Flutter Web Limitations

a list of Flutter web limitations.

DateTime precision is limited to milliseconds:

Be careful when relying on microseconds precision, according to the Dart team:

The web Date object doesn't support microseconds. It's implemented using the JavaScript Date object which only supports millisecond precision. So, working as well as possible.

main() {
 final datetimeParsed = DateTime.parse('2021-02-05T22:37:12.933232+00:00');
 final datetimeMS = DateTime.fromMicrosecondsSinceEpoch(1612564632933232);
  
 print(datetimeParsed.toIso8601String()); // prints 2021-02-05T22:37:12.933Z
 print(datetimeMS.toUtc().toIso8601String()); // prints 2021-02-05T22:37:12.933Z
  
 print(datetimeParsed.microsecondsSinceEpoch); // prints 1612564632933000
 print(datetimeMS.microsecondsSinceEpoch); // prints 1612564632933000 
}

see dartlang/issues/44876

Dart SDK libraries not available in Flutter:

• dart:io which includes Platform to detect current platform and File for transferring bytes.

see flutter/flutter/39998

Flutter Platform Detection

In Flutter web, you cannot use Platform from dart:io (e.g. Platform.iOS or Platform.Android) as mentioned above. In order to detect the platform, you need to import the foundation package such as:

import 'package:flutter/foundation.dart';

bool get isOS => defaultTargetPlatform == TargetPlatform.iOS;
bool get isAndroid => defaultTargetPlatform == TargetPlatform.android;
bool get isMacOS => defaultTargetPlatform == TargetPlatform.macOS;
bool get isWindows => defaultTargetPlatform == TargetPlatform.windows;
bool get isLinux => defaultTargetPlatform == TargetPlatform.linux;

// Extra: A handy way to detect to know if the client is on Web using Mobile:
bool get isWebMobile => kIsWeb &&
    (defaultTargetPlatform == TargetPlatform.iOS ||
        defaultTargetPlatform == TargetPlatform.android);

Flutter Web is not SEO friendly

Flutter Web is not SEO friendly see flutter/issues/46789 making it hard, if even possible, to motetize with ADS see flutter/issues/40376

Flutter Web JsonEncode behavior

In Web, using jsonEncode will convert double values to int when the fractional part is equal to zero as shown below:

import 'dart:convert';	
void main() {	
  final double x = 3.0;	
  final double y = 3.1;	
  final jsonString = jsonEncode({'x': x, 'y': y});	
  	
  print(jsonString); // prints {"x":3,"y":3.1}	
   	
  // Decoding jsonString above works fine in web. 	
  // Though decoding jsonString in iOS/Android will throw an error	
  // when the expected value is double	
  // e.g.	
  //    `DataClass.fromJson(jsonString);` 	
  //    where DataClass has x & y defined as doubles 	
}	

For more details, see dart-lang/issues/45160

Flutter Web does not support "Hot Reload"

Flutter web supports Hot Restart, but not Hot Reload, see issue flutter/issues/53041

Flutter Web does not support "dart-grpc"

Flutter Web does not support and seemingly doesn't plan to support dart-grpc flutter/issues/48054 this package doesn't figure in the List of dart sdk libraries not available in Flutter Web

GIFs have some issues in PWA (mobile):

GIFs in mobile browsers for Flutter Web have some ongoing issues (e.g., not playing at all or stops playing on scrolling and never play again, etc). see flutter/issues/78223