Adding an example to ProgressEvent constructor (mdn#31334)

* adding-a-event-progress-constructor-example

* Apply suggestions from code review

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* Update index.md

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Joshua Chen <[email protected]>

files/en-us/web/api/progressevent/progressevent/index.md

@@ -44,6 +44,28 @@ new ProgressEvent(type, options)
 
 A new {{domxref("ProgressEvent")}} object.
 
+## Example
+
+The example demonstrates how a `ProgressEvent` is built using a constructor. This is particularly useful for tracking the progress of processes like file uploads, downloads, or any long-running tasks.
+
+```js
+function updateProgress(loaded, total) {
+  const progressEvent = new ProgressEvent("progress", {
+    lengthComputable: true,
+    loaded: loaded,
+    total: total,
+  });
+
+  document.dispatchEvent(progressEvent);
+}
+
+document.addEventListener("progress", (event) => {
+  console.log(`Progress: ${event.loaded}/${event.total}`);
+});
+
+updateProgress(50, 100);
+```
+
 ## Specifications
 
 {{Specifications}}