Create a new empty C# project with the following configuration (via .csproj):

<Project Sdk="Microsoft.NET.Sdk.BlazorWebAssembly">

    <PropertyGroup>
        <TargetFramework>net6.0</TargetFramework>
        <LangVersion>10</LangVersion>
    </PropertyGroup>

    <ItemGroup>
        <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly" Version="6.0.0" />
        <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly.DevServer" Version="6.0.0" PrivateAssets="all" />
    </ItemGroup>

</Project>

Initialize Blazor JS runtime and specify the bindings:

namespace WasmTest;

public class Program
{
    private static IJSRuntime js;

    private static async Task Main (string[] args)
    {
        var builder = WebAssemblyHostBuilder.CreateDefault(args);
        var host = builder.Build();
        js = host.Services.GetRequiredService<IJSRuntime>();
        await host.RunAsync();
    }

    [JSInvokable]
    public static async Task<string> BuildMessage (string name)
    {
        var time = await GetTimeViaJS();
        return $"Hello {name}! Current time is {time}.";
    }

    public static async Task<DateTime> GetTimeViaJS ()
    {
        return await js.InvokeAsync<DateTime>("getTime");
    }
}

Publish with dotnet publish and use the C# library from JS:

<script src="_framework/blazor.webassembly.js" autostart="false"></script>

<script>
    window.getTime = () => {
        return new Date().toJSON();
    };
    window.onload = async function () {
        await Blazor.start();
        const msg = await DotNet.invokeMethodAsync("WasmTest", "BuildMessage", "John");
        console.log(msg);
    };
</script>

Alternatively, here is a solution, that allows compiling C# project into single-file UMD library, which can be consumed in any JavaScript environment: browsers, node, and custom restricted environments, such as VS Code's web extensions: https://github.com/Elringus/DotNetJS