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A cross-platform dart:io that works on all platforms, including browsers.

You can simply replace dart:io imports with package:universal_io/io.dart.

Licensed under the Apache License 2.0. Some of the source code was derived from Dart SDK, which was obtained under the BSD-style license of Dart SDK. See LICENSE file for details.

APIs added on top of "dart:io"

Other features

The following features may be deprecated in the future versions (3.x) of the package:

  • HttpClient
    • HttpClient() factory is changed so that it returns BrowserHttpClient on browsers.
  • Platform
    • The package makes methods like Platform.isAndroid and Platform.isMacOS work in the browsers too.
  • InternetAddress
    • The package makes it works in the browsers too.
  • BytesBuilder
    • The package makes it works in the browsers too.

Similar packages

Getting started

1.Add dependency

In pubspec.yaml:

  universal_io: ^2.2.2

2.Use HTTP client

import 'package:universal_io/io.dart';

Future<void> main() async {
  // HttpClient can be used in browser too!
  HttpClient httpClient = newUniversalHttpClient(); // Recommended way of creating HttpClient.
  final request = await httpClient.getUrl(Uri.parse(""));
  final response = await request.close();

HTTP client behavior

HTTP client is implemented with XMLHttpRequest (XHR) API on browsers.

XHR causes the following differences with dart:io:

  • HTTP connection is created only after request.close() has been called.
  • Same-origin policy limitations. For making cross-origin requests, see documentation below.

Helpful error messages

When requests fail and assertions are enabled, error messages contains descriptions how to fix possible issues such as missing cross-origin headers.

The error messages look like the following:

XMLHttpRequest error.
HTTP method:             PUT
HTTP URL:      
Cross-origin:            true
browserCredentialsMode:  false
browserResponseType:     arraybuffer

(For security reasons, browsers do not explain XHR errors.)

Is the server down? Did the server have an internal error?

Enabling credentials mode would enable use of some HTTP headers in both the
request and the response. For example, credentials mode is required for
sending/receiving cookies. If you think you need to enable 'credentials mode',
do the following:

    final httpClientRequest = ...;
    if (httpClientRequest is BrowserHttpClientRequest) {
      httpClientRequest.browserCredentialsMode = true;

Did the server respond to a cross-origin "preflight" (OPTIONS) request?

Did the server send the following headers?
  * Access-Control-Allow-Origin:
    * You can also use wildcard ("*").
    * Always required for cross-origin requests!
  * Access-Control-Allow-Methods: PUT
    * You can also use wildcard ("*").

Sometimes when you do cross-origin requests in browsers, you want to use CORS "credentials mode". This can be achieved with the following pattern:

Future<void> main() async {
    final client = HttpClient();
    final request = client.getUrl(Url.parse('http://example/url'));

    // Enable credentials mode
    if (request is BrowserHttpClientRequest) {
      request.browserCredentialsMode = true;

    // Close request
    final response = await request.close();
    // ...

Streaming text responses

The underlying XMLHttpRequest (XHR) API supports response streaming only when responseType is "text".

This package automatically uses responseType "text" based on value of the HTTP request header "Accept". These media types are defined BrowserHttpClient.defaultTextMimes:

  • "text/*" (any text media type)
  • "application/grpc-web"
  • "application/grpc-web+proto"

If you want to disable streaming, use the following pattern:

Future<void> main() async {
    final client = newUniversalHttpClient();
    if (client is BrowserHttpClient) {
      client.textMimes = const {};
    // ...


Cross-platform implementation of "dart:io" that works on browsers too.