javascript - Using var when declaring i in a for loop -

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• “var” or no “var” in javascript's “for-in” loop? 9 answers

i've seen javascript code has used 2 different ways of defining loop.

for (var i=0;i < x.length; i++) 

but it's been

for (i=0; < x.length; i++) 

the same thing has happened for-in loops

for (var in x) 

and

for (i in x) 

is there difference between declaring i var , saying i? there advantages of doing 1 on other? 1 these right way this? can tell, both act same, there has difference.

note: i'm not asking difference between for-in , for (i=0)

without var declaration somewhere in function, references i i property of global object. risks sorts of unpredictable behavior if code in body of for loop invokes code (say, inside called method) modifies global i.

note declaring var i in for loop initialization

for (var = ...) 

is equivalent declaring var i; before for loop:

var i; (i = ...) 

in particular, declaration of i hoisted top of enclosing scope.

edit: if enable strict mode, must declare loop variables (all variables, actually). referencing variable has not been declared var statement result in referenceerror being thrown (rather resulting in global variable coming existence).