Clarification of a JavaScript variable issue

Question

Having coded JavaScript since 1996, I have a very simple issue which I could not clearly prove/disprove using jsfiddle

In some JS attached to a CV I spotted some issues that I would like to verify - one of which is multiple declaration of variable in the same function

Testing it seems it is allowed in newer browsers (OSX Chrome16 Fx 10beta) - as far as I remember it used to give errors (Netscape/Mozilla/Fx1/IE5 or so) :

function something() {
  var var1 = "";
.
/* reams of code which scrolls the first declaration off the screen 
   so the author likely forgot the var was already declared earlier 
   in the same function */
.
  var var1 = ""; // could this result in an error in some browsers?
}

My fiddle is here

Rich O'Kelly · Accepted Answer · 2012-01-20 10:30:15Z

3

Not as far as I'm aware, I've seen javascript functions with multiple for loops each declaring their own var i for at least 6 years all without issue.

Having looked at the spec, there seems to be nothing concrete in this area, however since (particularly global) variable name overwrites (read: clashes) has been a feature since inception I would be very surprised if a restraint upon multiple declarations was imposed.

As it stands, I would suggest that it doesn't show particularly good knowledge of scoping (and hoisting) in javascript, but is valid code nonetheless.

edited Jan 20, 2012 at 10:30

answered Jan 20, 2012 at 10:03

Rich O'Kelly

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2 Comments

mplungjan Over a year ago

Thanks - I vaguely remember Netscape or Mozilla or Fx 1 or so giving me an error, but since it only happened once for me, I may have been wrong - I know it works in for loops but that is for me very local scope

Rich O'Kelly Over a year ago

@mplungjan The var i in for loops get hoisted to the top of a function just like any other variable declared in the function's scope - all the is in the loops refer to the same variable. The situation is analogous to the one you describe.

Paul Butcher · Accepted Answer · 2012-01-20 10:10:50Z

2

I've been playing with javascript for about as long as you, and I don't remember this ever being forbidden by an interpreter. Looking at the ECMAScript 1 spec, I can see no mention of ensuring that variable declarations are unique within a given scope; and it would be a strange feature to remove.

The absence of such a check, alongside the fact that variables are function~ and not block-scoped, is one of the things that seems to cause misunderstanding, where a variable "declared" in one block unexpectedly has the last value it had in a previous block.

answered Jan 20, 2012 at 10:10

Paul Butcher

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1 Comment

mplungjan Over a year ago

Thanks - I am pretty sure but I never have this issue myself so I thought I'd ask

Andreas Eriksson · Accepted Answer · 2012-01-20 10:17:10Z

2

I'm quite certain that the parser ignores the second "var"-declaration, since it is redundant - all it does is signify that the variable is limited to the local scope. There's no reason to use it twice in the same scope, as in the same function, but if you, as @rich.okelly pointed out, have loops or functions, you can indeed use "var" to create a local variable of the same name as a variable in a higher scope. It's not pretty, and certainly doesn't do any wonders for readability, but it's possible. I have never encountered a browser that hangs up on prepending a variable with "var" twice.

Example:

x = "Hello";

function test() { 
    alert(x); // Outputs "Hello"
}
function test2() { 
    var x = "local variable";
    alert(x); // ouputs "local variable"
    var x = "changed the variable"; // this does exactly the same thing as if you'd omitted "var"
}
alert(x); // Outputs "Hello"

answered Jan 20, 2012 at 10:17

Andreas Eriksson

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1 Comment

mplungjan Over a year ago

Yeah, that is what my fiddle did in Fx10/Chrome16 but that was not enough for me to take it at face value.

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