Flutter web Save as File

In flutter web, I created app.js file and used that in the flutter project.
When I release the project and publish it in the IIS javascript function not work.
It's working when I run the project debug or flutter run -d chrome --release.
https://developer.mozilla.org/en-US/docs/Web/API/window/showSaveFilePicker

index.html

<!DOCTYPE html>
<html>
<head>
 ...
 ...
<script src="app.js" defer></script>

<script>
// The value below is injected by flutter build, do not touch.
var serviceWorkerVersion = null;
</script>
<!-- This script adds the flutter initialization JS code -->
<script src="flutter.js" defer></script>
</head>
<body>
...
...
</body>
</html>

app.js

async function saveFile(blob, fileName) {
const opts = {
    suggestedName: fileName,
    types: [{
        description: 'Text file',
        accept: { 'text/plain': ['.txt'] },
    }],
};

// create a new handle
const newHandle = await window.showSaveFilePicker(opts);

// create a FileSystemWritableFileStream to write to
const writableStream = await newHandle.createWritable();

// write our file
await writableStream.write(blob);

// close the file and write the contents to disk.
await writableStream.close();
}

My Dart Function file_saver.dart

class FileSaver {
 Future saveAs2(List<String> finalLines, String fileName) async {
   await js.context.callMethod(
     'saveFile',
      [
        html.Blob(
        finalLines,
        'text/plain',
        ),
      fileName
      ],
   );
  }
}

Error :

app.js:19 Uncaught (in promise) TypeError: window.showSaveFilePicker is not a function
at saveFile (app.js:19:36)
at lz.la (main.dart.js:41242:19)
at main.dart.js:41902:19
at au6.a (main.dart.js:5401:62)
at au6.$2 (main.dart.js:37029:14)
at Object.H (main.dart.js:5387:10)
at a2R.TA (main.dart.js:41904:10)
at a2R.u9 (main.dart.js:41898:21)
at main.dart.js:42628:13
at au6.a (main.dart.js:5401:62

Solution

According to the docs, you must run your app on HTTPS hosts to use this function:

Is your hosting supports HTTPS?

Do you check your URL and are you sure it starts with https?

And do you run your app on a browser that supports showSaveFilePicker function?