[magicalist]

That looks like ActionScript 2, so the javascript convention was actually well in place by then. I can't speak confidently about the late 90s, but I wouldn't be surprised if it predated Actionscript 1 as well.

But doesn't it all trace back to C conventions anyways?

[zeroq]

Not at all.

AS1 was plain ES3.

movieclip._x or movieclip._alpha (as in position and alpha blending) were accessed by properties with underscore to denote that you were using accessors (getters/setters).

Internally properties like Object.prototype.__proto__ had two underscores to denote they were private.

In AS2 you had:

  class Foo extends Bar implements Xyz {
    private foo
    public function set x (value : Number) : void { ... }
    public function get x () : Number { }

AS2 was based on long forgotten ES4 specification [0]. Under the hood it was still, what we would call today, transpiled to ES3 bytecode, as it run basically on the same VM as AS1.

It's crazy to think it took us 10 years to reinvent the wheel with TypeScript.

AS3 was a complete rework, more akin to Java.

[0] https://evertpot.com/ecmascript-4-the-missing-version/

[cxr]

None of what you say here that is true moves the needle on what you say that isn't true.

Programmers using underscore to denote identifiers for internal use was well-established and in use long before ActionScript even appeared. To claim that it's a "direct descendant" of ActionScript is arbitrary, anachronistic, and odd.