Some days ago this problem was already addressed by PR #2525. It removed the duplicate class declarations.

@jpstotz I saw that, but it only removes duplicates when you export multiple functions at the same time.

With var you can just keep generating new snippets and pasting them into your main frida script without having to constantly delete the class definition line or changing it to var yourself.

So scenario:

1. Identify interesting function, generate frida script and paste into hook.js
2. Identify second interesting function based off result of first hook, generate frida script and paste into hook.js
3. => Clash, due to duplicate class definition

EDIT
Alternatively, we could just use Java.use() directly for the hook instead of putting it in a variable. It might have a tiny performance impact, but would make it work in all cases:

Java.use("com.google.android.material.appbar.AppBarLayout$BaseBehavior$a$a")["createFromParcel"].overload('android.os.Parcel', 'java.lang.ClassLoader').implementation = function (parcel, classLoader) {
    console.log(`C0049a.createFromParcel is called: parcel=${parcel}, classLoader=${classLoader}`);
    let result = this["createFromParcel"](parcel, classLoader);
    console.log(`C0049a.createFromParcel result=${result}`);
    return result;
};

Using var may seem convenient for copy-pasting multiple snippets, but it introduces several issues that can negatively affect script quality.

• var is function-scoped, not block-scoped, which can lead to unexpected variable leaks or overwriting in larger scripts.
• It allows re-declaration, which can silently override existing variables and make debugging harder.

I still think var makes sense since the full classname is used as the variable name, so it would be weird if any other value is assigned to it other than a Frida reference to that class.

So the choice is between:

• Generating scripts that break your frida script due to re-definition of let if you use the feature more than once
• Generating scripts that potentially introduce side-effects. I feel like this is not a problem, since you are the one choosing to copy code into existing code, and hence the risk is always there
• Generating scripts that never break by not using a variable in the first place
• Generating scripts that never break by wrapping them in a code block

Well, I agree, that let is better for cleaner and error-prone code, but Frida hooks is just a quick way to get some info, so I think it is fine to use var in hooks to simplify adding more methods.
And if such topic will be brought up again, we will use a workaround by inlining class variable as @TheDauntless suggested 🤣