ASP.NET Core Blazor JavaScript interoperability (JS interop)

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

JavaScript interop abstractions and features package

The @microsoft/dotnet-js-interop package (npmjs.com) (Microsoft.JSInterop NuGet package) 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.

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.

JavaScript class with a field of type function

A JavaScript class with a field of type function isn't supported by Blazor JS interop. Use Javascript functions in classes.

❌Unsupported: GreetingHelpers.sayHello in the following class as a field of type function isn't discovered by Blazor's JS interop and can't be executed from C# code:

export class GreetingHelpers {
  sayHello = function() {
    ...
  }
}

✔️Supported: GreetingHelpers.sayHello in the following class as a function is supported:

export class GreetingHelpers {
  sayHello() {
    ...
  }
}

Arrow functions are also supported:

export class GreetingHelpers {
  sayHello = () => {
    ...
  }
}

Avoid inline event handlers

A JavaScript function can be invoked directly from an inline event handler. In the following example, alertUser is a JavaScript function called when the button is selected by the user:

<button onclick="alertUser">Click Me!</button>

However, the use of inline event handlers is a poor design choice for calling JavaScript functions:

• Mixing HTML markup and JavaScript code often leads to unmaintainable code.
• Inline event handler execution may be blocked by a Content Security Policy (CSP) (MDN documentation).

We recommend avoiding inline event handlers in favor of approaches that assign handlers in JavaScript with addEventListener, as the following example demonstrates:

AlertUser.razor.js:

export function alertUser() {
  alert('The button was selected!');
}

export function addHandlers() {
  const btn = document.getElementById("btn");
  btn.addEventListener("click", alertUser);
}

AlertUser.razor:

@page "/alert-user"
@implements IAsyncDisposable
@inject IJSRuntime JS

<h1>Alert User</h1>

<p>
    <button id="btn">Click Me!</button>
</p>

@code {
    private IJSObjectReference? module;

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

            await module.InvokeVoidAsync("addHandlers");
        }
    }

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

In the preceding example, JSDisconnectedException is trapped during module disposal in case Blazor's SignalR circuit is lost. If the preceding code is used in a Blazor WebAssembly app, there's no SignalR connection to lose, so you can remove the try-catch block and leave the line that disposes the module (await module.DisposeAsync();). For more information, see ASP.NET Core Blazor JavaScript interoperability (JS interop).

For more information, see the following resources:

• JavaScript location in ASP.NET Core Blazor apps
• Introduction to events (MDN documentation)

Asynchronous JavaScript calls

JS interop calls are asynchronous, regardless of whether the called code is synchronous or asynchronous. Calls are asynchronous 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.

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.
• Complex framework types, such as KeyValuePair, might be trimmed away by the IL Trimmer on publish and not present for JS interop. We recommend creating custom types for types that the IL Trimmer trims away.
• Blazor always relies on reflection for JSON serialization, including when using C# source generation. Setting JsonSerializerIsReflectionEnabledByDefault to false in the app's project file results in an error when serialization is attempted.

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

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"
@implements IAsyncDisposable
@inject IJSRuntime JS

<h1>DOM Cleanup Example</h1>

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

@code {
    private IJSObjectReference? module;

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

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

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

In the preceding example, JSDisconnectedException is trapped during module disposal in case Blazor's SignalR circuit is lost. If the preceding code is used in a Blazor WebAssembly app, there's no SignalR connection to lose, so you can remove the try-catch block and leave the line that disposes the module (await module.DisposeAsync();). For more information, see ASP.NET Core Blazor JavaScript interoperability (JS interop).

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;

The preceding approaches attach the MutationObserver to target.parentNode, which works until parentNode itself is removed from the DOM. This is a common scenario, for example, when navigating to a new page, which causes the entire page component to be removed from the DOM. In such cases, any child components observing changes within the page aren't cleaned up properly.

Don't assume that observing document.body, instead of target.parentNode, is a better target. Observing document.body has performance implications because callback logic is executed for all DOM updates, whether or not they have anything to do with your element. Use either of the following approaches:

• In cases where you can identify a suitable ancestor node to observe, use MutationObserver with it. Ideally, this ancestor is scoped to the changes that you want to observe, rather than document.body.
• Instead of using MutationObserver, consider using a custom element and disconnectedCallback. The event always fires when your custom element is disconnected, no matter where it resides in the DOM relative to the DOM change.

JavaScript interop calls without a circuit

This section only applies to server-side apps.

JavaScript (JS) interop calls can't be issued after Blazor's 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 server-side component implements IAsyncDisposable.
• module is an IJSObjectReference for a JS module.
• 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 (module is not null)
        {
            await module.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 in a server-side Blazor app, 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.

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
  • 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.