Simon Willison’s Weblog

This actually isn't too crazy. I posted a comment on the actual post, but I wanted to post here also for people who only see this page.

There are function statements and function expressions in Javascript. Function statements define a function in the enclosing scope whereas function expressions can be assigned at the time you need them to be available.

Hsiu-Fan Wang - 3rd December 2008 09:53 - #

This isn't as weird as it sounds.

What do you think this will alert...

var hello = "hello";
(function() {
  if (0) {
    var hello = "HELLO";
  }
  alert(hello);
})();

It alerts 'undefined'. The 'hello' var is created inside closure, although it isn't assigned a value. This is correct according to the ECMA spec.

Considering the following works...

shout();
function shout() {
  alert("Oi!");
}

...I can kinda understand why IE would pick it up in an if(0)

Jake Archibald - 3rd December 2008 10:41 - #

In a similar note:

world(); // Most browsers get an undefined error here
hello(); // IE gets an undefined error here

var hello = function world() {
  alert("Hello world");
}

IE's behaviour makes more sense to me, considering the last example in my previous post. Although I'm sure it's incorrect according to the spec.

Jake Archibald - 3rd December 2008 11:11 - #

Just to muddy the waters further, what should we expect the following to do? It needs to be inside a script block, not executed in FireBug:

if (typeof really == "function") {
  function really() {
    alert("Yes, really");
  }
}

really();

function really() {
  alert("Original");
}

In this case, IE, Chrome and Opera say "Original", but Firefox says "Yes, really". I think Firefox gets this right, but it's all a bit dubious. There's definitely some ambiguity about what should happen when the top-level functions are initially parsed out, and what should happen at runtime.

This one is even weirder:

if (typeof really == "function") {
  function really() {
    alert("Yes, really");
  }
} else {
  function really() {
    alert("No way!");
  }
}

really();

{
  function really() {
    alert("Original");
  }
}

Firefox says "No way!"--seems that it defers evaluation of functions declared within any scope block. The others all say "Original". Pretty dodgy. Fortunately, I doubt anyone would code against this behaviour in the first place.

phl - 3rd December 2008 14:52 - #