in javascript, declare more than one variable in for loop

Question

have the following javascript code

// note: declaring i in this loop
for( var i=0; i<args.length; i++ ) {

   var elem = args[i];
   ...

   if( elem.attr == 'class' ) {

        // note declaring arr and i in this loop
        for( var arr=elem.val.split(' '), i=0; i<arr.length; i++ ) {

            element.classList.add(arr[classCt]);
        }
        continue;
    }
}

the problem is that i in the second for loop is the same i as declared in the first for loop.

thought that the var construct allowed multiple variables to be declared separated by commas.

when changed i to classCt in second loop, the code worked as expected

"when changed i to classCt in second loop, the code worked as expected" - so why don't you just go with that? — Niko
– Niko, Commented Mar 18, 2012 at 1:53
Changed i to classCt where? The i in second for loop is indeed the same as the first because you are not initializing it again with var. — user201788
– user201788, Commented Mar 18, 2012 at 1:55

Paul · Accepted Answer · 2012-03-18 01:54:29Z

7

You only have one scope there, so there can only be one variable with the same name. You're correct that var allows multiple variables to be declared separated by commas, but you can't declare two different variables with the same name in the same scope. You're just redeclaring a variable that already exists.

Either change it to classCt, or do what I do and use the variable j (and so on) for nested loop iterators:

var i, j, k, l;
for(i = 0; i < 10; i++){
    for(j = 0; j < 10; j++){
        for(k = 0; k < 10; k++){
            for(l = 0; l < 10; l++){
            }
        }
    }
}

answered Mar 18, 2012 at 1:54

Paul

142k28 gold badges285 silver badges272 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

cc young Over a year ago

you're right. a variable declared in a for loop is visible once the loop terminates, so the scope is not restricted to the for loop itself.

Paul Over a year ago

@ccyoung That's correct, Javascript is different some other languages such as C which use block scope. It uses function scope (every function has it's own scope and there is one global scope). Here's a link to read more: robertnyman.com/2008/10/09/…

cc young Over a year ago

@Niko - thought could create new scope with { } block - is this not true?

Niko Over a year ago

@ccyoung No, that is not true. As Paul explained, only functions create new scopes in javascript. This is important to know, as this is different in a lot of other languages like Java or C. See this fiddle for a demonstration: jsfiddle.net/5Dn7w

user166390 Over a year ago

@Niko Technically, with also suffices in certain cases ... but that is not recommended :)

Art · Accepted Answer · 2012-03-18 02:04:35Z

1

You are only working within one scope, the loop does not create it's own even if you use the var keyword. You are just overwriting your i variable within your current functional scope, so for example this:

for (var i = 0; i < 10; i++) {
        for (var i = 5; i < 10; i++) {
            console.log(i);
        }
}

Will just print 5,6,7,8,9.

If you want to create a new scope you would have to do it using functions as is typically done in javascript:

for (var i = 0; i < 10; i++) {
    (function(i) {
        for (var i = 5; i < 10; i++) {
            console.log(i);
        }
    })(i)
}

This will print 5,6,7,8,9 on their own lines 10 times.

answered Mar 18, 2012 at 2:04

Art

5,9223 gold badges32 silver badges32 bronze badges

1 Comment

Orwellophile Over a year ago

but does using 'var' restrict the scope to the function, and if so, how to declare 2 variables within a for loop? :p

Collectives™ on Stack Overflow

in javascript, declare more than one variable in for loop

2 Answers 2

5 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

5 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related