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I was trying out some simple JS code. I was aware that we should use var keyword to declare a loop variable inside the loop say for loop to avoid global variable declaration. However I realized that the loop variable exists after the execution of the loop too:

var a = [1, 2, 3, 4, 5, 6];
for (var i = 0; i < a.length; i++)
    document.write(a[i]); //123456
document.write(i);  //6

This is not inline (in fact it does not need to be, I know) with how loop variable of for loop in Object Oriented concepts behaves. I know I should not try to compare the JavaScript with in OO language for any aspect. However I am just guessing that the behavior in OO languages is better.

I think, if we put this code in directly in <script> tag then it actually adds the loop variable to global scope which is not desired. Is it like that? or I am thinking it wrong way?

We can achieve such behavior by creating explicit function scope:

var a1 = [1, 2, 3, 4, 5, 6];
(function () {
    for (var i1 = 0; i1 < a.length; i1++)
        document.write(a[i1]); //123456
})();

document.write(i1);  //undefined

But is it the standard way / followed in production? What other things can be done?

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  • What do for-loops (and their scoping behaviour) have to do with OOP? Commented Apr 14, 2013 at 13:22
  • @Bergi umm I think when we write for(int i=0;i<x;i++){} (in say Java, C#, C++) scopes i to for loop block only. Thats not seem to be the case in JavaScript. Commented Apr 14, 2013 at 13:25
  • JavaScript (and many other languages) does not have block scope. And that has nothing to do with whether a language follows some OOP principles or not. Commented Apr 14, 2013 at 13:29
  • you can also use a map or forEach Commented Apr 14, 2013 at 18:20

3 Answers 3

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We can achieve such behavior by creating explicit function scope. But is it the standard way / followed in production?

Yes. Functions are the only possibility to introduce a new scope in JavaScript (though that might change in future versions with let); and IIFEs are heavily used in production as well.

What other things can be done?

To limit the scope of i to the for-loop's block? None (let alone let). However, you hardly will need to do that, since nothing will interfere with it - var i is still scoped local to your function. Sometimes, you even might want to use i after the loop as an ongoing counter.

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4 Comments

But does that means every for need to be nested inside function scope?
No. Only if you need i to be scoped explicitly on the loop, but that's hardly the case.
rhetoric? may be yes, I tried to frame it neatly, but not just to cause huge responses and hits. Else every good question will be a rhetoric. I usually find related important stuff about the old topics I read while going through the web, far after initial reading. So just want to check if I missed anything. It is very possible with language like JS which is so flexible and has separate books dedicated just to patterns followed. The answer to the question can be No, there is no other known way or pattern just to address this issue, but still this will add some value to my understanding.
Maybe I did not understand what other things exactly you were asking for. Do you know closures already (the next thing related to function scope)? If not, read on them
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JavaScript has only function scope any variable defined using var in side function is available inside function only. for loop does not create any scope for variables.

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yes that I know, but many patterns, practices evolve to take advantages of OOP in JS or even to say to make JS behave like OO language. Just want to know if any standard/well-followed workaround/pattern is there to scope loop variable to loop block.
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Javascript is a bit strange and has, IMHO, a lot of defects that make it a poor language for middle/big projects.

"var" mean that the variable is only available in the current function scope: All variables are defined at the begening, before the function is run and set to "undefined". So:

alert(i);
var i=4;
alert(i);
var i=5;
alert(i);

internally mean:

var i=undefined;
alert(i);
i=4;
alert(i);
i=5;
alert(i);

In Structured programming, all code should be in functions, so that mean that your "var" keyword will work as expected. But if you write code in the global scope, "var" has no real sense.

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