What is a substatement function declaration?

JavaScript supports nested function declarations:

function outer() {
  function inner() { /* ... */ }
  // ...
}

But according to the standard, nested function declarations are only allowed to appear at the outermost level of their parent function (or script). If you nest a function declaration inside of another statement, you've waded into non-standard territory:

{
  // ...
  function f() { /* ... */ } // this is non-standard JavaScript!
  // ...
}

The function f is a substatement function declaration: it is a function declaration that appears inside a substatement of its parent function, rather than at its parent function's top level. Not only is this non-standard, but different JavaScript engines exhibit different behavior for this kind of code.

What does this browser do?

This browser exhibits scoping semantics for function declarations that are nested within substatements of their parent function or script.

What does all this mean?

static scoping

the function is in its scope regardless of whether its declaration is executed.

dynamic scoping

the function is added into its scope during execution, and only after its declaration is executed.

function-local scoping

the function's scope extends across its entire containing function (it is "hoisted" to its parent function's top level).

block-local scoping

the function's scope extends across its containing block.

Why should I care?

First of all, you should really care about your code behaving the same across different browsers! Second of all, dynamic scope is widely understood to be a bad thing. It has bad performance, and is harder to use correctly without accidentally introducing strange and subtle bugs. Firefox is the only browser that uses dynamic scoping for substatement function declarations, but Mozilla is considering trying to change this.

In the future, strict mode JavaScript will provide block-local scoping for substatement function declarations. So you should beware of depending on the hoisting of function scope, since migrating code to strict mode could change its behavior.