ASP.NET Core Blazor JavaScript interoperability (JS interop)
Σημείωση
This isn't the latest version of this article. For the current release, see the .NET 9 version of this article.
Προειδοποίηση
This version of ASP.NET Core is no longer supported. For more information, see .NET and .NET Core Support Policy. For the current release, see the .NET 8 version of this article.
Σημαντικό
This information relates to a pre-release product that may be substantially modified before it's commercially released. Microsoft makes no warranties, express or implied, with respect to the information provided here.
For the current release, see the .NET 9 version of this article.
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
Σημείωση
JavaScript [JSImport]
/[JSExport]
interop API is available for client-side components in ASP.NET Core in .NET 7 or later.
For more information, see JavaScript JSImport/JSExport interop with ASP.NET Core Blazor.
With compression, which is enabled by default, avoid creating secure (authenticated/authorized) interactive server-side components that render data from untrusted sources. Untrusted sources include route parameters, query strings, data from JS interop, and any other source of data that a third-party user can control (databases, external services). For more information, see ASP.NET Core Blazor SignalR guidance and Threat mitigation guidance for ASP.NET Core Blazor interactive server-side rendering.
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.
Σημείωση
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
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.
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 = () => {
...
}
}
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)
{
await module.DisposeAsync();
}
}
}
For more information, see the following resources:
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.
For more information, see the following articles:
For more information, see Call JavaScript functions from .NET methods in ASP.NET Core Blazor.
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
tofalse
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
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.
Blazor supports unmarshalled JS interop when a high volume of .NET objects are rapidly serialized or when large .NET objects or many .NET objects must be serialized. For more information, see Call JavaScript functions from .NET methods in ASP.NET Core Blazor.
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 anid
ofcleanupDiv
. 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 theDOMCleanup.razor.js
file and calls itscreateObserver
function to set up theMutationObserver
callback. These tasks are accomplished in theOnAfterRenderAsync
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)
{
await module.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;
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
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
andIAsyncDisposable
section describes how to implement disposal patterns in Razor components.
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:
- 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:
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.
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.
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