`new.target` Warning In Melange Raw JS: Causes & Solutions

Dec 3, 2025 by Alex Johnson 59 views

Understanding the `new.target` Warning in Melange Raw JS

Have you ever encountered an unexpected warning while working with Melange and raw JavaScript? Specifically, the "Warning: 103 [melange-ffi-warning] FFI warning: Unexpected token "." when using new.target within a mel.raw block? If so, you're not alone! This article dives deep into this issue, exploring why it happens and potential solutions to keep your code clean and warning-free.

What is `new.target`?

Before we delve into the specifics of the warning, let's quickly recap what new.target actually does in JavaScript. According to MDN Web Docs, new.target is a special property available inside constructors (classes or functions used with the new keyword). It holds a reference to the constructor that was directly invoked by new. This is particularly useful in inheritance scenarios to determine which class was originally instantiated.

For example, consider this simple JavaScript class:

class MyClass {
  constructor() {
    console.log(new.target); 
  }
}

new MyClass(); // Output: class MyClass

In this case, new.target inside the constructor of MyClass will log MyClass itself. If MyClass were extended by another class, new.target would reflect the subclass if it was instantiated directly.

The Melange and `mel.raw` Context

Melange is a delightful language that compiles to clean, readable, and performant JavaScript. It allows you to write OCaml code and seamlessly integrate it with existing JavaScript ecosystems. The mel.raw block is a powerful feature that lets you embed raw JavaScript code directly within your Melange code. This is incredibly useful when you need to interact with JavaScript libraries or leverage specific JavaScript features that don't have direct OCaml equivalents.

However, this is where the new.target warning comes into play. When you use new.target inside a mel.raw block, Melange's foreign function interface (FFI) might not correctly parse it, leading to the "Unexpected token "." warning. This doesn't necessarily mean your code is broken, but it's an indication that Melange's parser is stumbling over the syntax.

Why Does This Warning Occur?

The warning arises from how Melange processes the code within mel.raw blocks. Melange's parser, responsible for understanding the JavaScript code, might not fully recognize the new.target syntax, especially within the context of the FFI. This is similar to the issue reported in Flow (a static type checker for JavaScript) as mentioned in the original discussion: Flow Issue #1152. These types of issues often stem from the evolving nature of JavaScript syntax and the challenges of keeping parsers up-to-date with the latest features.

While the code might still function as expected, the warning can be bothersome and might mask other genuine issues. Therefore, it's essential to understand how to address it.

Reproducing the Warning

The original post provides a concise code snippet to reproduce the warning:

let () =
    [%mel.raw{| new.target |}]

You can even try this out directly in the Melange Playground. Simply paste the code into the editor, and you'll see the warning appear in the output.

Potential Solutions and Workarounds

So, how can you tackle this warning? Here are a few approaches you can consider:

1. Re-thinking the implementation

Sometimes the warning can be avoided by revisiting the original goal and implementation and re-writing the code in a Melange friendly way.

2. Using an Intermediate JavaScript Function:

One common workaround is to define a small JavaScript function outside the mel.raw block that encapsulates the new.target logic. You can then call this function from within your mel.raw code. This approach helps isolate the problematic syntax and allows Melange to parse the overall structure more effectively.

Here's how you might implement this:

module Internal = {
  %%raw(
    val get_new_target: unit => Js.nullable('a) = ""
    let get_new_target = () => new.target;
  )
}

let () = {
  let target = Internal.get_new_target()
  /* … */
}

In this example, get_new_target is a JavaScript function that simply returns new.target. By calling this function from within the mel.raw block, you avoid directly using new.target in a way that triggers the warning.

3. Conditional Compilation (If Applicable):

If the code involving new.target is only relevant in specific environments (e.g., when targeting a particular JavaScript runtime), you can use conditional compilation to exclude the code from other builds. This ensures that the warning doesn't appear in contexts where it's not applicable.

4. Ignoring the Warning (With Caution):

In some cases, the warning might be benign and not affect the actual functionality of your code. If you've thoroughly tested your code and are confident that the new.target usage is correct, you could choose to ignore the warning. However, this should be a last resort, and you should always document why you're ignoring the warning to avoid confusion later.

5. Check for Melange Updates:

The Melange team is continuously working on improving the language and its tooling. It's possible that future versions of Melange will address this parsing issue directly. Therefore, it's always a good idea to check for updates and see if the warning has been resolved.

Key Takeaways

The "Unexpected token "." warning when using new.target in Melange's mel.raw block is a known issue related to how Melange's parser handles this specific JavaScript syntax. While the code might still function correctly, addressing the warning is crucial for maintaining code clarity and avoiding potential masking of other issues.

The workarounds discussed, such as using intermediate JavaScript functions or conditional compilation, provide effective ways to mitigate the warning. Remember to choose the approach that best suits your specific use case and coding style.

By understanding the root cause of the warning and employing these solutions, you can continue to leverage the power of Melange and raw JavaScript while keeping your codebase clean and maintainable.

For further exploration of JavaScript features and best practices, consider visiting MDN Web Docs, a comprehensive resource for web developers.