Why can't I use a Javascript function before its definition inside a try block?

Question

As discussed here, function definitions can be used before they're defined. But as soon as a section of code is wrapped in a try block, this ceases to be the case.

This displays "Hello world":

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

But this displays "ReferenceError: hello is not defined":

try {   hello();   function hello() { alert("Hello world"); } } catch (err) {   alert(err); } 

So there is clearly something "special" about a try block with respect to function declarations. Is there any way to get around this behavior?

Answer 1

Firefox interprets function statements differently and apparently they broke declaration hoisting for the function declaration. ( A good read about named functions / declaration vs expression )

Why does Firefox interpret statements differently is because of the following code:

if ( true ) {     function test(){alert("YAY");} } else {     function test(){alert("FAIL");} } test(); // should alert FAIL 

Due to declaration hoisting, function test should always alert "fail", but not in Firefox. The above code actually alerts "YAY" in Firefox and I suspect the code that makes that happen finally broke declaration hoisting altogether.

I assume Firefox turns function declarations into var declarations when they are located in if/else or try/catch statements. Like so:

// firefox interpretted code var test; // hoisted if (true) {    test = function(){alert("yay")} } else {    test = function(){alert("fail")} } 

After a brief debate with Šime Vidas, I have to say that Firefox's dealing with function declarations is non-standard, because of:

The production SourceElement : Statement is processed for function declarations by taking no action.
The production SourceElement : Statement is evaluated as follows:

1. Evaluate Statement.
2. Return Result(1).

Both FunctionDeclaration and Statement are SourceElements, ergo, there should be no FunctionDeclarations inside a statement (if/else, try/catch). Give Šime Vidas a brownie!

Try/catch is basically another form of if/else and probably uses the same exception code.