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What's the scope of a Javascript variable declared in a for() loop?

Answer 1

+5 A:

In Javasript, the only "interesting" lexical scope boundary is the function body. Anything declared anywhere in a function (well, anywhere other than another nested function!) is at the same scope. There are also some weird things about the way that the declarations are interpreted.

Your anonymous function does act as a closure, but each function instantiated will share the same "i". A trick I use is to add another layer of function:

for (var i = 0; i < whatever; i++) {
  (function(idaho) {
    whatever(function() { alert("my own private " + idaho); });
  })(i);
}

At somepoint hopefully all the browsers will support the new "let" statement, which is a shorter, less weird-looking way to do basically the same thing.

Pointy 2010-04-28 21:58:48

Answer 2

+5 A:

The scope is the function that the variable is defined in (except there isn't one, so it is global).

The anonymous function you are passing is accessing the variable defined in the parent function's (again global) scope.

You need an actual closure.

alerts.push(
    function (foo) { 
        return function() { 
            document.write(foo + ', ');

        }
    }(i)
);

David Dorward 2010-04-28 22:00:11

Correct answer - thanks! - but I've accepted Pointy's answer because "my own private " + idaho made me laugh out loud...

Dylan Beattie 2010-04-28 22:14:07

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What's the scope of a Javascript variable declared in a for() loop?

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