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title author description monikerRange ms.author ms.custom ms.date no-loc uid
ASP.NET Core Blazor JavaScript interop
guardrex
Learn how to invoke JavaScript functions from .NET and .NET methods from JavaScript in Blazor apps.
>= aspnetcore-3.0
riande
mvc
12/02/2019
Blazor
blazor/javascript-interop

ASP.NET Core Blazor JavaScript interop

By Javier Calvarro Nelson, Daniel Roth, and Luke Latham

[!INCLUDE]

A Blazor app can invoke JavaScript functions from .NET and .NET methods from JavaScript code.

View or download sample code (how to download)

Invoke JavaScript functions from .NET methods

There are times when .NET code is required to call a JavaScript function. For example, a JavaScript call can expose browser capabilities or functionality from a JavaScript library to the app. This scenario is called JavaScript interoperability (JS interop).

To call into JavaScript from .NET, use the IJSRuntime abstraction. The InvokeAsync<T> method takes an identifier for the JavaScript function that you wish to invoke along with any number of JSON-serializable arguments. The function identifier is relative to the global scope (window). If you wish to call window.someScope.someFunction, the identifier is someScope.someFunction. There's no need to register the function before it's called. The return type T must also be JSON serializable. T should match the .NET type that best maps to the JSON type returned.

For Blazor Server apps:

  • Multiple user requests are processed by the Blazor Server app. Don't call JSRuntime.Current in a component to invoke JavaScript functions.
  • Inject the IJSRuntime abstraction and use the injected object to issue JS interop calls.
  • While a Blazor app is prerendering, calling into JavaScript isn't possible because a connection with the browser hasn't been established. For more information, see the Detect when a Blazor app is prerendering section.

The following example is based on TextDecoder, an experimental JavaScript-based decoder. The example demonstrates how to invoke a JavaScript function from a C# method. The JavaScript function accepts a byte array from a C# method, decodes the array, and returns the text to the component for display.

Inside the <head> element of wwwroot/index.html (Blazor WebAssembly) or Pages/_Host.cshtml (Blazor Server), provide a JavaScript function that uses TextDecoder to decode a passed array and return the decoded value:

[!code-html]

JavaScript code, such as the code shown in the preceding example, can also be loaded from a JavaScript file (.js) with a reference to the script file:

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

The following component:

  • Invokes the convertArray JavaScript function using JSRuntime when a component button (Convert Array) is selected.
  • After the JavaScript function is called, the passed array is converted into a string. The string is returned to the component for display.

[!code-cshtml]

Use of IJSRuntime

To use the IJSRuntime abstraction, adopt any of the following approaches:

  • Inject the IJSRuntime abstraction into the Razor component (.razor):

    [!code-cshtml]

    Inside the <head> element of wwwroot/index.html (Blazor WebAssembly) or Pages/_Host.cshtml (Blazor Server), provide a handleTickerChanged JavaScript function. The function is called with IJSRuntime.InvokeVoidAsync and doesn't return a value:

    [!code-html]

  • Inject the IJSRuntime abstraction into a class (.cs):

    [!code-csharp]

    Inside the <head> element of wwwroot/index.html (Blazor WebAssembly) or Pages/_Host.cshtml (Blazor Server), provide a handleTickerChanged JavaScript function. The function is called with JSRuntime.InvokeAsync and returns a value:

    [!code-html]

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

    [Inject]
IJSRuntime JSRuntime { get; set; }

In the client-side sample app that accompanies this topic, two JavaScript functions are available to the app that interact with the DOM to receive user input and display a welcome message:

  • showPrompt – Produces a prompt to accept user input (the user's name) and returns the name to the caller.
  • displayWelcome – Assigns a welcome message from the caller to a DOM object with an id of welcome.

wwwroot/exampleJsInterop.js:

[!code-javascript]

Place the <script> tag that references the JavaScript file in the wwwroot/index.html file (Blazor WebAssembly) or Pages/_Host.cshtml file (Blazor Server).

wwwroot/index.html (Blazor WebAssembly):

[!code-html]

Pages/_Host.cshtml (Blazor Server):

[!code-cshtml]

Don't place a <script> tag in a component file because the <script> tag can't be updated dynamically.

.NET methods interop with the JavaScript functions in the exampleJsInterop.js file by calling IJSRuntime.InvokeAsync<T>.

The IJSRuntime abstraction is asynchronous to allow for Blazor Server scenarios. If the app is a Blazor WebAssembly app and you want to invoke a JavaScript function synchronously, downcast to IJSInProcessRuntime and call Invoke<T> instead. We recommend that most JS interop libraries use the async APIs to ensure that the libraries are available in all scenarios.

The sample app includes a component to demonstrate JS interop. The component:

  • Receives user input via a JavaScript prompt.
  • Returns the text to the component for processing.
  • Calls a second JavaScript function that interacts with the DOM to display a welcome message.

Pages/JSInterop.razor:

[!code-cshtml]

  1. When TriggerJsPrompt is executed by selecting the component's Trigger JavaScript Prompt button, the JavaScript showPrompt function provided in the wwwroot/exampleJsInterop.js file is called.
  2. The showPrompt function accepts user input (the user's name), which is HTML-encoded and returned to the component. The component stores the user's name in a local variable, name.
  3. The string stored in name is incorporated into a welcome message, which is passed to a JavaScript function, displayWelcome, which renders the welcome message into a heading tag.

Call a void JavaScript function

JavaScript functions that return void(0)/void 0 or undefined are called with IJSRuntime.InvokeVoidAsync.

Detect when a Blazor app is prerendering

[!INCLUDE]

Capture references to elements

Some 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 focus 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 {
    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 3rd 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) because Blazor interacts with the DOM to populate this element's list items (<li>):

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

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.

As far as .NET code is concerned, an ElementReference is an opaque handle. The only thing you can do with ElementReference is pass it through to JavaScript code via JS interop. When you do so, the JavaScript-side code receives an HTMLElement instance, which it can use with normal DOM APIs.

For example, the following code defines a .NET extension method that enables setting the focus on an element:

exampleJsInterop.js:

window.exampleJsFunctions = {
  focusElement : function (element) {
    element.focus();
  }
}

To call a JavaScript function that doesn't return a value, use IJSRuntime.InvokeVoidAsync. The following code sets the focus on the username input by calling the preceding JavaScript function with the captured ElementReference:

[!code-cshtml]

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

public static async Task Focus(this ElementReference elementRef, IJSRuntime jsRuntime)
{
    await jsRuntime.InvokeVoidAsync(
        "exampleJsFunctions.focusElement", elementRef);
}

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

[!code-cshtml]

Important

The username variable is only populated after the component is rendered. If an unpopulated ElementReference is passed to JavaScript code, the JavaScript code receives a value of null. To manipulate element references after the component has finished rendering (to set the initial focus on an element) use the OnAfterRenderAsync or OnAfterRender component lifecycle methods.

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

public static ValueTask<T> GenericMethod<T>(this ElementReference elementRef, 
    IJSRuntime jsRuntime)
{
    return jsRuntime.InvokeAsync<T>(
        "exampleJsFunctions.doSomethingGeneric", elementRef);
}

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

[!code-cshtml]

Invoke .NET methods from JavaScript functions

Static .NET method call

To invoke a static .NET method from JavaScript, use the DotNet.invokeMethod or DotNet.invokeMethodAsync functions. Pass in the identifier of the static method you wish to call, the name of the assembly containing the function, and any arguments. The asynchronous version is preferred to support Blazor Server scenarios. To invoke a .NET method from JavaScript, the .NET method must be public, static, and have the [JSInvokable] attribute. By default, the method identifier is the method name, but you can specify a different identifier using the JSInvokableAttribute constructor. Calling open generic methods isn't currently supported.

The sample app includes a C# method to return an array of ints. The JSInvokable attribute is applied to the method.

Pages/JsInterop.razor:

[!code-cshtml]

JavaScript served to the client invokes the C# .NET method.

wwwroot/exampleJsInterop.js:

[!code-javascript]

When the Trigger .NET static method ReturnArrayAsync button is selected, examine the console output in the browser's web developer tools.

The console output is:

Array(4) [ 1, 2, 3, 4 ]

The fourth array value is pushed to the array (data.push(4);) returned by ReturnArrayAsync.

Instance method call

You can also call .NET instance methods from JavaScript. To invoke a .NET instance method from JavaScript:

  • Pass the .NET instance to JavaScript by wrapping it in a DotNetObjectReference instance. The .NET instance is passed by reference to JavaScript.
  • Invoke .NET instance methods on the instance using the invokeMethod or invokeMethodAsync functions. The .NET instance can also be passed as an argument when invoking other .NET methods from JavaScript.

Note

The sample app logs messages to the client-side console. For the following examples demonstrated by the sample app, examine the browser's console output in the browser's developer tools.

When the Trigger .NET instance method HelloHelper.SayHello button is selected, ExampleJsInterop.CallHelloHelperSayHello is called and passes a name, Blazor, to the method.

Pages/JsInterop.razor:

[!code-cshtml]

CallHelloHelperSayHello invokes the JavaScript function sayHello with a new instance of HelloHelper.

JsInteropClasses/ExampleJsInterop.cs:

[!code-csharp]

wwwroot/exampleJsInterop.js:

[!code-javascript]

The name is passed to HelloHelper's constructor, which sets the HelloHelper.Name property. When the JavaScript function sayHello is executed, HelloHelper.SayHello returns the Hello, {Name}! message, which is written to the console by the JavaScript function.

JsInteropClasses/HelloHelper.cs:

[!code-csharp]

Console output in the browser's web developer tools:

Hello, Blazor!

Share interop code in a class library

JS interop code can be included in a class library, which allows you to share the code in a NuGet package.

The class library handles embedding JavaScript resources in the built assembly. The JavaScript files are placed in the wwwroot folder. The tooling takes care of embedding the resources when the library is built.

The built NuGet package is referenced in the app's project file the same way that any NuGet package is referenced. After the package is restored, app code can call into JavaScript as if it were C#.

For more information, see xref:blazor/class-libraries.

Harden JS interop calls

JS interop may fail due to networking errors and should be treated as unreliable. By default, a Blazor Server app times out JS interop calls on the server after one minute. If an app can tolerate a more aggressive timeout, such as 10 seconds, set the timeout using one of the following approaches:

  • Globally in Startup.ConfigureServices, specify the timeout:

    services.AddServerSideBlazor(
    options => options.JSInteropDefaultCallTimeout = TimeSpan.FromSeconds({SECONDS}));

  • Per-invocation in component code, a single call can specify the timeout:

    var result = await JSRuntime.InvokeAsync<string>("MyJSOperation", 
    TimeSpan.FromSeconds({SECONDS}), new[] { "Arg1" });

For more information on resource exhaustion, see xref:security/blazor/server.

Additional resources