Table of Contents

Emulating IJSRuntime

It is common for Blazor components to use IJSRuntime to call JavaScript, and since bUnit does not run JavaScript, emulating IJSRuntime is needed for components that use it. In that regard, IJSRuntime is no different than other services that a component might depend on.

bUnit comes with it's own JSInterop, a tailor-made implementation of IJSRuntime that is active by default, allowing you to specify how JavaScript interop calls should be handled and what values the calls should return, and also allowing you to verify that they the calls have happened. The implementation runs in "strict mode", which means it will throw an exception if it receives an invocation it has not been configured to handle. See more about strict vs. loose mode in the following section.

If you prefer to use the same mocking framework for all mocking in your tests to keep things consistent, general-purpose mocking frameworks like Moq, JustMock Lite, or NSubstitute all work nicely with bUnit and can be used to mock IJSRuntime. In general, registering an implementation of IJSRuntime with bUnit's Services collection replaces bUnit's implementation.

The following sections show how to use the built-in implementation of IJSRuntime.


In the beta versions of bUnit you had to explicitly add the mock JSRuntime by calling Services.AddMockJSRuntime(). That is no longer needed, and indeed doesn't work any more.

Strict vs. loose mode

bUnit's JSInterop can run in two modes, strict or loose:

  • Loose mode configures the implementation to just return the default value when it receives an invocation that has not been explicitly set up, e.g. if a component calls InvokeAsync<int>(...) the mock will simply return default(int) back to it immediately.
  • Strict mode configures the implementation to throw an exception if it is invoked with a method call it has not been set up to handle explicitly. This is useful if you want to ensure that a component only performs a specific set of IJSRuntime invocations.

By default, the bUnit's JSInterop runs in Strict mode. To change the mode, do the following:

JSInterop.Mode = JSRuntimeMode.Loose;

Setting up invocations

Use the Setup<TResult>(...) and SetupVoid(...) methods to configure the implementation to handle calls from the matching InvokeAsync<TResult>(...) and InvokeVoidAsync(...) methods on IJSRuntime.

Use the parameterless Setup<TResult>() method to emulate any call to InvokeAsync<TResult>(...) with a given return type TResult and use the parameterless SetupVoid() to emulate any call to InvokeVoidAsync(...).

When an invocation is set up through of the Setup<TResult>(...) and SetupVoid(...) methods, a JSRuntimePlannedInvocation<TResult> object is returned. This can be used to set a result or an exception, to emulate what can happen during a JavaScript interop call in Blazor.

Similarly, when the parameterless Setup<TResult>() and SetupVoid() methods are used a JSRuntimeCatchAllPlannedInvocation<TResult> object is returned which can be used to set the result of invocation.

Here are two examples:

// Set up an invocation and specify the result value immediately
JSInterop.Setup<string>("getPageTitle").SetResult("bUnit is awesome");

// Set up an invocation without specifying the result
var plannedInvocation = JSInterop.SetupVoid("startAnimation");

// ... other test code

// Later in the test, mark the invocation as completed.
// SetResult() is not used in this case since InvokeVoidAsync
// only completes or throws, it doesn’t return a value.
// Any calls to InvokeVoidAsync(...) up till this point will
// have received an incomplete Task which the component 
// is awaiting until the call to SetVoidResult() below.

Verifying invocations

All calls to the InvokeAsync<TResult>(...) and InvokeVoidAsync(...) methods in bUnit's JSInterop are stored in its Invocations list, which can be inspected and asserted against. In addition to this, all planned invocations have their own Invocations lists which only contain their invocations.

Invocations are represented by the JSRuntimeInvocation type, which has three properties of interest when verifying an invocation happened as expected:

  • Identifier - the name of the function name/identifier passed to the invoke method.
  • Arguments - a list of arguments passed to the invoke method.
  • CancellationToken - the cancellation token passed to the invoke method (if any).

To verify these, just use the assertion methods you normally use.

Support for IJSInProcessRuntime and IJSUnmarshalledRuntime

bUnit's IJSRuntime supports being cast to the IJSInProcessRuntime and IJSUnmarshalledRuntime types, just like Blazor's IJSRuntime.

To set up a handler for an Invoke or InvokeUnmarshalled call, just use the regular Setup and SetupVoid methods on bUnit's JSInterop.

Support for importing JavaScript Modules

Since the .NET 5 release of Blazor, it has been possible to import JavaScript modules directly from components. This is supported by bUnit's JSInterop through the SetupModule methods, that sets up calls to InvokeAsync<IJSObjectReference>.

The SetupModule methods return a module JSInterop, which can be configured to handle JavaScript calls using the Setup and SetupVoid methods. For example, to configure bUnit's JSInterop to handle an import of the JavaScript module hello.js, and a call to the function world() in that model, do the following:

var moduleInterop = JSInterop.SetupModule("hello.js");

Module Interop Mode

By default, a module Interop inherits the Mode setting from the root JSInterop in bUnit. However, you can override it explicitly and have it in a different mode from another module's Interop or the root JSInterop. Just set the Mode property, e.g.:

var moduleInterop = JSInterop.SetupModule("hello.js");
moduleInterop.Mode = JSRuntimeMode.Loose;

Support for IJSInProcessObjectReference and IJSUnmarshalledObjectReference

bUnit's IJSObjectReference supports being cast to the IJSInProcessObjectReference and IJSUnmarshalledObjectReference types, just like Blazor's IJSObjectReference.

To set up a handler for an Invoke or InvokeUnmarshalled call, just use the regular Setup and SetupVoid methods on bUnit's JSInterop.

First-party JSInterop component emulation

Blazor comes out of the box with a few components that require a working JSInterop. bUnit's JSInterop is setup to emulate the JavaScript interactions of those components. The following sections describe how the interaction is emulated for the supported components.

JSInterop emulation

The <Virtualize> component requires JavaScript to notify it about the available screen space it is being rendered to, and also when the users scrolls the viewport, to trigger the loading of new data. bUnit emulates this interaction by telling the <Virtualize> component that the viewport is 1,000,000,000 pixels large. That should ensure that all items are loaded, which makes sense in a testing scenario.

To test the <Placeholder> template of the <Virtualize> component, create an items provider that doesn't return all items when queried.

FocusAsync JSInterop emulation

Support for the FocusAsync method on ElementReference in Blazor's .NET 5 release works by simply registering the invocations, which can then be verified to have occurred.

To verify that the FocusAsync has been called in the <ClickToFocus> component:

<input @ref="exampleInput" />

<button @onclick="ChangeFocus">Focus the Input Element</button>

@code {
    private ElementReference exampleInput;
    private async Task ChangeFocus()
        await exampleInput.FocusAsync();

Do the following:

var cut = RenderComponent<ClickToFocus>();
var inputElement = cut.Find("input");

cut.Find("button").Click(); // Triggers onclick handler that sets focus of input element

JSInterop.VerifyFocusAsyncInvoke() // Verifies that a FocusAsync call has happened
   .Arguments[0] // gets the first argument passed to the FocusAsync method
   .ShouldBeElementReferenceTo(inputElement); // verify that it is an element reference to the input element.

Support for IJSInProcessRuntime and IJSUnmarshalledRuntime

bUnit's IJSRuntime supports being cast to the IJSInProcessRuntime and IJSUnmarshalledRuntime types, just like Blazor's IJSRuntime.

To set up a handler for Invoke and InvokeUnmarshalled calls, just use the regular Setup and SetupVoid methods on bUnit's JSInterop.

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