JavaScript onload vs. onchange

In JavaScript, onload and onchange are event handlers used to listen for any specific action taking place and to respond to that event with the appropriate action. These event handlers are unique from one another and are used to accomplish different tasks.

The onload event handler

The onload event handler listens for the events associated with the document or the window object. It’s triggered when a web page has finished loading all its assets, including the style sheets, scripts, images, etc. It’s often used to execute some code when the web page has finished loading. For example, it could be used to create dynamic animations when the page has finished loading.

Note: The onload event handler works on other DOM elements, such as an image or a div, as well.

We can test out this functionality in the widget below:

From lines 8-13, we try to simulate a delayed page load by using the setTimeout function to wait for 3000 ms or 3 seconds, and after that time passes, we simply change the display property for the main <div> to block, and we also print the message to the console to know the timeout is finished. Finally, in line 15, we use the onload event handler to simulate this delayed load effect by calling the function delayedLoad() when the page finishes loading.

The onchange event handler

The onchange is an event handler that’s used with forms and elements such as <input>, <select>, and <textarea>. It listens for any changes in the state of these elements, such as when a user types in an input field, its state changes, and whenever the focus changes from the element (e.g., by clicking outside the textbox), it executes some code. This is often used to validate user input dynamically, or it could be used to interact with other parts of the web page as well.

If we run the above piece of code, type something into the text input field, and click anywhere on the screen to unfocus from the input field, it will display the current value in the input field on the console. This is how the onchange event handler works. In the code given above, we first select the input field by using its ID and then call a function detectChange() that displays the current value of the selected element to the console on lines 6–9. Finally, in line 11, we use the onchange event handler, which takes a function and performs the actions in that function whenever a change is detected in the element.

Use cases

We can see that both onload and onchange are different types of event handlers in JavaScript with specific functionalities that can be used to accomplish various tasks depending on what we need. Here are a few use cases for onload:

• It can be used to execute code to create animations when an image has finished loading.

• It can be used to ensure that a style sheet or a script has been loaded fully and executed before we move on with any further operations. This helps improve the performance of our web page

• We can use it to detect when an audio or video file has loaded fully and is ready to be played.

Below are a few use cases for the onchange event:

• It’s used to validate the form inputs in real time as a user fills out a form. For example, you can check if the email entered by a user is a valid email or not.

• It’s used to make dynamic forms where user input in one field can change the options in other elements, such as dropdown.

• It’s used to filter data in real time, for example, when the user types something in a search bar, we can use it to trigger a filtering function.

To conclude, in JavaScript the onload and onchange events serve different roles in web development. The onload event is used to trigger actions when external resources or the entire web page has finished loading, making it ideal for initializing scripts and handling external resourceData fetched from a remote server. dependencies. Meanwhile, the onchange event is tailored for form elements, allowing developers to respond to user interactions and alterations in input field values in real time, thereby enhancing user experience. These events are essential tools in creating interactive and responsive web applications, contributing to a more engaging and functional web environment.