progress
16 March, 2023
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About
The ProgressEvent interface is used to handle events that represent the progress of an operation, such as loading a resource, uploading a file, or downloading a large amount of data. The ProgressEvent provides information about the progress of the operation, including the total size of the resource, the amount of data that has been transferred, and the current state of the operation.
The ProgressEvent interface is commonly used in applications to provide feedback to the user about the progress of long-running operations. For example, when uploading a large file, the user may want to know how much of the file has been uploaded and how long it will take to complete the upload. The ProgressEvent interface can be used to display a progress bar or other visual indicator to the user to provide feedback on the progress of the operation.
In addition to providing information about the progress of an operation, the ProgressEvent interface also allows developers to handle errors and abort events. If an error occurs during an operation, the error event is triggered and can be handled by the application. Similarly, if the operation is aborted, the abort event is triggered and can be handled by the application.
Event listener
Here's an example of using the ProgressEvent interface to display the progress of uploading a file using the XMLHttpRequest object:
// Get the file input element from the HTML document.
const fileInput = document.getElementById('file-input');
// Add an event listener to the file input element to listen for changes.
fileInput.addEventListener('change', () => {
// Create a new XMLHttpRequest object.
const xhr = new XMLHttpRequest();
// Set up the event listeners for the XMLHttpRequest object.
xhr.upload.addEventListener('progress', handleUploadProgress);
xhr.upload.addEventListener('error', handleUploadError);
xhr.upload.addEventListener('abort', handleUploadAbort);
// Open the XMLHttpRequest object and specify the URL to send the data to.
xhr.open('POST', '/upload');
// Send the file data as a FormData object.
const formData = new FormData();
formData.append('file', fileInput.files[0]);
xhr.send(formData);
});
// Define a function to handle the upload progress event.
function handleUploadProgress(event) {
if (event.lengthComputable) {
const percentComplete = Math.round((event.loaded / event.total) * 100);
console.log(`Upload progress: ${percentComplete}%`);
}
}
// Define a function to handle the upload error event.
function handleUploadError(event) {
console.error('Upload error');
}
// Define a function to handle the upload abort event.
function handleUploadAbort(event) {
console.warn('Upload aborted');
}Here, we add an event listener to the file input element to listen for changes. When a file is selected, we create a new XMLHttpRequest object and set up event listeners for the progress, error, and abort events. We then open the XMLHttpRequest object and send the file data as a FormData object.
When the progress event is fired, we call the handleUploadProgress function to handle the event. This function checks if the total size of the resource is computable, and if so, calculates the percentage of the resource that has been loaded and logs it to the console. We also define handleUploadError and handleUploadAbort functions to handle the error and abort events, respectively.
By using the ProgressEvent interface, we can provide feedback to the user about the progress of the file upload operation, including the percentage of the file that has been uploaded.
Property
Here's an example of using the onprogress property to handle progress events:
const image = document.createElement('img');
image.src = 'image.jpg';
image.onprogress = function(event) {
if (event.lengthComputable) {
const percentComplete = Math.round((event.loaded / event.total) * 100);
console.log(`Image loading progress: ${percentComplete}%`);
}
};
document.body.appendChild(image);Here, we create a new img element and set its src attribute to the URL of an image file. We then set the onprogress property of the img element to a function that handles the progress event.
In the event handler function, we check if the total size of the resource is computable using the lengthComputable property of the event object. If it is, we calculate the percentage of the resource that has been loaded and log it to the console.
Finally, we append the img element to the body of the HTML document to initiate the loading of the image. As the image loads, the onprogress event handler will be called periodically to update the progress of the operation.
Inline
Here's an example of using the onprogress attribute inline in HTML:
HTML
<img src="image.jpg" onprogress="handleImageProgress(event)" />JavaScript
function handleImageProgress(event) {
if (event.lengthComputable) {
const percentComplete = Math.round((event.loaded / event.total) * 100);
console.log(`Image loading progress: ${percentComplete}%`);
}
}Here, we use the img element with its src attribute set to the URL of an image file. We also add the onprogress attribute to the img element, which specifies the name of the function to handle the progress event.
The handleImageProgress function is defined in a <script> tag and takes the event object as its argument. We check if the total size of the resource is computable using the lengthComputable property of the event object. If it is, we calculate the percentage of the resource that has been loaded and log it to the console.
As the image loads, the onprogress event is fired periodically, and the handleImageProgress function is called to update the progress of the operation.
Note that using inline event handlers like this is generally not recommended for maintainability reasons, but it can be useful for quick prototyping or experimentation.
Programmatic trigger
The ProgressEvent and its related onprogress event cannot be triggered programmatically using the progress() method. The ProgressEvent is a type of event object that is created by the browser and dispatched when certain events, such as file uploads or downloads, are in progress.
The progress() method is used in a different context, specifically with the XMLHttpRequest object, to update the progress of a file upload or download operation. When an XMLHttpRequest object is used to transfer data, it emits a progress event periodically to indicate the amount of data that has been transferred and the total size of the resource being transferred.
You can use the XMLHttpRequest object to trigger the progress event programmatically by setting the onprogress property to a function that handles the event and then calling the send() method of the XMLHttpRequest object. However, note that the progress event is typically triggered automatically by the browser as the data is being transferred, so there is usually no need to trigger it manually.
In short, the ProgressEvent and its related onprogress event cannot be triggered programmatically using the progress() method, but the XMLHttpRequest object can be used to trigger the progress event programmatically when uploading or downloading files.
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