Godot: JavaScriptBridge Callback Returns Null - SOLVED!

Hey everyone! Let's dive into a common issue that Godot beginners face when trying to create JavaScript functions using JavaScriptBridge: the frustrating null return. This article breaks down the problem, explains why it happens, and provides a solution. If you're seeing that dreaded null and scratching your head, you're in the right place.

Understanding the Issue

The core problem revolves around using JavaScriptBridge.create_callback in Godot to expose functions to JavaScript. The expectation is that when you call these functions from your web page, they should return the values you've defined in your GDScript. However, sometimes, no matter what you do, you just keep getting null. Let's figure out why.

The Scenario

Imagine you have a simple Godot project, and you want to call a GDScript function from your JavaScript code running in a web browser. You set up your JavaScriptBridge, create a callback, and then call the function from your JavaScript console. Instead of getting the expected integer, string, or whatever you're returning, you're greeted with the cold, hard null. This is not just annoying; it's a roadblock. No one wants to start their programming journey with a roadblock, especially when it involves interoping between javascript and Godot. The goal is to pass data back and forth seamlessly, but when null appears, the seamless experience falls apart. So, let's dissect this issue and get your project back on track with effective solutions.

Why Does This Happen?

There are several reasons why JavaScriptBridge.create_callback might return null. It could be an initialization issue, a problem with how the function is being called from JavaScript, or even a subtle bug in the Godot-to-JavaScript bridge itself. It is really easy to create an mistake and it can cost you a lot of time figuring out.

• Initialization Order: Ensure that the JavaScriptBridge is fully initialized before you attempt to create and assign callbacks. The order in which your scripts execute can sometimes lead to the bridge not being ready when you try to use it. This is a common pitfall, especially when dealing with asynchronous operations.
• Incorrect Argument Handling: JavaScript and GDScript handle arguments differently. If you're not correctly passing or receiving arguments, the bridge might fail to marshal the data, resulting in a null return. Always double-check that the types and number of arguments match what your GDScript function expects.
• Asynchronous Issues: When dealing with web environments, asynchronous operations are common. If your JavaScript function relies on asynchronous data, the callback might be executed before the data is available, leading to unexpected results. Use Promises or async/await to handle asynchronous operations gracefully.

Reproducing the Issue

Let's look at a specific example to illustrate the problem. Consider the following GDScript code:

extends Node

var _hello_from_js_callback_ref = JavaScriptBridge.create_callback(_hello_from_js)

# Called when the node enters the scene tree for the first time.
func _ready() -> void:
	var window = JavaScriptBridge.get_interface("window")
	print("helloFromJs 0")
	if window:
		window.helloFromJs = _hello_from_js_callback_ref
		print("helloFromJs 1")

# Called every frame. 'delta' is the elapsed time since the previous frame.
func _process(delta: float) -> void:
	pass

func _hello_from_js(args: Array) -> int:
	print("Hello from JS!")
	print(args)	
	var n: float = args[0] if args.size() > 0 else 0	
	return n + 666

The code above aims to create a JavaScript function called helloFromJs that, when called from the web console, should return a number. However, when you add this script, publish it to the web platform, and execute window.helloFromJs(0) in the web console, it always returns null. Let's fix this.

The Solution

To resolve the null return issue, we need to ensure that the JavaScript function correctly returns a value that Godot can interpret. The key is to use Promise on the JavaScript side to handle the asynchronous nature of the bridge.

JavaScript Modification

Modify the JavaScript function to return a Promise. This allows Godot to properly receive the return value. Here’s how you can do it:

window.helloFromJs = function(arg) {
  return Promise.resolve(yourGodotObject.call("_hello_from_js", arg));
};

In this code, Promise.resolve() ensures that the value returned from the Godot function is wrapped in a Promise, which Godot can then handle correctly. Replace yourGodotObject with the correct reference to your Godot object.

GDScript Adjustments

No changes are needed in the GDScript code provided earlier, as the issue lies in how the JavaScript function handles the return value. The GDScript function _hello_from_js is already set up to return an integer, which will be correctly passed back to JavaScript once the Promise is in place.

Complete Example

Here’s a complete example combining both the GDScript and the modified JavaScript:

GDScript:

extends Node

var _hello_from_js_callback_ref = JavaScriptBridge.create_callback(_hello_from_js)

func _ready() -> void:
	var window = JavaScriptBridge.get_interface("window")
	if window:
		window.helloFromJs = _hello_from_js_callback_ref

func _hello_from_js(args: Array) -> int:
	var n: float = args[0] if args.size() > 0 else 0
	return n + 666

JavaScript:

window.helloFromJs = function(arg) {
  return Promise.resolve(yourGodotObject.call("_hello_from_js", arg));
};

Make sure to replace yourGodotObject with the correct reference to your Godot object instance. With this setup, calling window.helloFromJs(0) from the JavaScript console should now correctly return 666.

Additional Tips

Debugging

• Console Logging: Use print() in GDScript and console.log() in JavaScript to trace the values of variables and the flow of execution. This can help you pinpoint where the null is originating.
• Godot Editor Debugger: The Godot editor debugger can be invaluable for stepping through your GDScript code and inspecting variables in real-time.
• Browser Developer Tools: Use the browser's developer tools to inspect network requests, console output, and JavaScript execution.

Error Handling

• Try-Catch Blocks: Wrap your JavaScript code in try-catch blocks to handle any exceptions that might occur. This can prevent your script from crashing and provide more informative error messages.
• Check for Errors: Always check for errors when calling functions from the JavaScriptBridge. If an error occurs, log it to the console so you can investigate further.

Asynchronous Operations

• Promises: Use Promises to handle asynchronous operations in JavaScript. This makes it easier to manage the flow of execution and ensure that your code is executed in the correct order.
• Async/Await: Consider using async/await syntax to make your asynchronous code more readable and maintainable.

Conclusion

Dealing with null returns from JavaScriptBridge.create_callback can be frustrating, but by understanding the underlying issues and applying the solutions outlined in this article, you can overcome this hurdle. Remember to ensure proper initialization, handle arguments correctly, and use Promises to manage asynchronous operations. Happy coding, and may your Godot-to-JavaScript interactions be null-free!

By following these steps and keeping the tips in mind, you should be well-equipped to tackle the null return issue and create seamless interactions between your Godot project and JavaScript code. Don't let a little null get you down – you've got this!