[ldjb]

I was expecting this post to explain why it's an underscore specifically, as opposed to a dollar sign, an asterisk, a caret, a tilde or some other special character.

I can only assume it's a holdover of languages like C where the standard library has some reserved names that start with an underscore.

https://devblogs.microsoft.com/oldnewthing/20230109-00/?p=10...

[lelandfe]

As another borrowed convention, JS developers of yore (and likely some today still) used an _ prefix to denote “private” function/methods. Quotes as it’s just a convention - today JS supports # for actual private members in class syntax.

[zeroq]

This is a direct descendant of ActionScript. It introduced the convention of Instance._getter and Instance.__private.

[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.

[DidYaWipe]

What about Lingo?

[xd1936]

Another weird case: In Google's server-side JS environment Apps Script, function names that end with _ are treated as private functions that cannot be called directly by the user or enumerated.

1. https://developers.google.com/apps-script/guides/html/commun...

[BurningFrog]

In the Python world I was in, trailing underscores is used to work around the ban on reserved words. The language grabs some of the best names for itself!

So a variable that really ought to be named `class` you name `class_`.

[smallnix]

In java usually clazz

[BurningFrog]

That's what I used to do before discovering this.

[phist_mcgee]

I use underscore to prevent shadowing of variables names. Not sure where I picked up that habit though.

[lelandfe]

I've seen that before.

A related conventional use of underscores in JS variable names is when "discarding" values in positional settings, like destructuring arrays.

E.g. `const [_, a, b] = triple`

In general, underscores in JS seem to be used to help call out what to ignore or what is less important.

[fennecbutt]

You've been able to `const [, a, b] = tuple;` for a wee while now FYI.

But yeah for every other time most linters will accept _ as "ignore this".

[phist_mcgee]

I had no idea that was a thing!

Thanks for teaching me something today.

[lelandfe]

Oh yeah, I always forget about that.

[layer8]

I wouldn’t say it’s a “holdover”, it continues to be a common convention for marking internal or reserved use.

[paradox460]

Some modern languages even turn _ flagged variables/parameters into black holes

[anoldperson]

I do it every day, so I see it from your perspective.

I'm willing to bet there are kids who do not though. Given how many crazy colours my IDE shows everything in, I could definitely believe there are people that go, 'Why bother, aren't those private members blue anyway?', or some such similar train of thought.

Of course if your IDE is little more than notepad, then such things are still important. For me, that's the Arduino IDE. I have to admit I really like writing micro controller code in it, it's a bit like writing code 30 years ago (both the good and bad parts).

[kazinator]

Making it a dollar sign would cause countless bugs for people who generate HTML using templating languages whose variables are indicated by dollar sign. From time to time, someone would add target="$blank" to a templates, forgetting that the $ must be escaped to be literal.

They might have to resort to predefining a reserved variable called blank, whose value is "$blank". :)

Similar reasoning applies to most other special characters.

Given how HTML gets generated by preprocessors which use special characters in this manner or that, its best not to come up with new schemes within HTML itself involving special characters.

Carving out a reserved space within an existing namespace is safe.

[robocat]

Your reason is unlikely to be a cause.

The _ comes from the W3C in 1995 well before JavaScript was commonly used for templating HTML.

Scripting has used $ for variables for a long time: I think the most relevant history line for $variable is PHP comes from Perl comes from shell scripts. I also remember finding $ ugly on Vax.

There were a huge variety of templating syntaxes for server side and HTML generation was virtually all server side in the 1900s.

Server side languages were very rarely JavaScript before Node in 2009.

JavaScript wasn't used much for HTML generation before Ajax. There were soon after many many client side templating syntaxes.

I'm guessing only Brendan Eich could say why $ was accepted for JavaScript variable names.

Timelines are hard because the foundations were compressed within a decade: JavaScript 1995, PHP 1996, DHTML 1997, Ajax early 2000s, jQuery 2006.

Syntaxes tend to be extremely path dependent, and every developer cribs from everything they use.

[kazinator]

Shell script here documents can generate HTML:

  cat <<!
  <div>
    ...
    <a href="$url" target="_blank" ...
  ...
  !

A form of CGI existed as far back as 1993 on the NCSA web server.