A better for-loop

[Jason Orendorff]

This is so neat I have to share...

Brendan tweeted that TC39 is considering making this code:

for (let i = 0; i < 10; i++)
setTimeout(function () { alert(i); }, 0);

work as expected, alerting each digit in turn, instead of alerting "10"
ten times.

I think this is really easy to implement in SpiderMonkey! Currently we
emit bytecode like this:

for (let V = INIT; TEST; UPD) STMT

INIT
enterlet V
goto L2
L1: STMT
UPD
L2: TEST
iftrue L1
leaveblock

The new way only requires one tiny change: add a "reenterblock"
instruction between STMT and UPD! The new instruction would just exit
the current block, then do the exact same thing as "enterlet V". All the
code already exists.

-j

[Wes Garland]

\o/