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

This article explains how to invoke JavaScript (JS) functions from .NET.

For information on how to call .NET methods from JS, see Call .NET methods from JavaScript functions in ASP.NET Core Blazor.

Invoke JS functions

IJSRuntime is registered by the Blazor framework. To call into JS from .NET, inject the IJSRuntime abstraction and call one of the following methods:

• IJSRuntime.InvokeAsync
• JSRuntimeExtensions.InvokeAsync
• JSRuntimeExtensions.InvokeVoidAsync

For the preceding .NET methods that invoke JS functions:

• The function identifier (String) is relative to the global scope (window). To call window.someScope.someFunction, the identifier is someScope.someFunction. There's no need to register the function before it's called.
• Pass any number of JSON-serializable arguments in Object[] to a JS function.
• The cancellation token (CancellationToken) propagates a notification that operations should be canceled.
• TimeSpan represents a time limit for a JS operation.
• The TValue return type must also be JSON serializable. TValue should match the .NET type that best maps to the JSON type returned.
• A JS Promise is returned for InvokeAsync methods. InvokeAsync unwraps the Promise and returns the value awaited by the Promise.

For Blazor apps with prerendering enabled, which is the default for server-side apps, calling into JS isn't possible during prerendering. For more information, see the Prerendering section.

The following example is based on TextDecoder, a JS-based decoder. The example demonstrates how to invoke a JS function from a C# method that offloads a requirement from developer code to an existing JS API. The JS function accepts a byte array from a C# method, decodes the array, and returns the text to the component for display.

<script>
  window.convertArray = (win1251Array) => {
    var win1251decoder = new TextDecoder('windows-1251');
    var bytes = new Uint8Array(win1251Array);
    var decodedArray = win1251decoder.decode(bytes);
    return decodedArray;
  };
</script>

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

The following component:

• Invokes the convertArray JS function with InvokeAsync when selecting a button (Convert Array).
• After the JS function is called, the passed array is converted into a string. The string is returned to the component for display (text).

CallJs1.razor:

@page "/call-js-1"
@inject IJSRuntime JS

<PageTitle>Call JS 1</PageTitle>

<h1>Call JS Example 1</h1>

<p>
    <button @onclick="ConvertArray">Convert Array</button>
</p>

<p>
    @text
</p>

<p>
    Quote ©2005 <a href="https://www.uphe.com">Universal Pictures</a>: 
    <a href="https://www.uphe.com/movies/serenity-2005">Serenity</a><br>
    <a href="https://www.imdb.com/name/nm0472710/">David Krumholtz on IMDB</a>
</p>

@code {
    private MarkupString text;

    private uint[] quoteArray = 
        new uint[]
        {
            60, 101, 109, 62, 67, 97, 110, 39, 116, 32, 115, 116, 111, 112, 32,
            116, 104, 101, 32, 115, 105, 103, 110, 97, 108, 44, 32, 77, 97,
            108, 46, 60, 47, 101, 109, 62, 32, 45, 32, 77, 114, 46, 32, 85, 110,
            105, 118, 101, 114, 115, 101, 10, 10,
        };

    private async Task ConvertArray() => 
        text = new(await JS.InvokeAsync<string>("convertArray", quoteArray));
}

JavaScript API restricted to user gestures

This section applies to server-side components.

Some browser JavaScript (JS) APIs can only be executed in the context of a user gesture, such as using the Fullscreen API (MDN documentation). These APIs can't be called through the JS interop mechanism in server-side components because UI event handling is performed asynchronously and generally no longer in the context of the user gesture. The app must handle the UI event completely in JavaScript, so use onclick instead of Blazor's @onclick directive attribute.

Invoke JavaScript functions without reading a returned value (InvokeVoidAsync)

Use InvokeVoidAsync when:

• .NET isn't required to read the result of a JavaScript (JS) call.
• JS functions return void(0)/void 0 or undefined.

Provide a displayTickerAlert1 JS function. The function is called with InvokeVoidAsync and doesn't return a value:

<script>
  window.displayTickerAlert1 = (symbol, price) => {
    alert(`${symbol}: $${price}!`);
  };
</script>

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

Component (.razor) example (InvokeVoidAsync)

TickerChanged calls the handleTickerChanged1 method in the following component.

CallJs2.razor:

@page "/call-js-2"
@inject IJSRuntime JS

<PageTitle>Call JS 2</PageTitle>

<h1>Call JS Example 2</h1>

<p>
    <button @onclick="SetStock">Set Stock</button>
</p>

@if (stockSymbol is not null)
{
    <p>@stockSymbol price: @price.ToString("c")</p>
}

@code {
    private string? stockSymbol;
    private decimal price;

    private async Task SetStock()
    {
        stockSymbol = 
            $"{(char)('A' + Random.Shared.Next(0, 26))}" +
            $"{(char)('A' + Random.Shared.Next(0, 26))}";
        price = Random.Shared.Next(1, 101);
        await JS.InvokeVoidAsync("displayTickerAlert1", stockSymbol, price);
    }
}

Class (.cs) example (InvokeVoidAsync)

JsInteropClasses1.cs:

using Microsoft.JSInterop;

namespace BlazorSample;

public class JsInteropClasses1(IJSRuntime js) : IDisposable
{
    private readonly IJSRuntime js = js;

    public async ValueTask TickerChanged(string symbol, decimal price) => 
        await js.InvokeVoidAsync("displayTickerAlert1", symbol, price);

    // Calling SuppressFinalize(this) prevents derived types that introduce 
    // a finalizer from needing to re-implement IDisposable.
    public void Dispose() => GC.SuppressFinalize(this);
}

TickerChanged calls the handleTickerChanged1 method in the following component.

CallJs3.razor:

@page "/call-js-3"
@implements IDisposable
@inject IJSRuntime JS

<PageTitle>Call JS 3</PageTitle>

<h1>Call JS Example 3</h1>

<p>
    <button @onclick="SetStock">Set Stock</button>
</p>

@if (stockSymbol is not null)
{
    <p>@stockSymbol price: @price.ToString("c")</p>
}

@code {
    private string? stockSymbol;
    private decimal price;
    private JsInteropClasses1? jsClass;

    protected override void OnInitialized() => jsClass = new(JS);

    private async Task SetStock()
    {
        if (jsClass is not null)
        {
            stockSymbol = 
                $"{(char)('A' + Random.Shared.Next(0, 26))}" +
                $"{(char)('A' + Random.Shared.Next(0, 26))}";
            price = Random.Shared.Next(1, 101);
            await jsClass.TickerChanged(stockSymbol, price);
        }
    }

    public void Dispose() => jsClass?.Dispose();
}

Invoke JavaScript functions and read a returned value (InvokeAsync)

Use InvokeAsync when .NET should read the result of a JavaScript (JS) call.

Provide a displayTickerAlert2 JS function. The following example returns a string for display by the caller:

<script>
  window.displayTickerAlert2 = (symbol, price) => {
    if (price < 20) {
      alert(`${symbol}: $${price}!`);
      return "User alerted in the browser.";
    } else {
      return "User NOT alerted.";
    }
  };
</script>

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

Component (.razor) example (InvokeAsync)

TickerChanged calls the handleTickerChanged2 method and displays the returned string in the following component.

CallJs4.razor:

@page "/call-js-4"
@inject IJSRuntime JS

<PageTitle>Call JS 4</PageTitle>

<h1>Call JS Example 4</h1>

<p>
    <button @onclick="SetStock">Set Stock</button>
</p>

@if (stockSymbol is not null)
{
    <p>@stockSymbol price: @price.ToString("c")</p>
}

@if (result is not null)
{
    <p>@result</p>
}

@code {
    private string? stockSymbol;
    private decimal price;
    private string? result;

    private async Task SetStock()
    {
        stockSymbol = 
            $"{(char)('A' + Random.Shared.Next(0, 26))}" +
            $"{(char)('A' + Random.Shared.Next(0, 26))}";
        price = Random.Shared.Next(1, 101);
        var interopResult = 
            await JS.InvokeAsync<string>("displayTickerAlert2", stockSymbol, price);
        result = $"Result of TickerChanged call for {stockSymbol} at " +
            $"{price.ToString("c")}: {interopResult}";
    }
}

Class (.cs) example (InvokeAsync)

JsInteropClasses2.cs:

using Microsoft.JSInterop;

namespace BlazorSample;

public class JsInteropClasses2(IJSRuntime js) : IDisposable
{
    private readonly IJSRuntime js = js;

    public async ValueTask<string> TickerChanged(string symbol, decimal price) => 
        await js.InvokeAsync<string>("displayTickerAlert2", symbol, price);

    // Calling SuppressFinalize(this) prevents derived types that introduce 
    // a finalizer from needing to re-implement IDisposable.
    public void Dispose() => GC.SuppressFinalize(this);
}

TickerChanged calls the handleTickerChanged2 method and displays the returned string in the following component.

CallJs5.razor:

@page "/call-js-5"
@implements IDisposable
@inject IJSRuntime JS

<PageTitle>Call JS 5</PageTitle>

<h1>Call JS Example 5</h1>

<p>
    <button @onclick="SetStock">Set Stock</button>
</p>

@if (stockSymbol is not null)
{
    <p>@stockSymbol price: @price.ToString("c")</p>
}

@if (result is not null)
{
    <p>@result</p>
}

@code {
    private string? stockSymbol;
    private decimal price;
    private JsInteropClasses2? jsClass;
    private string? result;

    protected override void OnInitialized() => jsClass = new(JS);

    private async Task SetStock()
    {
        if (jsClass is not null)
        {
            stockSymbol = 
                $"{(char)('A' + Random.Shared.Next(0, 26))}" +
                $"{(char)('A' + Random.Shared.Next(0, 26))}";
            price = Random.Shared.Next(1, 101);
            var interopResult = await jsClass.TickerChanged(stockSymbol, price);
            result = $"Result of TickerChanged call for {stockSymbol} at " +
                $"{price.ToString("c")}: {interopResult}";
        }
    }

    public void Dispose() => jsClass?.Dispose();
}

Dynamic content generation scenarios

For dynamic content generation with BuildRenderTree, use the [Inject] attribute:

[Inject]
IJSRuntime JS { get; set; }

Prerendering

This section applies to server-side apps that prerender Razor components. Prerendering is covered in Prerender ASP.NET Core Razor components.

Note

Internal navigation for interactive routing in Blazor Web Apps doesn't involve requesting new page content from the server. Therefore, prerendering doesn't occur for internal page requests. If the app adopts interactive routing, perform a full page reload for component examples that demonstrate prerendering behavior. For more information, see Prerender ASP.NET Core Razor components.

During prerendering, calling into JavaScript (JS) isn't possible. The following example demonstrates how to use JS interop as part of a component's initialization logic in a way that's compatible with prerendering.

The following scrollElementIntoView function:

• Scrolls to the passed element with scrollIntoView.
• Returns the element's top property value from the getBoundingClientRect method.

window.scrollElementIntoView = (element) => {
  element.scrollIntoView();
  return element.getBoundingClientRect().top;
}

Where IJSRuntime.InvokeAsync calls the JS function in component code, the ElementReference is only used in OnAfterRenderAsync and not in any earlier lifecycle method because there's no HTML DOM element until after the component is rendered.

StateHasChanged (reference source) is called to enqueue rerendering of the component with the new state obtained from the JS interop call (for more information, see ASP.NET Core Razor component rendering). An infinite loop isn't created because StateHasChanged is only called when scrollPosition is null.

PrerenderedInterop.razor:

@page "/prerendered-interop"
@using Microsoft.AspNetCore.Components
@using Microsoft.JSInterop
@inject IJSRuntime JS

<PageTitle>Prerendered Interop</PageTitle>

<h1>Prerendered Interop Example</h1>

<div @ref="divElement" style="margin-top:2000px">
    Set value via JS interop call: <strong>@scrollPosition</strong>
</div>

@code {
    private ElementReference divElement;
    private double? scrollPosition;

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender && scrollPosition is null)
        {
            scrollPosition = await JS.InvokeAsync<double>(
                "scrollElementIntoView", divElement);

            StateHasChanged();
        }
    }
}

The preceding example pollutes the client with a global function. For a better approach in production apps, see JavaScript isolation in JavaScript modules.

Synchronous JS interop in client-side components

This section only applies to client-side components.

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 render modes. On the server, all JS interop calls must be asynchronous because they're sent over a network connection.

If you know for certain that your component only runs on WebAssembly, you can choose to make synchronous JS interop calls. This has slightly less overhead than making asynchronous calls and can result in fewer render cycles because there's no intermediate state while awaiting results.

To make a synchronous call from .NET to JavaScript in a client-side component, cast IJSRuntime to IJSInProcessRuntime to make the JS interop call:

@inject IJSRuntime JS

...

@code {
    protected override void HandleSomeEvent()
    {
        var jsInProcess = (IJSInProcessRuntime)JS;
        var value = jsInProcess.Invoke<string>("javascriptFunctionIdentifier");
    }
}

When working with IJSObjectReference in ASP.NET Core 5.0 or later client-side components, you can use IJSInProcessObjectReference synchronously instead. IJSInProcessObjectReference implements IAsyncDisposable/IDisposable and should be disposed for garbage collection to prevent a memory leak, as the following example demonstrates:

@inject IJSRuntime JS
@implements IAsyncDisposable

...

@code {
    ...
    private IJSInProcessObjectReference? module;

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            module = await JS.InvokeAsync<IJSInProcessObjectReference>("import", 
            "./scripts.js");
        }
    }

    ...

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

JavaScript location

Load JavaScript (JS) code using any of approaches described by the article on JavaScript location:

• 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

For information on isolating scripts in JS modules, see the JavaScript isolation in JavaScript modules section.

Warning

Only place a <script> tag in a component file (.razor) if the component is guaranteed to adopt static server-side rendering (static SSR) because the <script> tag can't be updated dynamically.

JavaScript isolation in JavaScript modules

Blazor enables JavaScript (JS) isolation in standard JavaScript modules (ECMAScript specification). JavaScript module loading works the same way in Blazor as it does for other types of web apps, and you're free to customize how modules are defined in your app. For a guide on how to use JavaScript modules, see MDN Web Docs: JavaScript modules.

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.

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.

For example, the following JS module exports a JS function for showing a browser window prompt. Place the following JS code in an external JS file.

wwwroot/scripts.js:

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

Add the preceding JS module to an app or class library as a static web asset in the wwwroot folder and then import the module into the .NET code by calling InvokeAsync on the IJSRuntime instance.

IJSRuntime imports the module as an IJSObjectReference, which represents a reference to a JS object from .NET code. Use the IJSObjectReference to invoke exported JS functions from the module.

CallJs6.razor:

@page "/call-js-6"
@implements IAsyncDisposable
@inject IJSRuntime JS

<PageTitle>Call JS 6</PageTitle>

<h1>Call JS Example 6</h1>

<p>
    <button @onclick="TriggerPrompt">Trigger browser window prompt</button>
</p>

<p>
    @result
</p>

@code {
    private IJSObjectReference? module;
    private string? result;

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

    private async Task TriggerPrompt() => result = await Prompt("Provide text");

    public async ValueTask<string?> Prompt(string message) =>
        module is not null ? 
            await module.InvokeAsync<string>("showPrompt", message) : null;

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

In the preceding example:

• By convention, the import identifier is a special identifier used specifically for importing a JS module.
• Specify the module's external JS file using its stable static web asset path: ./{SCRIPT PATH AND FILE NAME (.js)}, where:
  • The path segment for the current directory (./) is required in order to create the correct static asset path to the JS file.
  • The {SCRIPT PATH AND FILE NAME (.js)} placeholder is the path and file name under wwwroot.
• Disposes the IJSObjectReference for garbage collection in IAsyncDisposable.DisposeAsync.

Dynamically importing a module requires a network request, so it can only be achieved asynchronously by calling InvokeAsync.

IJSInProcessObjectReference represents a reference to a JS object whose functions can be invoked synchronously in client-side components. For more information, see the Synchronous JS interop in client-side components section.

Note

When the external JS file is supplied by a Razor class library, specify the module's 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. In the following example, the library's assembly name is ComponentLibrary and the library's project file doesn't specify <PackageId>.
• The {SCRIPT PATH AND FILE NAME (.js)} placeholder is the path and file name under wwwroot. In the following example, the external JS file (script.js) is placed in the class library's wwwroot folder.
• module is a private nullable IJSObjectReference of the component class (private IJSObjectReference? module;).

module = await js.InvokeAsync<IJSObjectReference>(
    "import", "./_content/ComponentLibrary/scripts.js");

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

Throughout the Blazor documentation, examples use the .js file extension for module files, not the newer .mjs file extension (RFC 9239). Our documentation continues to use the .js file extension for the same reasons the Mozilla Foundation's documentation continues to use the .js file extension. For more information, see Aside — .mjs versus .js (MDN documentation).

Capture references to elements

Some JavaScript (JS) interop scenarios require references to HTML elements. For example, a UI library may require an element reference for initialization, or you might need to call command-like APIs on an element, such as click or play.

Capture references to HTML elements in a component using the following approach:

• Add an @ref attribute to the HTML element.
• Define a field of type ElementReference whose name matches the value of the @ref attribute.

The following example shows capturing a reference to the username <input> element:

<input @ref="username" ... />

@code {
    private ElementReference username;
}

Warning

Only use an element reference to mutate the contents of an empty element that doesn't interact with Blazor. This scenario is useful when a third-party API supplies content to the element. Because Blazor doesn't interact with the element, there's no possibility of a conflict between Blazor's representation of the element and the DOM.

In the following example, it's dangerous to mutate the contents of the unordered list (ul) using MyList via JS interop because Blazor interacts with the DOM to populate this element's list items (<li>) from the Todos object:

<ul @ref="MyList">
    @foreach (var item in Todos)
    {
        <li>@item.Text</li>
    }
</ul>

Using the MyList element reference to merely read DOM content or trigger an event is supported.

If JS interop mutates the contents of element MyList and Blazor attempts to apply diffs to the element, the diffs won't match the DOM. Modifying the contents of the list via JS interop with the MyList element reference is not supported.

For more information, see ASP.NET Core Blazor JavaScript interoperability (JS interop).

An ElementReference is passed through to JS code via JS interop. The JS code receives an HTMLElement instance, which it can use with normal DOM APIs. For example, the following code defines a .NET extension method (TriggerClickEvent) that enables sending a mouse click to an element.

The JS function clickElement creates a click event on the passed HTML element (element):

window.interopFunctions = {
  clickElement : function (element) {
    element.click();
  }
}

To call a JS function that doesn't return a value, use JSRuntimeExtensions.InvokeVoidAsync. The following code triggers a client-side click event by calling the preceding JS function with the captured ElementReference:

@inject IJSRuntime JS

<button @ref="exampleButton">Example Button</button>

<button @onclick="TriggerClick">
    Trigger click event on <code>Example Button</code>
</button>

@code {
    private ElementReference exampleButton;

    public async Task TriggerClick()
    {
        await JS.InvokeVoidAsync(
            "interopFunctions.clickElement", exampleButton);
    }
}

To use an extension method, create a static extension method that receives the IJSRuntime instance:

public static async Task TriggerClickEvent(this ElementReference elementRef, 
    IJSRuntime js)
{
    await js.InvokeVoidAsync("interopFunctions.clickElement", elementRef);
}

The clickElement method is called directly on the object. The following example assumes that the TriggerClickEvent method is available from the JsInteropClasses namespace:

@inject IJSRuntime JS
@using JsInteropClasses

<button @ref="exampleButton">Example Button</button>

<button @onclick="TriggerClick">
    Trigger click event on <code>Example Button</code>
</button>

@code {
    private ElementReference exampleButton;

    public async Task TriggerClick()
    {
        await exampleButton.TriggerClickEvent(JS);
    }
}

Important

The exampleButton variable is only populated after the component is rendered. If an unpopulated ElementReference is passed to JS code, the JS code receives a value of null. To manipulate element references after the component has finished rendering, use the OnAfterRenderAsync or OnAfterRender component lifecycle methods.

When working with generic types and returning a value, use ValueTask<TResult>:

public static ValueTask<T> GenericMethod<T>(
        this ElementReference elementRef, IJSRuntime js) => 
    js.InvokeAsync<T>("{JAVASCRIPT FUNCTION}", elementRef);

The {JAVASCRIPT FUNCTION} placeholder is the JS function identifier.

GenericMethod is called directly on the object with a type. The following example assumes that the GenericMethod is available from the JsInteropClasses namespace:

@inject IJSRuntime JS
@using JsInteropClasses

<input @ref="username" />

<button @onclick="OnClickMethod">Do something generic</button>

<p>
    returnValue: @returnValue
</p>

@code {
    private ElementReference username;
    private string? returnValue;

    private async Task OnClickMethod()
    {
        returnValue = await username.GenericMethod<string>(JS);
    }
}

Reference elements across components

An ElementReference can't be passed between components because:

• The instance is only guaranteed to exist after the component is rendered, which is during or after a component's OnAfterRender/OnAfterRenderAsync method executes.
• An ElementReference is a struct, which can't be passed as a component parameter.

For a parent component to make an element reference available to other components, the parent component can:

• Allow child components to register callbacks.
• Invoke the registered callbacks during the OnAfterRender event with the passed element reference. Indirectly, this approach allows child components to interact with the parent's element reference.

<style>
    .red { color: red }
</style>

<script>
  function setElementClass(element, className) {
    var myElement = element;
    myElement.classList.add(className);
  }
</script>

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

CallJs7.razor (parent component):

@page "/call-js-7"

<PageTitle>Call JS 7</PageTitle>

<h1>Call JS Example 7</h1>

<h2 @ref="title">Hello, world!</h2>

Welcome to your new app.

<SurveyPrompt Parent="this" Title="How is Blazor working for you?" />

CallJs7.razor.cs:

using Microsoft.AspNetCore.Components;

namespace BlazorSample.Pages;

public partial class CallJs7 : 
    ComponentBase, IObservable<ElementReference>, IDisposable
{
    private bool disposing;
    private readonly List<IObserver<ElementReference>> subscriptions = [];
    private ElementReference title;

    protected override void OnAfterRender(bool firstRender)
    {
        base.OnAfterRender(firstRender);

        foreach (var subscription in subscriptions)
        {
            subscription.OnNext(title);
        }
    }

    public void Dispose()
    {
        disposing = true;

        foreach (var subscription in subscriptions)
        {
            try
            {
                subscription.OnCompleted();
            }
            catch (Exception)
            {
            }
        }

        subscriptions.Clear();

        // The following prevents derived types that introduce a
        // finalizer from needing to re-implement IDisposable.
        GC.SuppressFinalize(this);
    }

    public IDisposable Subscribe(IObserver<ElementReference> observer)
    {
        if (disposing)
        {
            throw new InvalidOperationException("Parent being disposed");
        }

        subscriptions.Add(observer);

        return new Subscription(observer, this);
    }

    private class Subscription(IObserver<ElementReference> observer,
        CallJs7 self) : IDisposable
    {
        public IObserver<ElementReference> Observer { get; } = observer;
        public CallJs7 Self { get; } = self;

        public void Dispose() => Self.subscriptions.Remove(Observer);
    }
}

In the preceding example, the namespace of the app is BlazorSample. If testing the code locally, update the namespace.

SurveyPrompt.razor (child component):

<div class="alert alert-secondary mt-4">
    <span class="oi oi-pencil me-2" aria-hidden="true"></span>
    <strong>@Title</strong>

    <span class="text-nowrap">
        Please take our
        <a target="_blank" class="font-weight-bold link-dark" href="https://go.microsoft.com/fwlink/?linkid=2186158">brief survey</a>
    </span>
    and tell us what you think.
</div>

@code {
    // Demonstrates how a parent component can supply parameters
    [Parameter]
    public string? Title { get; set; }
}

SurveyPrompt.razor.cs:

using Microsoft.AspNetCore.Components;
using Microsoft.JSInterop;

namespace BlazorSample.Components;

public partial class SurveyPrompt : 
    ComponentBase, IObserver<ElementReference>, IDisposable
{
    private IDisposable? subscription = null;

    [Parameter]
    public IObservable<ElementReference>? Parent { get; set; }

    [Inject]
    public IJSRuntime? JS {get; set;}

    protected override void OnParametersSet()
    {
        base.OnParametersSet();

        subscription?.Dispose();
        subscription = Parent?.Subscribe(this);
    }

    public void OnCompleted() => subscription = null;

    public void OnError(Exception error) => subscription = null;

    public void OnNext(ElementReference value) =>
        _ = (JS?.InvokeAsync<object>("setElementClass", [value, "red"]));

    public void Dispose()
    {
        subscription?.Dispose();

        // The following prevents derived types that introduce a
        // finalizer from needing to re-implement IDisposable.
        GC.SuppressFinalize(this);
    }
}

In the preceding example, the namespace of the app is BlazorSample with shared components in the Shared folder. If testing the code locally, update the namespace.

Harden JavaScript interop calls

This section only applies to Interactive Server components, but client-side components may also set JS interop timeouts if conditions warrant it.

In server-side apps with server interactivity, JavaScript (JS) interop may fail due to networking errors and should be treated as unreliable. Blazor apps use a one minute timeout for JS interop calls. If an app can tolerate a more aggressive timeout, set the timeout using one of the following approaches.

Set a global timeout in the Program.cs with CircuitOptions.JSInteropDefaultCallTimeout:

builder.Services.AddRazorComponents()
    .AddInteractiveServerComponents(options => 
        options.JSInteropDefaultCallTimeout = {TIMEOUT});

The {TIMEOUT} placeholder is a TimeSpan (for example, TimeSpan.FromSeconds(80)).

Set a per-invocation timeout in component code. The specified timeout overrides the global timeout set by JSInteropDefaultCallTimeout:

var result = await JS.InvokeAsync<string>("{ID}", {TIMEOUT}, new[] { "Arg1" });

In the preceding example:

• The {TIMEOUT} placeholder is a TimeSpan (for example, TimeSpan.FromSeconds(80)).
• The {ID} placeholder is the identifier for the function to invoke. For example, the value someScope.someFunction invokes the function window.someScope.someFunction.

Although a common cause of JS interop failures are network failures with server-side components, per-invocation timeouts can be set for JS interop calls for client-side components. Although no SignalR circuit exists for a client-side component, JS interop calls might fail for other reasons that apply.

For more information on resource exhaustion, see Threat mitigation guidance for ASP.NET Core Blazor interactive server-side rendering.

Avoid circular object references

Objects that contain circular references can't be serialized on the client for either:

• .NET method calls.
• JavaScript method calls from C# when the return type has circular references.

JavaScript libraries that render UI

Sometimes you may wish to use JavaScript (JS) libraries that produce visible user interface elements within the browser DOM. At first glance, this might seem difficult because Blazor's diffing system relies on having control over the tree of DOM elements and runs into errors if some external code mutates the DOM tree and invalidates its mechanism for applying diffs. This isn't a Blazor-specific limitation. The same challenge occurs with any diff-based UI framework.

Fortunately, it's straightforward to embed externally-generated UI within a Razor component UI reliably. The recommended technique is to have the component's code (.razor file) produce an empty element. As far as Blazor's diffing system is concerned, the element is always empty, so the renderer does not recurse into the element and instead leaves its contents alone. This makes it safe to populate the element with arbitrary externally-managed content.

The following example demonstrates the concept. Within the if statement when firstRender is true, interact with unmanagedElement outside of Blazor using JS interop. For example, call an external JS library to populate the element. Blazor leaves the element's contents alone until this component is removed. When the component is removed, the component's entire DOM subtree is also removed.

<h1>Hello! This is a Razor component rendered at @DateTime.Now</h1>

<div @ref="unmanagedElement"></div>

@code {
    private ElementReference unmanagedElement;

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            ...
        }
    }
}

Consider the following example that renders an interactive map using open-source Mapbox APIs.

The following JS module is placed into the app or made available from a Razor class library.

Note

To create the Mapbox map, obtain an access token from Mapbox Sign in and provide it where the {ACCESS TOKEN} appears in the following code.

wwwroot/mapComponent.js:

import 'https://api.mapbox.com/mapbox-gl-js/v1.12.0/mapbox-gl.js';

mapboxgl.accessToken = '{ACCESS TOKEN}';

export function addMapToElement(element) {
  return new mapboxgl.Map({
    container: element,
    style: 'mapbox://styles/mapbox/streets-v11',
    center: [-74.5, 40],
    zoom: 9
  });
}

export function setMapCenter(map, latitude, longitude) {
  map.setCenter([longitude, latitude]);
}

To produce correct styling, add the following stylesheet tag to the host HTML page.

Add the following <link> element to the <head> element markup (location of <head> content):

<link href="https://api.mapbox.com/mapbox-gl-js/v1.12.0/mapbox-gl.css" 
    rel="stylesheet" />

CallJs8.razor:

@page "/call-js-8"
@implements IAsyncDisposable
@inject IJSRuntime JS

<PageTitle>Call JS 8</PageTitle>

<HeadContent>
    <link href="https://api.mapbox.com/mapbox-gl-js/v1.12.0/mapbox-gl.css" 
        rel="stylesheet" />
</HeadContent>

<h1>Call JS Example 8</h1>

<div @ref="mapElement" style='width:400px;height:300px'></div>

<button @onclick="() => ShowAsync(51.454514, -2.587910)">Show Bristol, UK</button>
<button @onclick="() => ShowAsync(35.6762, 139.6503)">Show Tokyo, Japan</button>

@code
{
    private ElementReference mapElement;
    private IJSObjectReference? mapModule;
    private IJSObjectReference? mapInstance;

    protected override async Task OnAfterRenderAsync(bool firstRender)
    {
        if (firstRender)
        {
            mapModule = await JS.InvokeAsync<IJSObjectReference>(
                "import", "./mapComponent.js");
            mapInstance = await mapModule.InvokeAsync<IJSObjectReference>(
                "addMapToElement", mapElement);
        }
    }

    private async Task ShowAsync(double latitude, double longitude)
    {
        if (mapModule is not null && mapInstance is not null)
        {
            await mapModule.InvokeVoidAsync("setMapCenter", mapInstance, 
                latitude, longitude);
        }
    }

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

        if (mapModule is not null)
        {
            await mapModule.DisposeAsync();
        }
    }
}

The preceding example produces an interactive map UI. The user:

• Can drag to scroll or zoom.
• Select buttons to jump to predefined locations.

In the preceding example:

• The <div> with @ref="mapElement" is left empty as far as Blazor is concerned. The mapbox-gl.js script can safely populate the element and modify its contents. Use this technique with any JS library that renders UI. You can embed components from a third-party JS SPA framework inside Razor components, as long as they don't try to reach out and modify other parts of the page. It is not safe for external JS code to modify elements that Blazor does not regard as empty.
• When using this approach, bear in mind the rules about how Blazor retains or destroys DOM elements. The component safely handles button click events and updates the existing map instance because DOM elements are retained where possible. If you were rendering a list of map elements from inside a @foreach loop, you want to use @key to ensure the preservation of component instances. Otherwise, changes in the list data could cause component instances to retain the state of previous instances in an undesirable manner. For more information, see how to use the @key directive attribute to preserve the relationship among elements, components, and model objects.
• The example encapsulates JS logic and dependencies within a JavaScript module and loads the module dynamically using the import identifier. For more information, see JavaScript isolation in JavaScript modules.

Byte array support

Blazor supports optimized byte array JavaScript (JS) interop that avoids encoding/decoding byte arrays into Base64. The following example uses JS interop to pass a byte array to JavaScript.

Provide a receiveByteArray JS function. The function is called with InvokeVoidAsync and doesn't return a value:

<script>
  window.receiveByteArray = (bytes) => {
    let utf8decoder = new TextDecoder();
    let str = utf8decoder.decode(bytes);
    return str;
  };
</script>

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

CallJs9.razor:

@page "/call-js-9"
@inject IJSRuntime JS

<h1>Call JS Example 9</h1>

<p>
    <button @onclick="SendByteArray">Send Bytes</button>
</p>

<p>
    @result
</p>

<p>
    Quote &copy;2005 <a href="https://www.uphe.com">Universal Pictures</a>:
    <a href="https://www.uphe.com/movies/serenity-2005">Serenity</a><br>
    <a href="https://www.imdb.com/name/nm0821612/">Jewel Staite on IMDB</a>
</p>

@code {
    private string? result;

    private async Task SendByteArray()
    {
        var bytes = new byte[] { 0x45, 0x76, 0x65, 0x72, 0x79, 0x74, 0x68, 0x69,
            0x6e, 0x67, 0x27, 0x73, 0x20, 0x73, 0x68, 0x69, 0x6e, 0x79, 0x2c,
            0x20, 0x43, 0x61, 0x70, 0x74, 0x69, 0x61, 0x6e, 0x2e, 0x20, 0x4e,
            0x6f, 0x74, 0x20, 0x74, 0x6f, 0x20, 0x66, 0x72, 0x65, 0x74, 0x2e };

        result = await JS.InvokeAsync<string>("receiveByteArray", bytes);
    }
}

Stream from .NET to JavaScript

Blazor supports streaming data directly from .NET to JavaScript (JS). Streams are created using a DotNetStreamReference.

DotNetStreamReference represents a .NET stream and uses the following parameters:

• stream: The stream sent to JS.
• leaveOpen: Determines if the stream is left open after transmission. If a value isn't provided, leaveOpen defaults to false.

In JS, use an array buffer or a readable stream to receive the data:

• Using an ArrayBuffer:

  async function streamToJavaScript(streamRef) {
    const data = await streamRef.arrayBuffer();
  }
  

• Using a ReadableStream:

  async function streamToJavaScript(streamRef) {
    const stream = await streamRef.stream();
  }
  

In C# code:

var streamRef = new DotNetStreamReference(stream: {STREAM}, leaveOpen: false);
await JS.InvokeVoidAsync("streamToJavaScript", streamRef);

In the preceding example:

• The {STREAM} placeholder represents the Stream sent to JS.
• JS is an injected IJSRuntime instance.

Disposing a DotNetStreamReference instance is usually unnecessary. When leaveOpen is set to its default value of false, the underlying Stream is automatically disposed after transmission to JS.

If leaveOpen is true, then disposing a DotNetStreamReference doesn't dispose its underlying Stream. The app's code determines when to dispose the underlying Stream. When deciding how to dispose the underlying Stream, consider the following:

• Disposing a Stream while it's being transmitted to JS is considered an application error and may cause an unhandled exception to occur.
• Stream transmission begins as soon as the DotNetStreamReference is passed as an argument to a JS interop call, regardless of whether the stream is actually used in JS logic.

Given these characteristics, we recommend disposing the underlying Stream only after it's fully consumed by JS (the promise returned by arrayBuffer or stream resolves). It follows that a DotNetStreamReference should only be passed to JS if it's unconditionally going to be consumed by JS logic.

Call .NET methods from JavaScript functions in ASP.NET Core Blazor covers the reverse operation, streaming from JavaScript to .NET.

ASP.NET Core Blazor file downloads covers how to download a file in Blazor.

Catch JavaScript exceptions

To catch JS exceptions, wrap the JS interop in a try-catch block and catch a JSException.

In the following example, the nonFunction JS function doesn't exist. When the function isn't found, the JSException is trapped with a Message that indicates the following error:

Could not find 'nonFunction' ('nonFunction' was undefined).

CallJs11.razor:

@page "/call-js-11"
@inject IJSRuntime JS

<PageTitle>Call JS 11</PageTitle>

<h1>Call JS Example 11</h1>

<p>
    <button @onclick="CatchUndefinedJSFunction">Catch Exception</button>
</p>

<p>
    @result
</p>

<p>
    @errorMessage
</p>

@code {
    private string? errorMessage;
    private string? result;

    private async Task CatchUndefinedJSFunction()
    {
        try
        {
            result = await JS.InvokeAsync<string>("nonFunction");
        }
        catch (JSException e)
        {
            errorMessage = $"Error Message: {e.Message}";
        }
    }
}

Abort a long-running JavaScript function

Use a JS AbortController with a CancellationTokenSource in the component to abort a long-running JavaScript function from C# code.

The following JS Helpers class contains a simulated long-running function, longRunningFn, to count continuously until the AbortController.signal indicates that AbortController.abort has been called. The sleep function is for demonstration purposes to simulate slow execution of the long-running function and wouldn't be present in production code. When a component calls stopFn, the longRunningFn is signalled to abort via the while loop conditional check on AbortSignal.aborted.

<script>
  class Helpers {
    static #controller = new AbortController();

    static async #sleep(ms) {
      return new Promise(resolve => setTimeout(resolve, ms));
    }

    static async longRunningFn() {
      var i = 0;
      while (!this.#controller.signal.aborted) {
        i++;
        console.log(`longRunningFn: ${i}`);
        await this.#sleep(1000);
      }
    }

    static stopFn() {
      this.#controller.abort();
      console.log('longRunningFn aborted!');
    }
  }

  window.Helpers = Helpers;
</script>

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

The following component:

• Invokes the JS function longRunningFn when the Start Task button is selected. A CancellationTokenSource is used to manage the execution of the long-running function. CancellationToken.Register sets a JS interop call delegate to execute the JS function stopFn when the CancellationTokenSource.Token is cancelled.
• When the Cancel Task button is selected, the CancellationTokenSource.Token is cancelled with a call to Cancel.
• The CancellationTokenSource is disposed in the Dispose method.

CallJs12.razor:

@page "/call-js-12"
@inject IJSRuntime JS

<h1>Cancel long-running JS interop</h1>

<p>
    <button @onclick="StartTask">Start Task</button>
    <button @onclick="CancelTask">Cancel Task</button>
</p>

@code {
    private CancellationTokenSource? cts;

    private async Task StartTask()
    {
        cts = new CancellationTokenSource();
        cts.Token.Register(() => JS.InvokeVoidAsync("Helpers.stopFn"));

        await JS.InvokeVoidAsync("Helpers.longRunningFn");
    }

    private void CancelTask()
    {
        cts?.Cancel();
    }

    public void Dispose()
    {
        cts?.Cancel();
        cts?.Dispose();
    }
}

A browser's developer tools console indicates the execution of the long-running JS function after the Start Task button is selected and when the function is aborted after the Cancel Task button is selected:

longRunningFn: 1
longRunningFn: 2
longRunningFn: 3
longRunningFn aborted!

JavaScript [JSImport]/[JSExport] interop

This section applies to client-side components.

As an alternative to interacting with JavaScript (JS) in client-side components using Blazor's JS interop mechanism based on the IJSRuntime interface, a JS [JSImport]/[JSExport] interop API is available to apps targeting .NET 7 or later.

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

Unmarshalled JavaScript interop

This section applies to client-side components.

Unmarshalled interop using the IJSUnmarshalledRuntime interface is obsolete and should be replaced with JavaScript [JSImport]/[JSExport] interop.

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

Disposal of JavaScript interop object references

Examples throughout the JavaScript (JS) interop articles demonstrate typical object disposal patterns:

• When calling JS from .NET, as described in this article, dispose any created IJSObjectReference/IJSInProcessObjectReference/JSObjectReference either from .NET or from JS to avoid leaking JS memory.

• When calling .NET from JS, as described in Call .NET methods from JavaScript functions in ASP.NET Core Blazor, dispose of a created DotNetObjectReference either from .NET or from JS to avoid leaking .NET memory.

JS interop object references are implemented as a map keyed by an identifier on the side of the JS interop call that creates the reference. When object disposal is initiated from either the .NET or JS side, Blazor removes the entry from the map, and the object can be garbage collected as long as no other strong reference to the object is present.

At a minimum, always dispose objects created on the .NET side to avoid leaking .NET managed memory.

DOM cleanup tasks during component disposal

For more information, see ASP.NET Core Blazor JavaScript interoperability (JS interop).

JavaScript interop calls without a circuit

For more information, see ASP.NET Core Blazor JavaScript interoperability (JS interop).

Additional resources

• Call .NET methods from JavaScript functions in ASP.NET Core Blazor
• InteropComponent.razor example (dotnet/AspNetCore GitHub repository main branch): The main branch represents the product unit's current development for the next release of ASP.NET Core. To select the branch for a different release (for example, release/5.0), use the Switch branches or tags dropdown list to select the branch.
• Blazor samples GitHub repository (dotnet/blazor-samples) (how to download)
• Handle errors in ASP.NET Core Blazor apps (JavaScript interop section)
• Threat mitigation: JavaScript functions invoked from .NET