vars on the first parameter of loops

Question

I am aware that I can simplify:

var a;
var b;

into:

var a, b;

But can I do the same on loops?

for(var a=0,b=0;a<3;a++){
    //code
}

Or I need to write var a=0, var b=0?

why don't you test it yourself.a simple 2 lines code will give you answer — Umesh Awasthi, Jan 06 '12 at 05:42
I don't know how. And don't make me open another question saying "how can I test if the 'b' in `for(var a,b;a<3;a++){}` is defined — , Jan 06 '12 at 05:43
@TuxedoKnight Chess there is this black magic called [jslint](http://www.jslint.com/) — Bakudan, Jan 06 '12 at 06:21

score 2 · Accepted Answer · answered Jan 06 '12 at 05:42

2

for(var a=0,b=0;a<3;a++) is fine.

answered Jan 06 '12 at 05:42

Ivan Castellanos

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score 0 · Answer 2 · answered Jan 06 '12 at 05:42

0

it is perfectly valid javascript and it will work as expected.

answered Jan 06 '12 at 05:42

asawilliams

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score 0 · Answer 3 · answered Jan 06 '12 at 05:43

0

Yes,you can do like that. Just check it.

answered Jan 06 '12 at 05:43

Sukanya

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score 0 · Answer 4 · edited May 23 '17 at 11:48

0

for(var a=0,b=0;a<3;a++) In other programming languages like C/C++ it is widely used. There is no need to use var twice - you've already stated that you are declaring variables with the first one.

You can also use (not a good practice):

var a = x, b = y; // or var a = x; var b = y;
for ( ; a < b; a++) {
   //code
}

This is mostly used when the length property is used. Thus not calling it on every iteration. Here is a related question, jsperf test, jsperf test.

for(var i = 0, len = obj.length; i < len; i++) {
    //code
}

edited May 23 '17 at 11:48

Community

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answered Jan 06 '12 at 05:49

Bakudan

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@TuxedoKnightChess - Huh? This answers your question and then goes on to give a practical example - why would you want to downvote it? – nnnnnn Jan 06 '12 at 06:03

nnnnnn · Answer 5 · 2012-01-06T06:00:39.110

The syntax for using var in the initialisation part of a for statement is the same as using var elsewhere. So the following will work fine:

for(var a=0,b=0; a<3; a++){ }

Note that this does not limit the scope of variables declared like that to just the for statement's block as would happen in some other languages. JavaScript has only global and function scope, not block scope. The a and b variables declared in the example above will be accessible anywhere in the current scope (including above the for statement, though the values will be undefined).

Note also that you can't say:

for(var a=0; var b=0; a<3; a++) { }

(If you want to declare multiple variables in the for statement's initialisation expression you need to use the comma syntax.)

vars on the first parameter of loops

5 Answers5