AFAICT gaia doesn't use let in its core code (since let is not available
in non-chrome js yet) but it does use it in its build scripts etc.
However, let is pretty heavily used in add-ons. CCing Jorge who can
probably help with what to do with that. But have we considered not
changing the behavior for chrome JS?
Cheers,
Ehsan
On 2014-08-13, 5:29 PM, Shu-yu Guo wrote:
Hello all,
We are in the process of making JS 'let' semantics ES6-compliant in
SpiderMonkey. I hope to land bug 1001090 sometime this month or early next
month (I've been told there's a B2G uplift on Sept 1st), which is one of many
for ES6 'let'-compliance. It changes 'let' semantics in two non-backward
compatible ways:
1. 'let' bindings may no longer redeclare existing bindings in function
scope. (In ES6, this is also true for global code, but global 'let'
changes are distinct from bug 1001090 and will be addressed by bug
589199.)
Currently, 'let' bindings at the function body-level are semantically
equivalent to 'var' bindings. This means that the following JS are legal:
function f(x) { let x = x || ""; }
function g() { let x = 42; let x = 43; }
When bug 1001090 lands, these will be syntax errors.
2. 'let' bindings may not be accessed in any fashion before initialization.
Initialization means execution of the |let x| or |let x = rhs|
forms. Before initialization, the 'let' bindings are in a "temporal dead
zone" (TDZ). The TDZ is enforced dynamically, throwing a ReferenceError.
When bug 1001090 lands, the following JS throw:
function f() { x = 42; let x; }
function g() { function inner() { x = 42; } inner(); let x; }
function h() { switch (c) { case 0: let x; break; default: x = 42;
break; } }
These changes wreak havoc on our existing chrome JS, largely due to item 1
above. It will be a rough landing, so I am raising awareness of these breaking
changes now. The changes will likely initially land under a Nightly-only
#ifdef.
I can fix (or can find someone to fix) code that is in mozilla-central. But
code that is outside of that tree will be problematic. Namely, Gaia and addons.
Can someone own updating Gaia JS to be ES6-compliant? It is unclear to me what
should be done about addons.
The takeaway for developers is to refrain from using the patterns I've listed
above starting now in any new code, before bug 1001090 lands. The patterns in
item 2 above are bad form and should never have been used, but those from item
1 have seen a fair bit of use.
P.S. I am not subscribed to dev-gaia. For those on that list but not on
dev-platform, please reply to me at my email address.
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