Saturday, 19 May 2018

Call a Javascript function from Blazor page

This article will show how a Js function can be called from a Blazor page. First off, create a button like this:
 <button class="btn btn-warning" onclick="@SayHelloToBlazor">Click me!</button>
Then define the .Net method to handle the onclick event.
@using Microsoft.AspNetCore.Blazor.Browser.Interop

 private async void ShowAlert()
    if (RegisteredFunction.Invoke("showAlert", "Hello World!"))
        Console.WriteLine("The Js function showAlert was called!");
The code invokes a registered function with the invoke method and passing in the method name and an argument. Note that you must add the Interop namespace to Blazor in AspNetCore. We then add the Js function, but Blazor will give you a compiler error if you put the Js function in the same file as the Blazor page. Instead, add it in the index.html file under wwwroot folder of your Blazor project (check the wwwroot folder). You can define the Js function in a .js file or right into the index.html file.

    Blazor.registerFunction('showAlert', (msg) => {
        return true;


Note that if you refactor the Js method, your reference in the .DotNet code of Blazor will of course go stale, and you must update it. Since we return true (as Blazor wants you to do), we can act upon that in the Blazor code as a callback (check the async modifier of the DotNet method). That is what you need to do to get started with calling Javascript from Blazor page running in DotNetCore 2.1 and later.