Learning Blazor JavaScript Interop: A Beginner's Guide

Mastering Blazor JavaScript Interop


Welcome to our comprehensive guide on integrating Blazor with JavaScript and external libraries. In this post, we aim to unravel the complexities of Blazor, a powerful web framework by Microsoft, and its interoperability with JavaScript. Whether you are a seasoned developer or just starting, this guide will walk you through the key concepts, practical examples, and best practices for leveraging JavaScript within Blazor applications.

Blazor, a feature of ASP.NET, allows developers to build interactive web UIs using C#. The unique aspect of Blazor is its ability to run C# code directly in the browser alongside JavaScript. This capability brings together the robustness of C# and the versatility of JavaScript, making it a preferred choice for modern web development. In the following sections, we will explore the JavaScript interop, learn how to call JavaScript functions from C# and vice versa, and delve into the integration with popular JavaScript libraries.

Understanding JavaScript Interop in Blazor

JavaScript Interop (interoperability) in Blazor is a mechanism that enables interaction between JavaScript and C# code. This capability is vital in scenarios where Blazor lacks certain functionalities that JavaScript libraries offer. JavaScript interop fills this gap, allowing developers to harness the strengths of both languages.

Why is JavaScript Interop Important?

Blazor applications, by nature, are executed on the .NET runtime. However, the web ecosystem is replete with JavaScript-based functionalities, from DOM manipulation to utilizing external libraries. JavaScript interop in Blazor bridges this gap, providing the flexibility to tap into JavaScript’s capabilities whenever needed.

Basic Concepts of JavaScript Interop

  1. Invocation Direction: The interop can be initiated from either C# to JavaScript or vice versa.
  2. Interop Methods: Blazor provides built-in methods to facilitate this communication.
  3. Serialization: Data exchanged between C# and JavaScript is serialized and deserialized as JSON.

Examples and Use Cases

Imagine you need to implement a feature that’s not readily available in Blazor, like a complex animation. With JavaScript interop, you can seamlessly call a JavaScript function to handle this, all from your C# code.

// Example of calling a JavaScript function from C#
await JSRuntime.InvokeVoidAsync("yourJavascriptFunction", args);

This code snippet demonstrates a basic call to a JavaScript function from C#. JSRuntime is the Blazor service that handles JavaScript interop, InvokeVoidAsync is the method to call a JavaScript function, and "yourJavascriptFunction" is the JavaScript function being called.

Calling JavaScript Functions from C#

Integrating JavaScript functions into a Blazor application can significantly enhance its capabilities. This section will guide you through the process of invoking JavaScript functions from C# code in a Blazor application, complete with practical examples.

How to Call JavaScript Functions from C#

To call a JavaScript function from C#, Blazor uses the IJSRuntime interface. This process involves two primary steps: defining the JavaScript function and invoking it from C#.

1. Defining the JavaScript Function

First, define the JavaScript function you intend to call. This function should be accessible globally, typically defined in a script tag or an external JavaScript file. For example:

// In your .html or .js file
function showAlert(message) {

This JavaScript function, showAlert, takes a message as a parameter and displays it using the browser’s alert function.

2. Invoking the JavaScript Function from C#

With the JavaScript function defined, you can now invoke it from your C# code using IJSRuntime. Here’s how you can do it:

@inject IJSRuntime JSRuntime

@code {
    private async Task CallJavaScriptFunction()
        await JSRuntime.InvokeVoidAsync("showAlert", "Hello from C#");

In this C# code snippet, IJSRuntime is injected into the Blazor component. The CallJavaScriptFunction method uses InvokeVoidAsync to call the showAlert JavaScript function, passing “Hello from C#” as an argument.

Practical Use Cases

  • Integrating third-party libraries for features not available in Blazor.
  • Performing operations on the DOM that are outside the scope of Blazor.
  • Leveraging existing JavaScript code within a Blazor application.

Invoking C# Methods from JavaScript

While calling JavaScript from C# is common, sometimes the reverse is necessary. Blazor also supports invoking C# methods from JavaScript, which can be particularly useful for handling callbacks or integrating with complex JavaScript libraries.

How to Invoke C# Methods from JavaScript

The process of calling C# from JavaScript involves exposing a C# method and then invoking it from your JavaScript code.

1. Exposing a C# Method

To expose a C# method to JavaScript, use the [JSInvokable] attribute. This makes the method callable from JavaScript. Here’s an example:

@code {
    public static Task ShowMessage(string message)
        Console.WriteLine($"Message from JavaScript: {message}");
        return Task.CompletedTask;

In this example, the ShowMessage method is marked with [JSInvokable], indicating that it can be called from JavaScript.

2. Calling the C# Method from JavaScript

After exposing the C# method, you can invoke it from JavaScript using DotNet.invokeMethod or DotNet.invokeMethodAsync. Here’s how:

DotNet.invokeMethodAsync('YourAssemblyName', 'ShowMessage', 'Hello from JavaScript')
    .then(data => {

This JavaScript code calls the ShowMessage C# method, passing a string argument.

Practical Applications

  • Handling JavaScript events in C#.
  • Integrating Blazor components with complex JavaScript UI interactions.
  • Utilizing C# logic in response to actions taken in JavaScript-driven components.

Integrating with External JavaScript Libraries

Blazor’s ability to work alongside JavaScript opens up a world of possibilities, allowing developers to leverage the vast ecosystem of JavaScript libraries. In this section, we’ll explore how to integrate popular libraries into your Blazor projects.

Steps for Integrating JavaScript Libraries in Blazor

1. Include the JavaScript Library

First, include the JavaScript library in your Blazor project. This can typically be done by adding a script tag in your _Layout.cshtml (for server-side Blazor) or index.html (for client-side Blazor). For instance:

<script src="https://cdn.jsdelivr.net/npm/your-javascript-library@version"></script>

2. Calling Library Functions from C#

Once included, you can call functions from the JavaScript library in a similar way to calling any JavaScript function from C#.

await JSRuntime.InvokeVoidAsync("libraryFunction", args);

Example: Integrating a Chart Library

Let’s take an example of integrating a chart library, like Chart.js, into a Blazor application.

1. Adding Chart.js

Include Chart.js by adding the following script tag to your index.html or _Layout.cshtml:

<script src="https://cdn.jsdelivr.net/npm/chart.js"></script>

2. Creating a Chart in JavaScript

Define a JavaScript function to create a chart. This function can be placed in a separate JavaScript file or within a script tag:

function createChart(chartId, chartType, chartData) {
    var ctx = document.getElementById(chartId).getContext('2d');
    new Chart(ctx, {
        type: chartType,
        data: chartData,
        options: {}

3. Calling the JavaScript Function from C#

Invoke the createChart function from your Blazor component:

@inject IJSRuntime JSRuntime

@code {
    private async Task InitializeChart()
        var chartData = new
            // ... define your chart data here

        await JSRuntime.InvokeVoidAsync("createChart", "myChart", "bar", chartData);

Best Practices and Tips

  • Keep the JavaScript code minimal and only for functionalities not available in Blazor.
  • Ensure the JavaScript library is compatible with Blazor and doesn’t conflict with its lifecycle.
  • Consider using JavaScript isolation in Blazor for better maintainability.

Benefits of Using JavaScript Libraries in Blazor

  • Access to a wide range of functionalities not natively available in Blazor.
  • Enhanced user experience with rich UI components.
  • Leveraging existing JavaScript code and libraries.


In this post, we’ve explored the powerful capability of Blazor to interoperate with JavaScript, enabling developers to harness the best of both worlds – the robustness of C# and the flexibility of JavaScript. We covered the essentials of JavaScript interop in Blazor, demonstrated how to call JavaScript functions from C# and vice versa, and provided insights into integrating external JavaScript libraries.

Remember, the key to successful integration lies in understanding the strengths of each technology and using them complementarily. Blazor and JavaScript together open up a realm of possibilities for building rich, interactive web applications.

As you embark on your journey to integrate JavaScript with Blazor, experiment with the concepts and examples provided, and don’t hesitate to dive into the vast resources available online for deeper understanding and more complex scenarios.

Blazor is an ever-evolving framework, and staying abreast of the latest developments will undoubtedly benefit your web development endeavors. Happy coding!

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