# Intent to ship: Hashbang syntax
## Introduction
As JS metastasizes from the web to Node and shell scripts and the like, it
becomes useful to run scripts directly in the shell and let the shell decide
how. The traditional way to do this is with an initial hashbang (or shebang)
line indicating a path to an interpreter:
```
#!/usr/bin/env node
print("Hello world");
```
Thus many JS shells manually grunge past an initial "#!" line as convenience.
SpiderMonkey's shell has done this since, well, before some people reading this
were born. :-) https://dxr.mozilla.org/classic/source/js/src/js.c#143 But
this means every implementation has micro-forked the language in perhaps subtly
different manner. (It's easy to mess up line numbers doing this.)
So it makes sense to standardize this as a micro-tax implementations will all
pay consistently. Hashbang isn't very valuable on the web -- polyglot web
scripts/shell scripts are likely rare now -- but it's a minor nicety.
## How it works
If eval code, an entire script, or a module (but *not* the code string passed
to the Function constructor, nor the contents of a block in normal JS source
text) -- at its *very start* begins with the two code points U+0023 NUMBER SIGN
followed by U+0021 EXCLAMATION MARK, all code points up to a LineTerminator are
treated as a single-line comment -- exactly as if "//" were encountered instead
of "#!".
Hashbang only begins a comment at the very start of a script; it's a syntax
error elsewhere, including after any amount of whitespace.
A minor wrinkle: if you have an inline script element with the <script> on one
line, then you have a linebreak and *then* your source text starts, that isn't
necessarily the very start of the script data. In XHTML it definitely *isn't*
the start of the script -- "#!" would have to appear immediately after the ">"
at end of the <script> element open. But in plain old HTML *possibly* a single
linebreak is ignored. Memory hazy, too lazy to look up.
## Tracking bugs
https://bugzilla.mozilla.org/show_bug.cgi?id=hashbang
## Standard
https://github.com/tc39/proposal-hashbang
The standard is "stage 3" in TC39, the ECMAScript standardization committee:
https://github.com/tc39/proposals#stage-3.
## Platform coverage
All
## Estimated or target release
67 (soft freeze 11 march, train leaving 18 march)
## MDN
...none yet, we'll find a place. This feature is low-level enough the place to
slot docs in is not immediately obvious.
## Support in other browser / JS engines
https://github.com/v8/v8/commit/0cd67eb7c57e109aa4cbc8b8bdff6b9eaf8c1286
suggests v8 and Chrome are shipping this as of this week.
I couldn't find Safari/JavaScriptCore implementation discussion, but I didn't
look much.
Node and other shells may already support something nigh-indistinguishable from
this as an extension.
## test262 coverage
The relevant test262 tests will land in our tree shortly. There aren't many,
but there are enough. (And honestly some of them verge on silliness: "what if
the script starts with literally \u0023\u0021" is not a realistic
implementation mistake. :-) )
https://github.com/tc39/test262/tree/master/test/language/comments/hashbang
## Where to send your bugs
https://bugzilla.mozilla.org/enter_bug.cgi?product=Core&component=JavaScript%20Engine
and feel free to CC me. :-) But my good friend George Washington says you
won't find any. (What can I say? I'm a high roller.)
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