Function declarations cannot be nested within non-function blocks

Question

I am reading about Function Declarations vs. Function Expressions, and I cannot figure out the meaning of following statement:

Function Declarations occur as standalone constructs and cannot be nested within non-function blocks.

Someone please to explain with an exemple what does the author means, precisely by: "...cannot be nested within non-function blocks".

Link is: https://javascriptweblog.wordpress.com/2010/07/06/function-declarations-vs-function-expressions/

Answer 1

I dont know the author meant it was physically impossible or more of it shouldn't be done. From my understanding what the author was saying is that this:

var y = true;
if (y) {
    function example() {
    alert('hi');
    return true;
   }
}

Here the function is declared inside a conditional statement, which is fine since x is true, but if it were false that function would never be declared and when we do want to the call the example function nothing will happen because it was never declared. So it should be

function example() {
"use strict";
return true;
}
var y = true;
if (y) {
    example();
}

In the above code we still call the example function if the condition is met, however since example is defined outside the condition statement we can use it regardless of the conditional statement. This Post has more information about it. Hopefully this is what you meant

Answer 2

Taken at face value, the statement:

Function Declarations occur as standalone constructs and cannot be nested within non-function blocks.

is wrong. It's possible to put function declarations inside blocks, as examples in the article show. The reason that it's warned against is that the behaviour differs in different browsers. In most browsers (not certain versions of IE and Firefox), such functions are declared regardless of whether execution enters the block or not, e.g.:

if (false) {
  function foo(){}
}

foo is declared and available within the outer scope. This is exactly the same with variable declarations:

if (false) {
  var x = 3;
}

In the above, x is declared regardless of whether the block is executed or not. The assignment of the value, however, only occurs if the block is entered.

Back to functions. The reason function declarations in blocks is warned against is that firstly, it infers that the function is only created if the block is entered, which is incorrect for most browsers but not all. Secondly, and more importantly, it's because different browsers have different behaviour.

Some interesting reading:

• Richard Cornford: FunctionExpressions and memory consumption
• Kangax: Function statements
• What are the precise semantics of block-level functions in ES6?

Also note that function statements are warned against in ES5 strict mode and may be introduced in some future version of ECMAScript.

Finally, this behaviour is addressed directly in ECMA-262 ed 6 in Appendix B 3.3.3 and Appendix B 3.4.