Question

Some static languages like Java seem to have very special rules for variables defined in the first argument of a for loop. They are accessible only by the given loop, which makes them behave pretty much like javascript functions’ local variables and arguments. I mean stuff like this:

class ForVariable {
  public static void main(String[] args) {
    for(int i = 0; i != 0; i++) {}
    System.out.println(i); // Throws an Exception
  }
}

Javascript doesn’t behave like that, which makes nesting loops quite a messy business. My question is: Is it valid to declare variables in the subsequent loops via the var keyword? In other words – which of the following examples is valid?

for(var i = 0, j; i < 5; i++) {
  for(j = 0; j < 10; j++) <do some stuff>;
}

OR

for(var i = 0; i < 5; i++) {
  for(var j = 0; j < 10; j++) <do some stuff>;
}

Clearly it is wrong to declare a variable several times, which would make the 2nd example a no-go, but given the fact that the 1st example is the way loops nesting is done in most languages I know, I’m rather hesitant to declare the winner.

Answer 1

Its not wrong to declare a variable several times. For instance there is really no problem with:

var i = 0;
var i = 1;

That’s valid JavaScript. Good tools like the Closure Compiler will generate a warning though because you typically don’t intend to do that.

That being said, even the Closure Compiler won’t generate a warning for your example #2. It’s just common convention in JS even if you are technically re-declaring.

Either of your two examples is fine but the second one is a little more sensible to parse. I wouldn’t worry about it either way.