Lift var declarations to the loop initializer

[kangax]

Currently, this code:

for (var i = 0; i < 0; i++) {
  var j = jj();
  console.log(j);
}

transforms to:

for(var i=0;0>i;i++){var j=jj();console.log(j)}

but if we lift var declarations to the loop, we can get it down to:

for(var i=0,j;0>i;i++){j=jj();console.log(j)}

This becomes irrelevant with multiple declarations so we might only consider doing this in certain cases but not others:

for (var i = 0; i < 0; i++) {
  var j = jj(), k = kk(), l = ll();
  console.log(j, k, l);
}

transforms to:

for(var i=0;0>i;i++){var j=jj(),k=kk(),l=ll();console.log(j,k,l)}

and is same length with lifted:

for(var i=0,j,k,l;0>i;i++){j=jj(),k=kk(),l=ll();console.log(j,k,l)}

[boopathi]

for(var i=0;;){var j=foo;}
for(var i=0,j;;){j=foo;}

for(var i=0;;){var j=foo,k=bar;}
for(var i=0,j,k;;){j=foo,k=bar;}

// and we can stop at 2 or even 1
for(var i=0;;){var j=foo,k=bar,l=baz;}
for(var i=0,j,k,l;;){j=foo,k=bar,l=baz;}

The gotcha I see here is the order of transformations.

for (var i = 0;;) { var j = foo; var k = bar; }

merging sibling variables vs merging to previous for-loop.

[boopathi]

We can do this as a part of merging-sibling-variables only ... and not worry about order. !!!

[vigneshshanmugam]

@kangax @boopathi I can work on this if you guys are okay with it :)

[kangax]

@vigneshshanmugam sure, any help is welcome!