Calling JavaScript functions from a Unity C# class in Unity can be achieved when you're working with a WebGL build, where Unity allows interaction between the Unity environment and the surrounding browser JavaScript. Below are the steps to do this:

1. Using ExternalEval (Deprecated)

In older versions of Unity, you could use Application.ExternalEval. However, this is deprecated, and you should use Application.ExternalCall or Application.ExternalEval.

2. Using Application.ExternalCall (Deprecated)

This was also deprecated but worked similarly to ExternalEval.

3. Using JavaScriptInterface (Recommended)

Unity introduced a more robust way to interact with JavaScript using the JavaScriptInterface for WebGL. The approach is to define a JavaScript function in your web page and call it from your Unity C# script.

4. Using WebGLPlugin (Recommended)

This is the most current and preferred method. Unity provides a way to call JavaScript functions directly from C# using WebGL plugins.

Step-by-Step Guide:

1. Create Your JavaScript File:

First, create a JavaScript file with the functions you want to call. For example, let's create a file called unityFunctions.js:

function ShowAlert(message) {
    alert(message);
}

function AddNumbers(a, b) {
    return a + b;
}

2. Include the JavaScript in Your WebGL Template:

When building for WebGL, include this JavaScript file in your WebGL template or HTML file. Make sure that the JavaScript file is correctly referenced:

<script src="unityFunctions.js"></script>

3. Create a C# Wrapper in Unity:

In Unity, create a C# class that will call these JavaScript functions:

using UnityEngine;
using System.Runtime.InteropServices;

public class WebGLInteraction : MonoBehaviour
{
    // Import the JavaScript functions using DLLImport
    [DllImport("__Internal")]
    private static extern void ShowAlert(string message);

    [DllImport("__Internal")]
    private static extern int AddNumbers(int a, int b);

    // Method to call the JavaScript ShowAlert function
    public void CallShowAlert()
    {
        ShowAlert("Hello from Unity!");
    }

    // Method to call the JavaScript AddNumbers function
    public void CallAddNumbers()
    {
        int result = AddNumbers(5, 3);
        Debug.Log("Result from JavaScript: " + result);
    }
}

4. Call the Functions from Your C# Script:

You can now call these methods in your Unity scripts just like any other C# method:

public class GameManager : MonoBehaviour
{
    void Start()
    {
        WebGLInteraction webGLInteraction = new WebGLInteraction();
        webGLInteraction.CallShowAlert();
        webGLInteraction.CallAddNumbers();
    }
}

Important Notes:

Only for WebGL Builds: This method works only in Unity WebGL builds. If you try to call these methods in the Unity editor or in a non-WebGL build, it will throw errors.
JavaScript Functions Must Be Global: The JavaScript functions that you want to call from Unity must be globally accessible in the web page where your Unity WebGL content is running.
DllImport("__Internal"): This attribute is used to indicate that the methods are external JavaScript functions when targeting WebGL.

How to call javascript functions from Unity C# class in unity?