Flutter web app initialization

This page details the initialization process for Flutter web apps and how it can be customized.

The flutter build web command produces a script called flutter_bootstrap.js in the build output directory (build/web). This file contains the JavaScript code needed to initialize and run your Flutter app. You can use this script by placing an async-script tag for it in your index.html file in the web subdirectory of your Flutter app:

html

<html>
  <body>
    <script src="flutter_bootstrap.js" async></script>
  </body>
</html>

Alternatively, you can inline the entire contents of the flutter_bootstrap.js file by inserting the template token {{flutter_bootstrap_js}} in your index.html file:

html

<html>
  <body>
    <script>
      {{flutter_bootstrap_js}}
    </script>
  </body>
</html>

The {{flutter_bootstrap_js}} token is replaced with the contents of the flutter_bootstrap.js file when the index.html file is copied to the output directory (build/web) during the build step.

By default, flutter build web generates a flutter_bootstrap.js file that does a simple initialization of your Flutter app. However, in some scenarios, you might have a reason to customize this initialization process, such as:

Setting a custom Flutter configuration for your app.
Changing the settings for the Flutter service worker.
Writing custom JavaScript code to run at different stages of the startup process.

To write your own custom bootstrapping logic instead of using the default script produced by the build step, you can place a flutter_bootstrap.js file in the web subdirectory of your project, which is copied over and used instead of the default script produced by the build. This file is also templated, and you can insert several special tokens that the build step substitutes at build time when copying the flutter_bootstrap.js file to the output directory. The following table lists the tokens that the build step will substitute in either the flutter_bootstrap.js or index.html files:

Token	Replaced with
`{{flutter_js}}`	The JavaScript code that makes the `FlutterLoader` object available in the `_flutter.loader` global variable. (See the `_flutter.loader.load() API` section below for more details.)
`{{flutter_build_config}}`	A JavaScript statement that sets metadata produced by the build process which gives the `FlutterLoader` information needed to properly bootstrap your application.
`{{flutter_service_worker_version}}`	A unique number representing the build version of the service worker, which can be passed as part of the service worker configuration (see the "Common warning" info below).
`{{flutter_bootstrap_js}}`	As mentioned above, this inlines the contents of the `flutter_bootstrap.js` file directly into the `index.html` file. Note that this token can only be used in the `index.html` and not the `flutter_bootstrap.js` file itself.

Any custom flutter_bootstrap.js script needs to have three components in order to successfully start your Flutter app:

A {{flutter_js}} token, to make _flutter.loader available.
A {{flutter_build_config}} token, which provides information about the build to the FlutterLoader needed to start your app.
A call to _flutter.loader.load(), which actually starts the app.

The most basic flutter_bootstrap.js file would look something like this:

{{flutter_js}}
{{flutter_build_config}}

_flutter.loader.load();

The _flutter.loader.load() JavaScript API can be invoked with optional arguments to customize initialization behavior:

Name	Description	JS type
`config`	The Flutter configuration of your app.	`Object`
`onEntrypointLoaded`	The function called when the engine is ready to be initialized. Receives an `engineInitializer` object as its only parameter.	`Function`

The config argument is an object that can have the following optional fields:

Name	Description	Dart type
`assetBase`	The base URL of the `assets` directory of the app. Add this when Flutter loads from a different domain or subdirectory than the actual web app. You might need this when you embed Flutter web into another app, or when you deploy its assets to a CDN.	`String`
`canvasKitBaseUrl`	The base URL from where `canvaskit.wasm` is downloaded.	`String`
`canvasKitVariant`	The CanvasKit variant to download. Your options cover: 1. `auto` : Downloads the optimal variant for the browser. The option defaults to this value. 2. `full` : Downloads the full variant of CanvasKit that works in all browsers. 3. `chromium` : Downloads a smaller variant of CanvasKit that uses Chromium compatible APIs. *Warning* : Don't use the `chromium` option unless you plan on only using Chromium-based browsers.	`String`
`canvasKitForceCpuOnly`	When `true`, forces CPU-only rendering in CanvasKit (the engine won't use WebGL).	`bool`
`canvasKitMaximumSurfaces`	The maximum number of overlay surfaces that the CanvasKit renderer can use.	`double`
`debugShowSemanticNodes`	If `true`, Flutter visibly renders the semantics tree onscreen (for debugging).	`bool`
`entrypointBaseUrl`	The base URL of your Flutter app's entrypoint. Defaults to "/".	`String`
`hostElement`	HTML Element into which Flutter renders the app. When not set, Flutter web takes over the whole page.	`HtmlElement`
`renderer`	Specifies the web renderer for the current Flutter application, either `"canvaskit"` or `"skwasm"` .	`String`

The following example shows a custom flutter_bootstrap.js that allows the user to select a renderer by providing a renderer query parameter, e.g. ?renderer=skwasm, in the URL of their website:

{{flutter_js}}
{{flutter_build_config}}

const searchParams = new URLSearchParams(window.location.search);
const renderer = searchParams.get('renderer');
const userConfig = renderer ? {'renderer': renderer} : {};
_flutter.loader.load({
  config: userConfig,
});

This script evaluates the URLSearchParams of the page to determine whether the user passed a renderer query parameter and then changes the user configuration of the Flutter app.

You can also pass an onEntrypointLoaded callback into the load API in order to perform custom logic at different parts of the initialization process. The initialization process is split into the following stages:

Loading the entrypoint script: The load function calls the onEntrypointLoaded callback once the Service Worker is initialized, and the main.dart.js entrypoint has been downloaded and run by the browser. Flutter also calls onEntrypointLoaded on every hot restart during development.
Initializing the Flutter engine: The onEntrypointLoaded callback receives an engine initializer object as its only parameter. Use the engine initializer initializeEngine() function to set the run-time configuration, like multiViewEnabled: true, and start the Flutter web engine.
Running the app: The initializeEngine() function returns a Promise that resolves with an app runner object. The app runner has a single method, runApp(), that runs the Flutter app.
Adding views to (or removing views from) an app: The runApp() method returns a flutter app object. In multi-view mode, the addView and removeView methods can be used to manage app views from the host app. To learn more, check out Embedded mode.

To give the user of your application feedback during the initialization process, use the hooks provided for each stage to update the DOM:

{{flutter_js}}
{{flutter_build_config}}

const loading = document.createElement('div');
document.body.appendChild(loading);
loading.textContent = "Loading Entrypoint...";
_flutter.loader.load({
  onEntrypointLoaded: async function(engineInitializer) {
    loading.textContent = "Initializing engine...";
    const appRunner = await engineInitializer.initializeEngine();

    loading.textContent = "Running app...";
    await appRunner.runApp();
  }
});

If you experience a warning similar to the following:

Warning: In index.html:37: Local variable for "serviceWorkerVersion" is deprecated.
Use "" template token instead.

You can fix this by deleting the following line in the web/index.html file:

web/index.html

html

var serviceWorkerVersion = null;

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Unless stated otherwise, the documentation on this site reflects Flutter 3.35.5. Page last updated on 2025-8-13. View source or report an issue.

Flutter web app initialization

Bootstrapping

Customize initialization

Write a custom bootstrap script

Customize the Flutter loader

Example: Customizing Flutter configuration based on URL query parameters

The onEntrypointLoaded callback

Example: Display a progress indicator

Common warning