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?
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.To limit the scope of
i
to the for-loop's block? None (let alonelet
). 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 usei
after the loop as an ongoing counter.JavaScript has only
function scope
any variable defined usingvar
in side function is available inside function only. for loop does not create any scope for variables.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:
internally mean:
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.