ASP.NET Core Blazor JavaScript interoperability (JS interop)

This article explains general concepts on how to interact with JavaScript in Blazor apps.

A Blazor app can invoke JavaScript (JS) functions from .NET methods and .NET methods from JS functions. These scenarios are called JavaScript interoperability (JS interop).

Further JS interop guidance is provided in the following articles:

• Call JavaScript functions from .NET methods in ASP.NET Core Blazor
• Call .NET methods from JavaScript functions in ASP.NET Core Blazor

Note

JavaScript [JSImport]/[JSExport] interop API is available for client-side components in ASP.NET Core 7.0 or later.

For more information, see JavaScript JSImport/JSExport interop with ASP.NET Core Blazor.

Throughout this article, the terms client/client-side and server/server-side are used to distinguish locations where app code executes:

• Client/client-side
  • Client-side rendering (CSR) of a Blazor Web App. The Program file is Program.cs of the client project (.Client). Blazor script start configuration is found in the App component (Components/App.razor) of the server project. Routable Interactive WebAssembly and Interactive Auto render mode components with an @page directive are placed in the client project's Pages folder. Place non-routable shared components at the root of the .Client project or in custom folders based on component functionality.
  • A Blazor WebAssembly app. The Program file is Program.cs. Blazor script start configuration is found in the wwwroot/index.html file.
• Server/server-side: Interactive server-side rendering (interactive SSR) of a Blazor Web App. The Program file is Program.cs of the server project. Blazor script start configuration is found in the App component (Components/App.razor). Only routable Interactive Server render mode components with an @page directive are placed in the Components/Pages folder. Non-routable shared components are placed in the server project's Components folder. Create custom folders based on component functionality as needed.

JavaScript interop abstractions and features package

The @microsoft/dotnet-js-interop package (npmjs.com) provides abstractions and features for interop between .NET and JavaScript (JS) code. Reference source is available in the dotnet/aspnetcore GitHub repository (/src/JSInterop folder). For more information, see the GitHub repository's README.md file.

Note

Documentation links to .NET reference source usually load the repository's default branch, which represents the current development for the next release of .NET. To select a tag for a specific release, use the Switch branches or tags dropdown list. For more information, see How to select a version tag of ASP.NET Core source code (dotnet/AspNetCore.Docs #26205).

Additional resources for writing JS interop scripts in TypeScript:

• TypeScript
• Tutorial: Create an ASP.NET Core app with TypeScript in Visual Studio
• Manage npm packages in Visual Studio

Interaction with the DOM

Only mutate the DOM with JavaScript (JS) when the object doesn't interact with Blazor. Blazor maintains representations of the DOM and interacts directly with DOM objects. If an element rendered by Blazor is modified externally using JS directly or via JS Interop, the DOM may no longer match Blazor's internal representation, which can result in undefined behavior. Undefined behavior may merely interfere with the presentation of elements or their functions but may also introduce security risks to the app or server.

This guidance not only applies to your own JS interop code but also to any JS libraries that the app uses, including anything provided by a third-party framework, such as Bootstrap JS and jQuery.

In a few documentation examples, JS interop is used to mutate an element purely for demonstration purposes as part of an example. In those cases, a warning appears in the text.

For more information, see Call JavaScript functions from .NET methods in ASP.NET Core Blazor.

Asynchronous JavaScript calls

JS interop calls are asynchronous by default, regardless of whether the called code is synchronous or asynchronous. Calls are asynchronous by default to ensure that components are compatible across server-side and client-side rendering models. When adopting server-side rendering, JS interop calls must be asynchronous because they're sent over a network connection. For apps that exclusively adopt client-side rendering, synchronous JS interop calls are supported.

For more information, see the following articles:

• Call JavaScript functions from .NET methods in ASP.NET Core Blazor
• Call .NET methods from JavaScript functions in ASP.NET Core Blazor

Object serialization

Blazor uses System.Text.Json for serialization with the following requirements and default behaviors:

• Types must have a default constructor, get/set accessors must be public, and fields are never serialized.
• Global default serialization isn't customizable to avoid breaking existing component libraries, impacts on performance and security, and reductions in reliability.
• Serializing .NET member names results in lowercase JSON key names.
• JSON is deserialized as JsonElement C# instances, which permit mixed casing. Internal casting for assignment to C# model properties works as expected in spite of any case differences between JSON key names and C# property names.

JsonConverter API is available for custom serialization. Properties can be annotated with a [JsonConverter] attribute to override default serialization for an existing data type.

For more information, see the following resources in the .NET documentation:

• JSON serialization and deserialization (marshalling and unmarshalling) in .NET
• How to customize property names and values with System.Text.Json
• How to write custom converters for JSON serialization (marshalling) in .NET

Blazor supports optimized byte array JS interop that avoids encoding/decoding byte arrays into Base64. The app can apply custom serialization and pass the resulting bytes. For more information, see Call JavaScript functions from .NET methods in ASP.NET Core Blazor.

DOM cleanup tasks during component disposal

Don't execute JS interop code for DOM cleanup tasks during component disposal. Instead, use the MutationObserver pattern in JavaScript (JS) on the client for the following reasons:

• The component may have been removed from the DOM by the time your cleanup code executes in Dispose{Async}.
• During server-side rendering, the Blazor renderer may have been disposed by the framework by the time your cleanup code executes in Dispose{Async}.

The MutationObserver pattern allows you to run a function when an element is removed from the DOM.

In the following example, the DOMCleanup component:

• Contains a <div> with an id of cleanupDiv. The <div> element is removed from the DOM along with the rest of the component's DOM markup when the component is removed from the DOM.
• Loads the DOMCleanup JS class from the DOMCleanup.razor.js file and calls its createObserver function to set up the MutationObserver callback. These tasks are accomplished in the OnAfterRenderAsync lifecycle method.

DOMCleanup.razor:

@page "/dom-cleanup"
@rendermode InteractiveServer
@implements IAsyncDisposable
@inject IJSRuntime JS

<h1>DOM Cleanup Example</h1>

<div id="cleanupDiv"></div>

@code {
    private IJSObjectReference? jsModule;

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            jsModule = await JS.InvokeAsync<IJSObjectReference>(
                "import", "./Components/Pages/DOMCleanup.razor.js");

            await jsModule.InvokeVoidAsync("DOMCleanup.createObserver");
        }
    }

    async ValueTask IAsyncDisposable.DisposeAsync()
    {
        if (jsModule is not null)
        {
            await jsModule.DisposeAsync();
        }
    }
}

In the following example, the MutationObserver callback is executed each time a DOM change occurs. Execute your cleanup code when the if statement confirms that the target element (cleanupDiv) was removed (if (targetRemoved) { ... }). It's important to disconnect and delete the MutationObserver to avoid a memory leak after your cleanup code executes.

DOMCleanup.razor.js placed side-by-side with the preceding DOMCleanup component:

export class DOMCleanup {
  static observer;

  static createObserver() {
    const target = document.querySelector('#cleanupDiv');

    this.observer = new MutationObserver(function (mutations) {
      const targetRemoved = mutations.some(function (mutation) {
        const nodes = Array.from(mutation.removedNodes);
        return nodes.indexOf(target) !== -1;
      });

      if (targetRemoved) {
        // Cleanup resources here
        // ...

        // Disconnect and delete MutationObserver
        this.observer && this.observer.disconnect();
        delete this.observer;
      }
    });

    this.observer.observe(target.parentNode, { childList: true });
  }
}

window.DOMCleanup = DOMCleanup;

JavaScript interop calls without a circuit

This section only applies to server-side apps.

JavaScript (JS) interop calls can't be issued after a SignalR circuit is disconnected. Without a circuit during component disposal or at any other time that a circuit doesn't exist, the following method calls fail and log a message that the circuit is disconnected as a JSDisconnectedException:

• JS interop method calls
  • IJSRuntime.InvokeAsync
  • JSRuntimeExtensions.InvokeAsync
  • JSRuntimeExtensions.InvokeVoidAsync)
• Dispose/DisposeAsync calls on any IJSObjectReference.

In order to avoid logging JSDisconnectedException or to log custom information, catch the exception in a try-catch statement.

For the following component disposal example:

• The component implements IAsyncDisposable.
• objInstance is an IJSObjectReference.
• JSDisconnectedException is caught and not logged.
• Optionally, you can log custom information in the catch statement at whatever log level you prefer. The following example doesn't log custom information because it assumes the developer doesn't care about when or where circuits are disconnected during component disposal.

async ValueTask IAsyncDisposable.DisposeAsync()
{
    try
    {
        if (objInstance is not null)
        {
            await objInstance.DisposeAsync();
        }
    }
    catch (JSDisconnectedException)
    {
    }
}

If you must clean up your own JS objects or execute other JS code on the client after a circuit is lost, use the MutationObserver pattern in JS on the client. The MutationObserver pattern allows you to run a function when an element is removed from the DOM.

For more information, see the following articles:

• Handle errors in ASP.NET Core Blazor apps: The JavaScript interop section discusses error handling in JS interop scenarios.
• ASP.NET Core Razor component lifecycle: The Component disposal with IDisposable and IAsyncDisposable section describes how to implement disposal patterns in Razor components.

JavaScript location

Load JavaScript (JS) code using any of the following approaches:

• Load a script in <head> markup (Not generally recommended)
• Load a script in <body> markup
• Load a script from an external JavaScript file (.js) collocated with a component
• Load a script from an external JavaScript file (.js)
• Inject a script before or after Blazor starts

Warning

Don't place a <script> tag in a Razor component file (.razor) because the <script> tag can't be updated dynamically by Blazor.

Note

Documentation examples usually place scripts in a <script> tag or load global scripts from external files. These approaches pollute the client with global functions. For production apps, we recommend placing JavaScript into separate JavaScript modules that can be imported when needed. For more information, see the JavaScript isolation in JavaScript modules section.

Load a script in <head> markup

The approach in this section isn't generally recommended.

Place the JavaScript (JS) tags (<script>...</script>) in the <head> element markup:

<head>
    ...

    <script>
      window.jsMethod = (methodParameter) => {
        ...
      };
    </script>
</head>

Loading JS from the <head> isn't the best approach for the following reasons:

• JS interop may fail if the script depends on Blazor. We recommend loading scripts using one of the other approaches, not via the <head> markup.
• The page may become interactive slower due to the time it takes to parse the JS in the script.

Load a script in <body> markup

Place the JavaScript (JS) tags (<script>...</script>) inside the closing </body> element after the Blazor script reference:

<body>
    ...

    <script src="{BLAZOR SCRIPT}"></script>
    <script>
      window.jsMethod = (methodParameter) => {
        ...
      };
    </script>
</body>

In the preceding example, the {BLAZOR SCRIPT} placeholder is the Blazor script path and file name. For the location of the script, see ASP.NET Core Blazor project structure.

Load a script from an external JavaScript file (.js) collocated with a component

Collocation of JavaScript (JS) files for pages, views, and Razor components is a convenient way to organize scripts in an app.

Collocate JS files using the following filename extension conventions:

• Pages of Razor Pages apps and views of MVC apps: .cshtml.js. Examples:
  • Pages/Index.cshtml.js for the Index page of a Razor Pages app at Pages/Index.cshtml.
  • Views/Home/Index.cshtml.js for the Index view of an MVC app at Views/Home/Index.cshtml.
• Razor components of Blazor apps: .razor.js. Example: Index.razor.js for the Index component.

Collocated JS files are publicly addressable using the path to the file in the project:

• Pages, views, and components from a collocated scripts file in the app:

  {PATH}/{PAGE, VIEW, OR COMPONENT}.{EXTENSION}.js

  • The {PATH} placeholder is the path to the page, view, or component.
  • The {PAGE, VIEW, OR COMPONENT} placeholder is the page, view, or component.
  • The {EXTENSION} placeholder matches the extension of the page, view, or component, either razor or cshtml.

  Razor Pages example:

  A JS file for the Index page is placed in the Pages folder (Pages/Index.cshtml.js) next to the Index page (Pages/Index.cshtml). In the Index page, the script is referenced at the path in the Pages folder:

  @section Scripts {
    <script src="~/Pages/Index.cshtml.js"></script>
  }
  

  When the app is published, the framework automatically moves the script to the web root. In the preceding example, the script is moved to bin\Release\{TARGET FRAMEWORK MONIKER}\publish\wwwroot\Pages\Index.cshtml.js, where the {TARGET FRAMEWORK MONIKER} placeholder is the Target Framework Moniker (TFM). No change is required to the script's relative URL in the Index page.

  Blazor example:

  A JS file for the Index component is placed next to the Index component (Index.razor). In the Index component, the script is referenced at its path.

  Index.razor.js:

  export function showPrompt(message) {
    return prompt(message, 'Type anything here');
  }
  

  In the OnAfterRenderAsync method of the Index component (Index.razor):

  module = await JS.InvokeAsync<IJSObjectReference>(
      "import", "./Components/Pages/Index.razor.js");
  

  When the app is published, the framework automatically moves the script to the web root. In the preceding example, the script is moved to bin\Release\{TARGET FRAMEWORK MONIKER}\publish\wwwroot\Components\Pages\Index.razor.js, where the {TARGET FRAMEWORK MONIKER} placeholder is the Target Framework Moniker (TFM). No change is required to the script's relative URL in the Index component.

• For scripts provided by a Razor class library (RCL):

  _content/{PACKAGE ID}/{PATH}/{PAGE, VIEW, OR COMPONENT}.{EXTENSION}.js

  • The {PACKAGE ID} placeholder is the RCL's package identifier (or library name for a class library referenced by the app).
  • The {PATH} placeholder is the path to the page, view, or component. If a Razor component is located at the root of the RCL, the path segment isn't included.
  • The {PAGE, VIEW, OR COMPONENT} placeholder is the page, view, or component.
  • The {EXTENSION} placeholder matches the extension of page, view, or component, either razor or cshtml.

  In the following Blazor app example:

  • The RCL's package identifier is AppJS.
  • A module's scripts are loaded for the Index component (Index.razor).
  • The Index component is in the Pages folder of the Components folder of the RCL.

  var module = await JS.InvokeAsync<IJSObjectReference>("import", 
      "./_content/AppJS/Components/Pages/Index.razor.js");
  

For more information on RCLs, see Consume ASP.NET Core Razor components from a Razor class library (RCL).

Load a script from an external JavaScript file (.js)

Place the JavaScript (JS) tags (<script>...</script>) with a script source (src) path inside the closing </body> element after the Blazor script reference:

<body>
    ...

    <script src="{BLAZOR SCRIPT}"></script>
    <script src="{SCRIPT PATH AND FILE NAME (.js)}"></script>
</body>

In the preceding example:

• The {BLAZOR SCRIPT} placeholder is the Blazor script path and file name. For the location of the script, see ASP.NET Core Blazor project structure.
• The {SCRIPT PATH AND FILE NAME (.js)} placeholder is the path and script file name under wwwroot.

In the following example of the preceding <script> tag, the scripts.js file is in the wwwroot/js folder of the app:

<script src="js/scripts.js"></script>

You can also serve scripts directly from the wwwroot folder if you prefer not to keep all of your scripts in a separate folder under wwwroot:

<script src="scripts.js"></script>

When the external JS file is supplied by a Razor class library, specify the JS file using its stable static web asset path: ./_content/{PACKAGE ID}/{SCRIPT PATH AND FILE NAME (.js)}:

• The path segment for the current directory (./) is required in order to create the correct static asset path to the JS file.
• The {PACKAGE ID} placeholder is the library's package ID. The package ID defaults to the project's assembly name if <PackageId> isn't specified in the project file.
• The {SCRIPT PATH AND FILE NAME (.js)} placeholder is the path and file name under wwwroot.

<body>
    ...

    <script src="{BLAZOR SCRIPT}"></script>
    <script src="./_content/{PACKAGE ID}/{SCRIPT PATH AND FILE NAME (.js)}"></script>
</body>

In the following example of the preceding <script> tag:

• The Razor class library has an assembly name of ComponentLibrary, and a <PackageId> isn't specified in the library's project file.
• The scripts.js file is in the class library's wwwroot folder.

<script src="./_content/ComponentLibrary/scripts.js"></script>

For more information, see Consume ASP.NET Core Razor components from a Razor class library (RCL).

Inject a script before or after Blazor starts

To ensure scripts load before or after Blazor starts, use a JavaScript initializer. For more information and examples, see ASP.NET Core Blazor startup.

JavaScript isolation in JavaScript modules

Blazor enables JavaScript (JS) isolation in standard JavaScript modules (ECMAScript specification).

JS isolation provides the following benefits:

• Imported JS no longer pollutes the global namespace.
• Consumers of a library and components aren't required to import the related JS.

For more information, see Call JavaScript functions from .NET methods in ASP.NET Core Blazor.

Dynamic import with the import() operator is supported with ASP.NET Core and Blazor:

if ({CONDITION}) import("/additionalModule.js");

In the preceding example, the {CONDITION} placeholder represents a conditional check to determine if the module should be loaded.

For browser compatibility, see Can I use: JavaScript modules: dynamic import.

Cached JavaScript files

JavaScript (JS) files and other static assets aren't generally cached on clients during development in the Development environment. During development, static asset requests include the Cache-Control header with a value of no-cache or max-age with a value of zero (0).

During production in the Production environment, JS files are usually cached by clients.

To disable client-side caching in browsers, developers usually adopt one of the following approaches:

• Disable caching when the browser's developer tools console is open. Guidance can be found in the developer tools documentation of each browser maintainer:
  • Chrome DevTools
  • Firefox Developer Tools
  • Microsoft Edge Developer Tools overview
• Perform a manual browser refresh of any webpage of the Blazor app to reload JS files from the server. ASP.NET Core's HTTP Caching Middleware always honors a valid no-cache Cache-Control header sent by a client.

For more information, see:

• ASP.NET Core Blazor environments
• Response caching in ASP.NET Core

Size limits on JavaScript interop calls

This section only applies to interactive components in server-side apps. For client-side components, the framework doesn't impose a limit on the size of JavaScript (JS) interop inputs and outputs.

For interactive components in server-side apps, JS interop calls passing data from the client to the server are limited in size by the maximum incoming SignalR message size permitted for hub methods, which is enforced by HubOptions.MaximumReceiveMessageSize (default: 32 KB). JS to .NET SignalR messages larger than MaximumReceiveMessageSize throw an error. The framework doesn't impose a limit on the size of a SignalR message from the hub to a client. For more information on the size limit, error messages, and guidance on dealing with message size limits, see ASP.NET Core Blazor SignalR guidance.

Determine where the app is running

If it's relevant for the app to know where code is running for JS interop calls, use OperatingSystem.IsBrowser to determine if the component is executing in the context of browser on WebAssembly.