Question

Possible Duplicates:
What is the difference between a function expression vs declaration in JavaScript?
Explain JavaScript's encapsulated anonymous function syntax

Why this:

(function () {
    //code
}());

and this:

var f = function () {
    //code
}();

works, while this:

function () {
    //code
}();

does not? It looks exactly the same – anonymous function defined, and immediately called. Can someone make a quotation from the JavaScript/ECMAScript standard which explains that?

UPDATE: Thanks for the answers everyone!
So it’s about function expression vs. function declaration. See this Stack Overflow answer, ECMAScript standard section 13, and this great article: Named function expressions demystified.

To recap answers:

1. The first snippet is interpreted as an expression because the grouping operator, (), is applied – see ECMAScript standard section 11.1.6.

2. In the second snippet, the function is interpreted as an expression because it’s on the right-hand part of assignment operator, =.

3. The third snippet doesn’t have anything which allows the interpreter to read the function as an expression, so it’s considered a declaration, which is invalid without an identifier (Gecko lets it pass however, but it chokes on following () grouping operator (as it thinks) applied to nothing).

Answer 1

The first two cases show function expressions, and can appear anywhere an expression like (1+1 or x*f(4)) would appear. Just like how 1+1, evaluates to 2, these expressions evaluate to a corresponding function.

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The third case is a function declation statement, and can appear anywhere you can have other statements (like an if or while statement).

There is not much point in trying to declare an anonymous function via a Funcion declaration statements, since otherwise noone would get a reference to the function afterwards.

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The reason you need the opening ( or the var x = like in the first two cases is that they force next bit to be parsed in an expression context. (just think how you cant do var x = if ..., for example). If you just put the function as the first thing it will be parsed as the declaration statement you don’t want.