Blazor JavaScript Interop: Practical Guide for .NET Developers

Have you ever felt stuck trying to bridge Blazor and JavaScript? I’ve been there. You’re deep in a Blazor project, but then you hit that one missing piece — a powerful JavaScript library that Blazor just can’t do without. What next? Well, let’s solve this properly.

In this post, we’re diving headfirst into Blazor and JavaScript interop. I’ll not only show you how to call JavaScript from Blazor and vice versa, but I’ll also explain best practices and real-world scenarios from my own projects. You’ll walk away knowing exactly how to integrate JS smoothly into your Blazor apps — with practical, tested code snippets you can copy straight into your project.

Why Blazor and JavaScript Need Each Other

Before we jump into the code, let’s get something clear. Blazor (both Server and WebAssembly) is powerful, but not omnipotent. Certain browser APIs and popular JS libraries (like Chart.js, Google Maps, or even basic localStorage access) aren’t natively supported in .NET.

This is where JavaScript interop becomes your secret weapon.

Common Use Cases:

• Working with the browser’s native API (Clipboard, LocalStorage, IndexedDB).
• Integrating JS libraries like Chart.js, Leaflet, etc.
• Handling complex DOM manipulations that Blazor doesn’t cover well.
• Performance optimization for some tasks that run faster in JS.

How to Call JavaScript Functions from Blazor

Let’s start with the most common scenario: calling JavaScript from your Blazor component.

Step 1: Prepare Your JavaScript Function

In your wwwroot/index.html (for WASM) or _Host.cshtml (for Server), define your JavaScript function:

<script>
  function showAlert(message) {
    alert(message);
  }
</script>

Step 2: Inject IJSRuntime and Call the Function

In your Blazor component, inject the JS runtime:

@inject IJSRuntime JS

<button @onclick="ShowJsAlert">Show Alert</button>

@code {
    private async Task ShowJsAlert()
    {
        await JS.InvokeVoidAsync("showAlert", "Hello from Blazor!");
    }
}

Explanation:

• We’re injecting the JS runtime service.
• InvokeVoidAsync is used because showAlert does not return a value.
• The parameters are passed directly into the JavaScript function.

Tip: If your JS function returns a value, use InvokeAsync<T> instead.

How to Call Blazor Methods from JavaScript

Interop works both ways! JavaScript can call your Blazor methods too.

Step 1: Add JS Function to Call .NET Method

<script>
  function callDotNetMethod(dotNetHelper) {
    dotNetHelper.invokeMethodAsync("ShowMessage").then(() => {
      console.log(".NET method called successfully!");
    });
  }
</script>

Step 2: Expose Blazor Method to JS

In your component:

@inject IJSRuntime JS

<button @onclick="TriggerJsFunction">Call .NET Method from JS</button>

@code {
    private DotNetObjectReference<YourComponent> objRef;

    protected override void OnInitialized()
    {
        objRef = DotNetObjectReference.Create(this);
    }

    private async Task TriggerJsFunction()
    {
        await JS.InvokeVoidAsync("callDotNetMethod", objRef);
    }

    [JSInvokable]
    public void ShowMessage()
    {
        Console.WriteLine("Called from JavaScript!");
    }

    public void Dispose()
    {
        objRef?.Dispose();
    }
}

Explanation:

• DotNetObjectReference.Create(this) exposes your Blazor component instance to JS.
• [JSInvokable] attribute makes the method callable from JS.
• Don’t forget to clean up references by disposing of objRef.

Tip: Always dispose of DotNetObjectReference to avoid memory leaks!

Pass Complex Data Between Blazor and JavaScript

Let’s go further. How do you pass complex objects (like JSON) between Blazor and JS?

Example: Passing Object to JS

In JS:

<script>
  function processData(data) {
    console.log("Received data:", data);
  }
</script>

In Blazor:

await JS.InvokeVoidAsync("processData", new { Name = "Blazor", Year = 2025 });

Explanation:

• Blazor serializes anonymous objects to JSON automatically.
• JS receives them as native objects.

Example: Returning Object from JS

In JS:

<script>
  function getData() {
    return { name: "JavaScript", version: "ES2025" };
  }
</script>

In Blazor:

var result = await JS.InvokeAsync<Dictionary<string, string>>("getData");
Console.WriteLine(result["name"]); // Outputs: JavaScript

Explanation:

• We deserialize the returned JS object into a .NET dictionary.
• This works great for simple data structures.

Best Practices for Blazor-JS Interop

• Use IJSRuntime.InvokeAsync<T> for async operations.
• Minimize JS dependencies — prefer .NET where possible.
• Dispose of DotNetObjectReference to prevent memory leaks.
• Wrap JS interop calls in services for better maintainability.
• Use try-catch blocks to handle JS errors gracefully.

My Experience: In my recent Blazor WASM project, wrapping JS interop in dedicated services dramatically improved testability and code readability. For example, I created a LocalStorageService to abstract all my local storage operations — and it saved me hours when debugging!

FAQ: Blazor-JavaScript Interop Questions Answered

Conclusion: Bring Blazor and JavaScript Together!

As you’ve seen, Blazor and JavaScript are better together. Whether you’re adding a sprinkle of JS magic to your Blazor app or integrating full libraries, interop is your bridge to a more powerful application.

My advice? Start small. Try creating a simple JS function and call it from Blazor today. Then expand as your app grows!

What’s your experience with Blazor and JS interop? Share your wins or struggles in the comments below! Let’s learn together.