual project rather than a simple little example. Still it is not much of a project, but it is something a bit more advanced that takes into account more than one type of event on top of just the on change event.
Here I have made a function that when called will create a container and inject some input elements and a canvas element into the container. The container element will then be appended to the actual html when called. I have on change events attached for both the input elements, as well as handlers for when they both gain and loose focus. When focus is gained the canvas element will be shown, when they loose focus the canvas element will no longer be shown. The canvas element can also be used as a way to set the values for the input elements as well.
This example might be kind of still and pointless, but taking just a moment to use my imagination of course there are all kinds of things that comes to might that might prove to be more practical, or interesting. An input element is a way of letting a user set some kind of value, what is to be done with that value depends on the nature of the application.
So the onchange event is useful for setting one or more callbacks for an element that will fire when the value of the element changes. The onchange event can be used in conjunction with a wide range of other events such as on blur, and on focus to define a user interface that can be used to do things like gather information that will be posted back to a server, or update the position of something in a canvas element, or anything else that can be set or changed with an input element.