Blazor Tutorial Invoke JavaScript Functions

Blazor is a frontend framework, but it has no direct access to the browser’s DOM API.

As web developers, we want to access browser APIs and existing JavaScript functions.
There are times when Blazor .NET code is required to call a JavaScript function.
For example, a JavaScript call can expose browser capabilities or functionality from a JavaScript library to the Blazor app.

JavaScript Interop

A Blazor app can invoke JavaScript functions from .NET and .NET methods from JavaScript code. This property of calling a JS method from C# code and vice versa is referred to as JavaScript Interop.

Blazor uses JavaScript Interop to handle DOM manipulation and browser API calls.
To call a JavaScript from .NET, use the IJSRuntime abstraction, which is accessible from JSRuntime.Current.

Let's add two JavaScript functions to index.html file.

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <meta name="viewport" content="width=device-width">
    <title>BlazorApplication</title>
    <base href="/" />
    <link href="css/bootstrap/bootstrap.min.css" rel="stylesheet" />
    <link href="css/site.css" rel="stylesheet" />
</head>
<body>
    <app>Loading...</app>

    <script src="_framework/blazor.webassembly.js"></script>
    <script type="blazor-boot">
    </script>
    <script>
       function JSMethod() {
            $("#demop").text("JavaScript function called from C#");
        }
    </script>
    <script>
        function CSMethod() {
            DotNet.invokeMethodAsync('BlazorApplication', 'CSCallBackMethod');
        }
    </script>
</body>
</html>

The JSMethod function will set the text of a <p> tag having id "demop" to "JavaScript function called from C#".
This CSMethod function will have a call back to our C# method CSCallBackMethod which we will define later.

Now create a new Blazor component JSInterop.cshtml and add the following functions.

@functions {
    protected static string message { get; set; }
    
    protected void CallJSMethod()
    {
        JSRuntime.Current.InvokeAsync<bool>("JSMethod");
    }

    protected void CallCSMethod()
    {
        JSRuntime.Current.InvokeAsync<bool>("CSMethod");
    }
    
    [JSInvokable]
    public static void CSCallBackMethod()
    {
        message = "C# function called from JavaScript";
    }
}

The CallJSMethod and CallCSMethod will call our JS functions JSMethod and CSMethod by using JSRuntime.Current.InvokeAsync method.
The JSRuntime.Current.InvokeAsync method can take two parameters; the JS function name and any parameter that needed to be supplied to the JS function. But, we are not passing any parameter to JS function.
The CSCallBackMethod is a static method, and it will be called from the JavaScript function CSMethod.
It is decorated with [JSInvokable] attribute.
This method will set the value of a string variable message, which will be displayed on the UI.

Add two buttons to the JSInterop.cshtml file will call the CallJSMethod and CallCSMethod method on the onclick event.

@page "/jsinterop"
@using BlazorApplication.Pages
@using Microsoft.JSInterop

<h1>JavaScript Interop Demo</h1>

<hr />

<button class="btn btn-primary" onclick="@CallJSMethod">Call JS Method</button>
<button class="btn btn-primary" onclick="@CallCSMethod">Call C# Method</button>
<br />
<p id="demop"></p>
<br />
<p>@message</p>

@functions {
    protected static string message { get; set; }
    
    protected void CallCSMethod()
    {
        JSRuntime.Current.InvokeAsync<bool>("CSMethod");
    }
    
    protected void CallJSMethod()
    {
        JSRuntime.Current.InvokeAsync<bool>("JSMethod");
    }
    
    [JSInvokable]
    public static void CSCallBackMethod()
    {
        message = "C# function called from JavaScript";
    }
}

Click on the buttons to call the JavaScript functions and C# method.

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