using var keyword in while loop definition

Question

In JavaScript for loop I can use var keyword in loop definition something like that:

for (var i=0; i<10; i++) ...

I know scope of the variable i is not inside the loop but inside function where loop is declared. This it a better notation than declaring local variable i outside of the loop (the worst notation is declare i variable in the beggining of function body):

var i;
for (i=0; i<10; i++) ...

My question is about while loop. I'm not able declare variable in while loop definition something like this:

while((var match = re.exec(pattern)) != null) ...

I have to use var keyword outside of the while loop.

var match;
while((match = re.exec(pattern)) != null) ...

Am I doing something wrong?

That is correct, you declare the variable before the while loop, not inside it. — adeneo
– adeneo, Commented Aug 10, 2014 at 22:55
what are you talking about "the worst notation is declare i variable in the beggining of function body". I'm pretty sure javascript hoists all variables to the beginning of the function body — Scott Mitchell
– Scott Mitchell, Commented Aug 10, 2014 at 23:03
Forget the "JavaScript hoists variables" part, it is a beginners mistake. I think a gave a proper/brief overview in stackoverflow.com/a/10792121. — Kijewski
– Kijewski, Commented Aug 10, 2014 at 23:06
Im confused on what you are saying. You are saying that javascript does NOT hoist variables to the top of the scope? Just would like to confirm — Scott Mitchell
– Scott Mitchell, Commented Aug 10, 2014 at 23:10

Felix Kling · Accepted Answer · 2014-08-10 23:01:23Z

5

Am I doing something wrong?

No you are not. That's just how the syntax is defined. The while loop only accepts an expression, while the for loop can also be used with a var declaration.

See https://es5.github.io/#x12.6:

while ( Expression ) Statement
for ( ExpressionNoIn_opt; Expression_opt ; Expression_opt ) Statement
for ( var VariableDeclarationListNoIn; Expression_opt ; Expression_opt ) Statement

edited Aug 10, 2014 at 23:01

answered Aug 10, 2014 at 22:56

Felix Kling

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