I understand the semantics that a closure holds a reference to a variable lengthen it's life cycle, makes primitive variables not limited by calling stack, and thus those variables captured by closures should be specially treated.
I also understand variables in same scope could be differently treated depends on whether it was captured by closures in now-days javascript engine. for example,
function foo(){
var a=2;
var b=new Array(a_very_big_number).join('+');
return function(){
console.log(a);
};
}
var b=foo();
as no one hold a reference to b in foo, there's no need to keep b in memory, thus memory used could be released as soon as foo returns(or even never created under furthur optimization).
My question is, why v8 seems to pack all variables referenced by all closures together in each calling context? for example,
function foo(){
var a=0,b=1,c=2;
var zig=function(){
console.log(a);
};
var zag=function(){
console.log(b);
};
return [zig,zag];
}
both zig and zag seems to hold a reference to a and b, even it's apparent that b is not available to zig. This could be awful when b is very big, and zig persists very long.
But stands on the point of view of the implementation, I can not understand why this is a must. Based on my knowledge, without calling eval, the scope chain can be determined before excution, thus the reference relationship can be determined. The engine should aware that when zig is no longer available, nether do a so the engine mark it as garbage.
Both chrome and firefox seems to obey the rule. Does standard say that any implementation must do this? Or this implementation is more practical, more efficient? I'm quite puzzling.
