Blazor WASM Javascript Interop and Isolation

Blazor WASM Javascript Interop and Isolation

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In Blazor WASM we should try to have minimal usage of Javascript. Though Blazor WASM runs on top of web assembly, we might sometime require access to javascript for certain operations like displaying alert, prompt setting theme on html element, focusing a suggestion from autocomplete input on up and down keyboard key press, getting viewport size or at few cases we might need to use the existing javascript library to render chart, grid, etc.

The above usecases can be solved in two different ways as mentioned below.

  1. Javascript Interop
    1. Calling JS functions from .Net
    2. Calling .Net methods from JS
  2. Javascript Isolation

Javascript Interop

To start using Javascript using JS interop, first we need our javascript code in a separate js file place inside wwwroot and we need to inject IJSRuntime in the component to invoke our javascript function.

Let's look on how to display a simple javascript alert in blazor. We'll start with defining a blazorInterop object just to make sure we don't pollute the global window object. Now let's register a simple displayAlert function which will take text and display it in javascript alert().

Code Sample - JS function to be called from .NET which returns no value

Calling JS from .Net

Befor we proceed on invoking the function make sure to add a reference to your jsvascript file in index.html. Now let's inject IJSRuntimr into our component and call blazorInterop.displayAlert("I ❤️ DotNet") using InvokeVoidAsync as we don't have any return value from the javascript function.

Code Sample - Alert Component to call JS from .NET which returns no value

Demo - Javascript Interop - Displaying Alert

Voila.. now we are using javascript logics in our blazor app. That's a fairly simple example. Let's look at another example where we need some values from javascript function. If we want to find the current viewport width and height, then we can levearge javascript to get window height and width and return it back to blazor. The steps are same like before except this time we need to call the javascript function using InvokeAsync<WindowDimensions>("blazorInterop.getViewportDimensions").

Code Sample - JS function to be called from .NET which returns value

Code Sample - JS function to be called from .NET which returns value

Demo - Javascript Interop - Finding Viewport Dimensions

Window Width: 0 px

Window Height: 0 px

Calling .Net from JS

There are situations where you need to call a .Net method from JS function. One example is clicking on a point in chart to display additional information. Now you need to pass some information from JS to .NET and make API call to retrieve additional information. You can call static method or .NET instance method from JS. Let's see how to do this in blazor.

Invoking Static Method

Calling a static method is pretty simple. All you need to do is to add [JSInvokable] attribute to your .NET static method and call the static .Net method from JS using DotNet.invokeMethod('Namespace', '.Net Static Method Name').

Code Sample - .NET static method to be called from JS

Code Sample - .NET static method to be called from JS

Demo - Javascript Interop - Calling .Net Static Method From JS

Invoking Instance Method

Calling an component instance method needs an .NET instance by reference to be passed to JS. This can be done by warpping the component instance inside DotNetObjectReference<ComponentInstance> and calling create on it. Then we can invoke .NET method from JS using InvokeMethod or InvokeMethodAsync from the passed instance reference. Finally we need to dispose the DotNetObjectReference.

Code Sample - .NET instance method to be called from JS

Code Sample - .NET instance method to be called from JS

Demo - Javascript Interop - Calling .Net Instance Method From JS

Javascript Isolation

Blazor supports JS isolation in standard JS modules. This has the following benefits:

  1. No need to add reference to js file in index.html
  2. Imported JS no longer pollutes the global namespace.
  3. Consumers of a library and components aren't required to import the related JS.

With Javascript Isolation, the steps are same but with an extra step to load JS Module using IJSRuntime into IJSObjectReference (module). After loading the module, we need to call JS functions using IJSObjectReference module InvokeAsync instead of IJSRuntime. We also need to make sure the module is disposed on component dispose.

Code Sample - .NET instance method to be called from JS Module

Code Sample - .NET instance method to be called from JS Module

Demo - Javascript Isolation - Calling Javascript prompt from JS Module

Your message:

  • Blazor
  • Javascript
  • Javascript Interop
  • Javascript Isolation