Javascript Syntax: Anonymous Self Calling Functions

Question

Possible Duplicate:
Self Executing function doesnt work?

In Javascript, why does

function() { console.log('hello'); }();

give an error, but

function() { console.log('hello'); }()

not?

Edit: some answers state that both give syntax errors in Firefox, but this does not seem to be the case in either Chrome or a Node.js REPL. Additionally,

(function() { console.log('hello'); }());

seems to work fine in Firefox.

In get an error in both cases... – Felix Kling Jul 31 '12 at 21:42 — Felix Kling, Jul 31 '12 at 21:42

meder omuraliev · Answer 1 · 2012-07-31T21:49:00.640

Actually both of those are SyntaxErrors.

You need to wrap parens around the anonymous function so it can be properly parsed.

(function(){alert('eyo')})();

This defines the function and immediately invokes it. It's not syntactically valid to define a function and invoke it without 'forming' the function by wrapping parens around it.

Good explanation here: http://benalman.com/news/2010/11/immediately-invoked-function-expression/

Fortunately, the SyntaxError “fix” is simple. The most widely accepted way to tell the parser to expect a function expression is just to wrap in in parens, because in JavaScript, parens can’t contain statements. At this point, when the parser encounters the function keyword, it knows to parse it as a function expression and not a function declaration.

Good job, but if you work at twitter you can also use `!` instead of parens. — Esailija, Jul 31 '12 at 21:52

score 0 · Answer 2 · answered Jul 31 '12 at 21:41

0

(function() { console.log('hello'); })();

You must invoke some function, in this case is "()"

No, any Syntax errors

answered Jul 31 '12 at 21:41

NiLL

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That's not really an explanation. *edit*: The OP has parenthesis after the function definition, shouldn't this count as "call"? – Felix Kling Jul 31 '12 at 21:42
Yes; this doesn't seem to answer the question at all. – Sam Jul 31 '12 at 21:42
Also you can do this (function() {console.log('hello');}()) – NiLL Jul 31 '12 at 21:46

score 0 · Answer 3 · answered Jul 31 '12 at 21:42

In Firefox, both lines generate the same error:

SyntaxError: function statement requires a name

To fix this, simply add parentheses:

(function() { console.log('hello'); })();

The error is probably raised because it indicates a syntax error:

function (value) {
    return value * value;
}

It's obvious that the function name is missing, and that no anonymous function was intended.

Therefore, anonymous functions always have to occur as expressions, e.g. to assign a variable:

var a = function() { };

Otherwise, you need the parentheses.

score 0 · Answer 4 · answered Jul 31 '12 at 21:45

0

an anonymous function that is followed by () is essentially you saying "immediately call this function" but your syntax is not correct because without the wrapping parents, the () doesn't attach to anything (there is an exception, that if you set your anonymous function equal to a variable, its ok, but you didn't do that... see comments).

so it must be like this: ( function(){} )()

think of it like this:

1. ( someExpressionReturnsFunction )(2)
2. is now: aFunction(2)

answered Jul 31 '12 at 21:45

Kristian

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That's not completely true. `var foo = function() { return 'bar'; }();` works perfectly. – Felix Kling Jul 31 '12 at 21:46
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@FelixKling lol I am loving your comments. – Esailija Jul 31 '12 at 21:47
1

@Esailija: I take it as a compliment :D – Felix Kling Jul 31 '12 at 21:49
he is the king, afterall – Kristian Jul 31 '12 at 21:49

score 0 · Answer 5 · answered Jul 31 '12 at 21:56

As the others pointed out, there's a difference in assignment vs. an expression

Self-executed functions are functions that were turned into expressions:

// self-executed due to both being an expression, no function declaration.
!function(foo) { console.log('test', foo); }('first');
(function(foo) { console.log('test', foo); })('second');

// long-hand examples of normal usage without expression conversions.
var bar = function(foo) { console.log('test', foo); };
bar('third');

You can only pass through arguments with the least parens "()" afterwards if the thing before is either a callable already-declared function or an expression.

edit:// I added the negotiator example, because that will cause the following keyword being converted in an expression. expressions always return true or false.

Javascript Syntax: Anonymous Self Calling Functions

5 Answers5