javascript function behaving differently

javascript function behaving differently - javascript

I have a function in javascript:
var a=function greet(){
console.log("Hello");
}
var b=a;
b();
While creating a new var b and assigning function a to b logs hello.However the code:
function greet(){
console.log("Hello");
}()
logs error.Why?
Also
(function greet(){
console.log("Hello");
})()
and
var a=function greet(){
console.log("Hello");
}()
logs "Hello".please explain the logic behind this.

function greet(){
console.log("Hello");
}()
There showing error because you invoked function immediately but not wrapped function body with '()'. So the invoke function immediately you should follow following pattern:
(fucntion(){ /* code*/ })()
But your first example is working because of you storing function in a variable and calling it later as a callback style.

As commented before, when you define a function expression, the function on RHS is treated as anonymous function. Reference of this function will not be registered.
Sample:
var a = function test(){
console.log("test");
}
test();
But when you declare a named function, it will be registered in heap and will retrieve its reference from there.
Now question is when is this action performed?
When ever its surrounding script is executed. i.e.
function test(){
// some code
} // when this line is evaluated, function reference will be added in heap.
But when you do
function test(){
console.log('test')
}()
when it reaches last line (}()), it will try to execute test(), but reference to this test is not defined yet and will throw error.
You may also ask why IIFE ((function (){...})()) works, thats is because, its surrounded by (). This makes it expression and outside this parenthesis, it becomes anonymous.
Note: As I'm not sure, making this as a community answer. Please add if anything is incorrect or missing.
As pointed out by #nnnnnn, my assumptions are incorrect. Please refer comments for more information.

In your first example you are assigning a function to variable a, then making b a reference to variable a. Then you are calling b which executes function a.
In your second example, you are creating a function, but you are not assigning it or calling it. If you simply call the function in your second example it will work as intended.
function greet(){
console.log("Hello");
}
greet();
Your third example is what is known as a 'self executing function' - meaning you don't have to call it for it to execute.
In your fourth example, you are assigning a function to variable a, but not executing it. The following adjustment will make example 4 work.
var a=function greet(){
console.log("Hello");
}
a();

Related

javascript auto execute anonymous function - parenthesis [duplicate]

I was reading some posts about closures and saw this everywhere, but there is no clear explanation how it works - everytime I was just told to use it...:
// Create a new anonymous function, to use as a wrapper
(function(){
// The variable that would, normally, be global
var msg = "Thanks for visiting!";
// Binding a new function to a global object
window.onunload = function(){
// Which uses the 'hidden' variable
alert( msg );
};
// Close off the anonymous function and execute it
})();
Ok I see that we will create new anonymous function and then execute it. So after that this simple code should work (and it does):
(function (msg){alert(msg)})('SO');
My question is what kind of magic happens here? I thought that when I wrote:
(function (msg){alert(msg)})
then a new unnamed function would be created like function ""(msg) ...
but then why doesn't this work?
(function (msg){alert(msg)});
('SO');
Why does it need to be in the same line?
Could you please point me some posts or give me an explanation?

Drop the semicolon after the function definition.
(function (msg){alert(msg)})
('SO');
Above should work.
DEMO Page: https://jsfiddle.net/e7ooeq6m/
I have discussed this kind of pattern in this post:
jQuery and $ questions
EDIT:
If you look at ECMA script specification, there are 3 ways you can define a function. (Page 98, Section 13 Function Definition)
1. Using Function constructor
var sum = new Function('a','b', 'return a + b;');
alert(sum(10, 20)); //alerts 30
2. Using Function declaration.
function sum(a, b)
{
return a + b;
}
alert(sum(10, 10)); //Alerts 20;
3. Function Expression
var sum = function(a, b) { return a + b; }
alert(sum(5, 5)); // alerts 10
So you may ask, what's the difference between declaration and expression?
From ECMA Script specification:
FunctionDeclaration :
function Identifier ( FormalParameterListopt ){ FunctionBody
}
FunctionExpression :
function Identifieropt ( FormalParameterListopt ){ FunctionBody
}
If you notice, 'identifier' is optional for function expression. And when you don't give an identifier, you create an anonymous function. It doesn't mean that you can't specify an identifier.
This means following is valid.
var sum = function mySum(a, b) { return a + b; }
Important point to note is that you can use 'mySum' only inside the mySum function body, not outside. See following example:
var test1 = function test2() { alert(typeof test2); }
alert(typeof(test2)); //alerts 'undefined', surprise!
test1(); //alerts 'function' because test2 is a function.
Live Demo
Compare this to
function test1() { alert(typeof test1) };
alert(typeof test1); //alerts 'function'
test1(); //alerts 'function'
Armed with this knowledge, let's try to analyze your code.
When you have code like,
function(msg) { alert(msg); }
You created a function expression. And you can execute this function expression by wrapping it inside parenthesis.
(function(msg) { alert(msg); })('SO'); //alerts SO.

It's called a self-invoked function.
What you are doing when you call (function(){}) is returning a function object. When you append () to it, it is invoked and anything in the body is executed. The ; denotes the end of the statement, that's why the 2nd invocation fails.

One thing I found confusing is that the "()" are grouping operators.
Here is your basic declared function.
Ex. 1:
var message = 'SO';
function foo(msg) {
alert(msg);
}
foo(message);
Functions are objects, and can be grouped. So let's throw parens around the function.
Ex. 2:
var message = 'SO';
function foo(msg) { //declares foo
alert(msg);
}
(foo)(message); // calls foo
Now instead of declaring and right-away calling the same function, we can use basic substitution to declare it as we call it.
Ex. 3.
var message = 'SO';
(function foo(msg) {
alert(msg);
})(message); // declares & calls foo
Finally, we don't have a need for that extra foo because we're not using the name to call it! Functions can be anonymous.
Ex. 4.
var message = 'SO';
(function (msg) { // remove unnecessary reference to foo
alert(msg);
})(message);
To answer your question, refer back to Example 2. Your first line declares some nameless function and groups it, but does not call it. The second line groups a string. Both do nothing. (Vincent's first example.)
(function (msg){alert(msg)});
('SO'); // nothing.
(foo);
(msg); //Still nothing.
But
(foo)
(msg); //works

An anonymous function is not a function with the name "". It is simply a function without a name.
Like any other value in JavaScript, a function does not need a name to be created. Though it is far more useful to actually bind it to a name just like any other value.
But like any other value, you sometimes want to use it without binding it to a name. That's the self-invoking pattern.
Here is a function and a number, not bound, they do nothing and can never be used:
function(){ alert("plop"); }
2;
So we have to store them in a variable to be able to use them, just like any other value:
var f = function(){ alert("plop"); }
var n = 2;
You can also use syntatic sugar to bind the function to a variable:
function f(){ alert("plop"); }
var n = 2;
But if naming them is not required and would lead to more confusion and less readability, you could just use them right away.
(function(){ alert("plop"); })(); // will display "plop"
alert(2 + 3); // will display 5
Here, my function and my numbers are not bound to a variable, but they can still be used.
Said like this, it looks like self-invoking function have no real value. But you have to keep in mind that JavaScript scope delimiter is the function and not the block ({}).
So a self-invoking function actually has the same meaning as a C++, C# or Java block. Which means that variable created inside will not "leak" outside the scope. This is very useful in JavaScript in order not to pollute the global scope.

It's just how JavaScript works. You can declare a named function:
function foo(msg){
alert(msg);
}
And call it:
foo("Hi!");
Or, you can declare an anonymous function:
var foo = function (msg) {
alert(msg);
}
And call that:
foo("Hi!");
Or, you can just never bind the function to a name:
(function(msg){
alert(msg);
})("Hi!");
Functions can also return functions:
function make_foo() {
return function(msg){ alert(msg) };
}
(make_foo())("Hi!");
It's worth nothing that any variables defined with "var" in the body of make_foo will be closed over by each function returned by make_foo. This is a closure, and it means that the any change made to the value by one function will be visible by another.
This lets you encapsulate information, if you desire:
function make_greeter(msg){
return function() { alert(msg) };
}
var hello = make_greeter("Hello!");
hello();
It's just how nearly every programming language but Java works.

The code you show,
(function (msg){alert(msg)});
('SO');
consist of two statements. The first is an expression which yields a function object (which will then be garbage collected because it is not saved). The second is an expression which yields a string. To apply the function to the string, you either need to pass the string as an argument to the function when it is created (which you also show above), or you will need to actually store the function in a variable, so that you can apply it at a later time, at your leisure. Like so:
var f = (function (msg){alert(msg)});
f('SO');
Note that by storing an anonymous function (a lambda function) in a variable, your are effectively giving it a name. Hence you may just as well define a regular function:
function f(msg) {alert(msg)};
f('SO');

In summary of the previous comments:
function() {
alert("hello");
}();
when not assigned to a variable, yields a syntax error. The code is parsed as a function statement (or definition), which renders the closing parentheses syntactically incorrect. Adding parentheses around the function portion tells the interpreter (and programmer) that this is a function expression (or invocation), as in
(function() {
alert("hello");
})();
This is a self-invoking function, meaning it is created anonymously and runs immediately because the invocation happens in the same line where it is declared. This self-invoking function is indicated with the familiar syntax to call a no-argument function, plus added parentheses around the name of the function: (myFunction)();.
There is a good SO discussion JavaScript function syntax.

My understanding of the asker's question is such that:
How does this magic work:
(function(){}) ('input') // Used in his example
I may be wrong. However, the usual practice that people are familiar with is:
(function(){}('input') )
The reason is such that JavaScript parentheses AKA (), can't contain statements and when the parser encounters the function keyword, it knows to parse it as a function expression and not a function declaration.
Source: blog post Immediately-Invoked Function Expression (IIFE)

examples without brackets:
void function (msg) { alert(msg); }
('SO');
(this is the only real use of void, afaik)
or
var a = function (msg) { alert(msg); }
('SO');
or
!function (msg) { alert(msg); }
('SO');
work as well. the void is causing the expression to evaluate, as well as the assignment and the bang. the last one works with ~, +, -, delete, typeof, some of the unary operators (void is one as well). not working are of couse ++, -- because of the requirement of a variable.
the line break is not necessary.

This answer is not strictly related to the question, but you might be interested to find out that this kind of syntax feature is not particular to functions. For example, we can always do something like this:
alert(
{foo: "I am foo", bar: "I am bar"}.foo
); // alerts "I am foo"
Related to functions. As they are objects, which inherit from Function.prototype, we can do things like:
Function.prototype.foo = function () {
return function () {
alert("foo");
};
};
var bar = (function () {}).foo();
bar(); // alerts foo
And you know, we don't even have to surround functions with parenthesis in order to execute them. Anyway, as long as we try to assign the result to a variable.
var x = function () {} (); // this function is executed but does nothing
function () {} (); // syntax error
One other thing you may do with functions, as soon as you declare them, is to invoke the new operator over them and obtain an object. The following are equivalent:
var obj = new function () {
this.foo = "bar";
};
var obj = {
foo : "bar"
};

There is one more property JavaScript function has. If you want to call same anonymous function recursively.
(function forInternalOnly(){
//you can use forInternalOnly to call this anonymous function
/// forInternalOnly can be used inside function only, like
var result = forInternalOnly();
})();
//this will not work
forInternalOnly();// no such a method exist

It is a self-executing anonymous function. The first set of brackets contain the expressions to be executed, and the second set of brackets executes those expressions.
(function () {
return ( 10 + 20 );
})();
Peter Michaux discusses the difference in An Important Pair of Parentheses.
It is a useful construct when trying to hide variables from the parent namespace. All the code within the function is contained in the private scope of the function, meaning it can't be accessed at all from outside the function, making it truly private.
See:
Closure (computer science)
JavaScript Namespacing
Important Pair of Javascript Parentheses

Another point of view
First, you can declare an anonymous function:
var foo = function(msg){
alert(msg);
}
Then you call it:
foo ('Few');
Because foo = function(msg){alert(msg);} so you can replace foo as:
function(msg){
alert(msg);
} ('Few');
But you should wrap your entire anonymous function inside pair of braces to avoid syntax error of declaring function when parsing. Then we have,
(function(msg){
alert(msg);
}) ('Few');
By this way, It's easy understand for me.

When you did:
(function (msg){alert(msg)});
('SO');
You ended the function before ('SO') because of the semicolon. If you just write:
(function (msg){alert(msg)})
('SO');
It will work.
Working example: http://jsfiddle.net/oliverni/dbVjg/

The simple reason why it doesn't work is not because of the ; indicating the end of the anonymous function. It is because without the () on the end of a function call, it is not a function call. That is,
function help() {return true;}
If you call result = help(); this is a call to a function and will return true.
If you call result = help; this is not a call. It is an assignment where help is treated like data to be assigned to result.
What you did was declaring/instantiating an anonymous function by adding the semicolon,
(function (msg) { /* Code here */ });
and then tried to call it in another statement by using just parentheses... Obviously because the function has no name, but this will not work:
('SO');
The interpreter sees the parentheses on the second line as a new instruction/statement, and thus it does not work, even if you did it like this:
(function (msg){/*code here*/});('SO');
It still doesn't work, but it works when you remove the semicolon because the interpreter ignores white spaces and carriages and sees the complete code as one statement.
(function (msg){/*code here*/}) // This space is ignored by the interpreter
('SO');
Conclusion: a function call is not a function call without the () on the end unless under specific conditions such as being invoked by another function, that is, onload='help' would execute the help function even though the parentheses were not included. I believe setTimeout and setInterval also allow this type of function call too, and I also believe that the interpreter adds the parentheses behind the scenes anyhow which brings us back to "a function call is not a function call without the parentheses".

(function (msg){alert(msg)})
('SO');
This is a common method of using an anonymous function as a closure which many JavaScript frameworks use.
This function called is automatically when the code is compiled.
If placing ; at the first line, the compiler treated it as two different lines. So you can't get the same results as above.
This can also be written as:
(function (msg){alert(msg)}('SO'));
For more details, look into JavaScript/Anonymous Functions.

The IIFE simply compartmentalizes the function and hides the msg variable so as to not "pollute" the global namespace. In reality, just keep it simple and do like below unless you are building a billion dollar website.
var msg = "later dude";
window.onunload = function(msg){
alert( msg );
};
You could namespace your msg property using a Revealing Module Pattern like:
var myScript = (function() {
var pub = {};
//myscript.msg
pub.msg = "later dude";
window.onunload = function(msg) {
alert(msg);
};
//API
return pub;
}());

Anonymous functions are functions that are dynamically declared at
runtime. They’re called anonymous functions because they aren’t
given a name in the same way as normal functions.
Anonymous functions are declared using the function operator instead
of the function declaration. You can use the function operator to
create a new function wherever it’s valid to put an expression. For
example, you could declare a new function as a parameter to a
function call or to assign a property of another object.
Here’s a typical example of a named function:
function flyToTheMoon() {
alert("Zoom! Zoom! Zoom!");
}
flyToTheMoon();
Here’s the same example created as an anonymous function:
var flyToTheMoon = function() {
alert("Zoom! Zoom! Zoom!");
}
flyToTheMoon();
For details please read http://helephant.com/2008/08/23/javascript-anonymous-functions/

Anonymous functions are meant to be one-shot deal where you define a function on the fly so that it generates an output from you from an input that you are providing. Except that you did not provide the input. Instead, you wrote something on the second line ('SO'); - an independent statement that has nothing to do with the function. What did you expect? :)

Different ways of creating functions in javascript

First of all - yes, I know there are plenty of posts regarding this, or at least very similar ones. Having looked through those I still haven't found the answer to what I'm looking for:
I've learned that there are two main ways to create functions in javascript:
var test = function(a){
console.log(a);
}
Which is created at runtime, and:
function test(a){
console.log(a);
}
Which is created before runtime.
Today I saw this one:
(function test(a){
console.log(a);
})();
I have never seen that before. What separates this one from the two above?

This is immediately-invoke function, it will call itself immediately after declaration.
You can read more about Immediately-invoked function expression at wikipedia.

What separates them is that the first 2 just define the function, whilst the last one defines and executes the function. Typical use for the last one is to reduce global namespace pollution as variables will be scoped to the function not the window.

Your function examples would be considered in Javascript as follows:
Function expression
Function declaration
Immediately invoked function also called a Immediately invoked function expression "IIFE" and self-executing anonymous function.
What happens in what you call 'runtime' is two things. The Javascript engine makes two passes over your code. First pass could be called the declaration pass where variables and function declaration are identified. However, a funny thing happens with variables in this first pass they are said to be 'hoisted' but not initialized. The second pass is the execution pass. Those are the two things that happen in what I think you are referring to as 'runtime'.
The first function what will happen is the same behavior that happens to variables since is assigned to a variable it will be "hoisted" to the top of it functional context and declared but not initialized. So JS will know its a variable but its recognized as undefined until it reaches the section of code that says 'oh' initialize this variable to a function. So you have to wait until after the initialization step to use.
/* This is how function expression looks after
first pass identified but not assigned(initialized) to function
its hoisted and undefined just like a reg variable. */
var test;
//test is undefined here
console.log(test);
//test is initialize here
var test = function (a) {
console.log(a);
}
//test is recognized as a function here
console.log(test);
The second function will be declared(identified) as function in the first pass and you can use it since JS will know right that this function is a function.
//this is recognized as function
console.log(test);
function test (a) {
console.log(a);
}
//test is recognized as function here as well
console.log(test);
The third function will execute itself right away after being declared in the first pass and create it own execution context, self containing.
//test is not recognized here
console.log(test);
(function test(a) {
console.log(a);
//surprise test is recognized here
console.log(test);
})();
//test is not recognized here
console.log(test);
You also have anonymous functions, and named function expressions. Functions are very powerful in JS. Happy Coding!!

JavaScript: What is an anonymous function? [duplicate]

I was reading some posts about closures and saw this everywhere, but there is no clear explanation how it works - everytime I was just told to use it...:
// Create a new anonymous function, to use as a wrapper
(function(){
// The variable that would, normally, be global
var msg = "Thanks for visiting!";
// Binding a new function to a global object
window.onunload = function(){
// Which uses the 'hidden' variable
alert( msg );
};
// Close off the anonymous function and execute it
})();
Ok I see that we will create new anonymous function and then execute it. So after that this simple code should work (and it does):
(function (msg){alert(msg)})('SO');
My question is what kind of magic happens here? I thought that when I wrote:
(function (msg){alert(msg)})
then a new unnamed function would be created like function ""(msg) ...
but then why doesn't this work?
(function (msg){alert(msg)});
('SO');
Why does it need to be in the same line?
Could you please point me some posts or give me an explanation?

Drop the semicolon after the function definition.
(function (msg){alert(msg)})
('SO');
Above should work.
DEMO Page: https://jsfiddle.net/e7ooeq6m/
I have discussed this kind of pattern in this post:
jQuery and $ questions
EDIT:
If you look at ECMA script specification, there are 3 ways you can define a function. (Page 98, Section 13 Function Definition)
1. Using Function constructor
var sum = new Function('a','b', 'return a + b;');
alert(sum(10, 20)); //alerts 30
2. Using Function declaration.
function sum(a, b)
{
return a + b;
}
alert(sum(10, 10)); //Alerts 20;
3. Function Expression
var sum = function(a, b) { return a + b; }
alert(sum(5, 5)); // alerts 10
So you may ask, what's the difference between declaration and expression?
From ECMA Script specification:
FunctionDeclaration :
function Identifier ( FormalParameterListopt ){ FunctionBody
}
FunctionExpression :
function Identifieropt ( FormalParameterListopt ){ FunctionBody
}
If you notice, 'identifier' is optional for function expression. And when you don't give an identifier, you create an anonymous function. It doesn't mean that you can't specify an identifier.
This means following is valid.
var sum = function mySum(a, b) { return a + b; }
Important point to note is that you can use 'mySum' only inside the mySum function body, not outside. See following example:
var test1 = function test2() { alert(typeof test2); }
alert(typeof(test2)); //alerts 'undefined', surprise!
test1(); //alerts 'function' because test2 is a function.
Live Demo
Compare this to
function test1() { alert(typeof test1) };
alert(typeof test1); //alerts 'function'
test1(); //alerts 'function'
Armed with this knowledge, let's try to analyze your code.
When you have code like,
function(msg) { alert(msg); }
You created a function expression. And you can execute this function expression by wrapping it inside parenthesis.
(function(msg) { alert(msg); })('SO'); //alerts SO.

It's called a self-invoked function.
What you are doing when you call (function(){}) is returning a function object. When you append () to it, it is invoked and anything in the body is executed. The ; denotes the end of the statement, that's why the 2nd invocation fails.

One thing I found confusing is that the "()" are grouping operators.
Here is your basic declared function.
Ex. 1:
var message = 'SO';
function foo(msg) {
alert(msg);
}
foo(message);
Functions are objects, and can be grouped. So let's throw parens around the function.
Ex. 2:
var message = 'SO';
function foo(msg) { //declares foo
alert(msg);
}
(foo)(message); // calls foo
Now instead of declaring and right-away calling the same function, we can use basic substitution to declare it as we call it.
Ex. 3.
var message = 'SO';
(function foo(msg) {
alert(msg);
})(message); // declares & calls foo
Finally, we don't have a need for that extra foo because we're not using the name to call it! Functions can be anonymous.
Ex. 4.
var message = 'SO';
(function (msg) { // remove unnecessary reference to foo
alert(msg);
})(message);
To answer your question, refer back to Example 2. Your first line declares some nameless function and groups it, but does not call it. The second line groups a string. Both do nothing. (Vincent's first example.)
(function (msg){alert(msg)});
('SO'); // nothing.
(foo);
(msg); //Still nothing.
But
(foo)
(msg); //works

An anonymous function is not a function with the name "". It is simply a function without a name.
Like any other value in JavaScript, a function does not need a name to be created. Though it is far more useful to actually bind it to a name just like any other value.
But like any other value, you sometimes want to use it without binding it to a name. That's the self-invoking pattern.
Here is a function and a number, not bound, they do nothing and can never be used:
function(){ alert("plop"); }
2;
So we have to store them in a variable to be able to use them, just like any other value:
var f = function(){ alert("plop"); }
var n = 2;
You can also use syntatic sugar to bind the function to a variable:
function f(){ alert("plop"); }
var n = 2;
But if naming them is not required and would lead to more confusion and less readability, you could just use them right away.
(function(){ alert("plop"); })(); // will display "plop"
alert(2 + 3); // will display 5
Here, my function and my numbers are not bound to a variable, but they can still be used.
Said like this, it looks like self-invoking function have no real value. But you have to keep in mind that JavaScript scope delimiter is the function and not the block ({}).
So a self-invoking function actually has the same meaning as a C++, C# or Java block. Which means that variable created inside will not "leak" outside the scope. This is very useful in JavaScript in order not to pollute the global scope.

It's just how JavaScript works. You can declare a named function:
function foo(msg){
alert(msg);
}
And call it:
foo("Hi!");
Or, you can declare an anonymous function:
var foo = function (msg) {
alert(msg);
}
And call that:
foo("Hi!");
Or, you can just never bind the function to a name:
(function(msg){
alert(msg);
})("Hi!");
Functions can also return functions:
function make_foo() {
return function(msg){ alert(msg) };
}
(make_foo())("Hi!");
It's worth nothing that any variables defined with "var" in the body of make_foo will be closed over by each function returned by make_foo. This is a closure, and it means that the any change made to the value by one function will be visible by another.
This lets you encapsulate information, if you desire:
function make_greeter(msg){
return function() { alert(msg) };
}
var hello = make_greeter("Hello!");
hello();
It's just how nearly every programming language but Java works.

The code you show,
(function (msg){alert(msg)});
('SO');
consist of two statements. The first is an expression which yields a function object (which will then be garbage collected because it is not saved). The second is an expression which yields a string. To apply the function to the string, you either need to pass the string as an argument to the function when it is created (which you also show above), or you will need to actually store the function in a variable, so that you can apply it at a later time, at your leisure. Like so:
var f = (function (msg){alert(msg)});
f('SO');
Note that by storing an anonymous function (a lambda function) in a variable, your are effectively giving it a name. Hence you may just as well define a regular function:
function f(msg) {alert(msg)};
f('SO');

In summary of the previous comments:
function() {
alert("hello");
}();
when not assigned to a variable, yields a syntax error. The code is parsed as a function statement (or definition), which renders the closing parentheses syntactically incorrect. Adding parentheses around the function portion tells the interpreter (and programmer) that this is a function expression (or invocation), as in
(function() {
alert("hello");
})();
This is a self-invoking function, meaning it is created anonymously and runs immediately because the invocation happens in the same line where it is declared. This self-invoking function is indicated with the familiar syntax to call a no-argument function, plus added parentheses around the name of the function: (myFunction)();.
There is a good SO discussion JavaScript function syntax.

My understanding of the asker's question is such that:
How does this magic work:
(function(){}) ('input') // Used in his example
I may be wrong. However, the usual practice that people are familiar with is:
(function(){}('input') )
The reason is such that JavaScript parentheses AKA (), can't contain statements and when the parser encounters the function keyword, it knows to parse it as a function expression and not a function declaration.
Source: blog post Immediately-Invoked Function Expression (IIFE)

examples without brackets:
void function (msg) { alert(msg); }
('SO');
(this is the only real use of void, afaik)
or
var a = function (msg) { alert(msg); }
('SO');
or
!function (msg) { alert(msg); }
('SO');
work as well. the void is causing the expression to evaluate, as well as the assignment and the bang. the last one works with ~, +, -, delete, typeof, some of the unary operators (void is one as well). not working are of couse ++, -- because of the requirement of a variable.
the line break is not necessary.

This answer is not strictly related to the question, but you might be interested to find out that this kind of syntax feature is not particular to functions. For example, we can always do something like this:
alert(
{foo: "I am foo", bar: "I am bar"}.foo
); // alerts "I am foo"
Related to functions. As they are objects, which inherit from Function.prototype, we can do things like:
Function.prototype.foo = function () {
return function () {
alert("foo");
};
};
var bar = (function () {}).foo();
bar(); // alerts foo
And you know, we don't even have to surround functions with parenthesis in order to execute them. Anyway, as long as we try to assign the result to a variable.
var x = function () {} (); // this function is executed but does nothing
function () {} (); // syntax error
One other thing you may do with functions, as soon as you declare them, is to invoke the new operator over them and obtain an object. The following are equivalent:
var obj = new function () {
this.foo = "bar";
};
var obj = {
foo : "bar"
};

There is one more property JavaScript function has. If you want to call same anonymous function recursively.
(function forInternalOnly(){
//you can use forInternalOnly to call this anonymous function
/// forInternalOnly can be used inside function only, like
var result = forInternalOnly();
})();
//this will not work
forInternalOnly();// no such a method exist

It is a self-executing anonymous function. The first set of brackets contain the expressions to be executed, and the second set of brackets executes those expressions.
(function () {
return ( 10 + 20 );
})();
Peter Michaux discusses the difference in An Important Pair of Parentheses.
It is a useful construct when trying to hide variables from the parent namespace. All the code within the function is contained in the private scope of the function, meaning it can't be accessed at all from outside the function, making it truly private.
See:
Closure (computer science)
JavaScript Namespacing
Important Pair of Javascript Parentheses

Another point of view
First, you can declare an anonymous function:
var foo = function(msg){
alert(msg);
}
Then you call it:
foo ('Few');
Because foo = function(msg){alert(msg);} so you can replace foo as:
function(msg){
alert(msg);
} ('Few');
But you should wrap your entire anonymous function inside pair of braces to avoid syntax error of declaring function when parsing. Then we have,
(function(msg){
alert(msg);
}) ('Few');
By this way, It's easy understand for me.

When you did:
(function (msg){alert(msg)});
('SO');
You ended the function before ('SO') because of the semicolon. If you just write:
(function (msg){alert(msg)})
('SO');
It will work.
Working example: http://jsfiddle.net/oliverni/dbVjg/

The simple reason why it doesn't work is not because of the ; indicating the end of the anonymous function. It is because without the () on the end of a function call, it is not a function call. That is,
function help() {return true;}
If you call result = help(); this is a call to a function and will return true.
If you call result = help; this is not a call. It is an assignment where help is treated like data to be assigned to result.
What you did was declaring/instantiating an anonymous function by adding the semicolon,
(function (msg) { /* Code here */ });
and then tried to call it in another statement by using just parentheses... Obviously because the function has no name, but this will not work:
('SO');
The interpreter sees the parentheses on the second line as a new instruction/statement, and thus it does not work, even if you did it like this:
(function (msg){/*code here*/});('SO');
It still doesn't work, but it works when you remove the semicolon because the interpreter ignores white spaces and carriages and sees the complete code as one statement.
(function (msg){/*code here*/}) // This space is ignored by the interpreter
('SO');
Conclusion: a function call is not a function call without the () on the end unless under specific conditions such as being invoked by another function, that is, onload='help' would execute the help function even though the parentheses were not included. I believe setTimeout and setInterval also allow this type of function call too, and I also believe that the interpreter adds the parentheses behind the scenes anyhow which brings us back to "a function call is not a function call without the parentheses".

(function (msg){alert(msg)})
('SO');
This is a common method of using an anonymous function as a closure which many JavaScript frameworks use.
This function called is automatically when the code is compiled.
If placing ; at the first line, the compiler treated it as two different lines. So you can't get the same results as above.
This can also be written as:
(function (msg){alert(msg)}('SO'));
For more details, look into JavaScript/Anonymous Functions.

The IIFE simply compartmentalizes the function and hides the msg variable so as to not "pollute" the global namespace. In reality, just keep it simple and do like below unless you are building a billion dollar website.
var msg = "later dude";
window.onunload = function(msg){
alert( msg );
};
You could namespace your msg property using a Revealing Module Pattern like:
var myScript = (function() {
var pub = {};
//myscript.msg
pub.msg = "later dude";
window.onunload = function(msg) {
alert(msg);
};
//API
return pub;
}());

Anonymous functions are functions that are dynamically declared at
runtime. They’re called anonymous functions because they aren’t
given a name in the same way as normal functions.
Anonymous functions are declared using the function operator instead
of the function declaration. You can use the function operator to
create a new function wherever it’s valid to put an expression. For
example, you could declare a new function as a parameter to a
function call or to assign a property of another object.
Here’s a typical example of a named function:
function flyToTheMoon() {
alert("Zoom! Zoom! Zoom!");
}
flyToTheMoon();
Here’s the same example created as an anonymous function:
var flyToTheMoon = function() {
alert("Zoom! Zoom! Zoom!");
}
flyToTheMoon();
For details please read http://helephant.com/2008/08/23/javascript-anonymous-functions/

Anonymous functions are meant to be one-shot deal where you define a function on the fly so that it generates an output from you from an input that you are providing. Except that you did not provide the input. Instead, you wrote something on the second line ('SO'); - an independent statement that has nothing to do with the function. What did you expect? :)

Different forms of defining a function

What is the difference between following forms of function definition:
$.myFunction = function() {
//function body
};
var myFunction = function() {
//function body
};
$(myFunction = function() {
//function body
});

First one adds a function myFunction to $ which can be any of the JavaScript libraries such as jQuery, mooTools, Prototype, etc that use $ internally.
In first function, you are simply adding your own function to javascript library designated by $.
Second one is Function Expression as oppossed to Function Declaration which has this form:
function myFunction() {
//function body
}
Third one looks weired but it is also Function Expression used in the context of a JavaScript library due to $ then whatever that library is meaning by that.
To know more about the differece between Function Expression and Function Declaration, check out this excellent post:
Named function expressions demystified
The most important differece between Function Expression and Function Declaration for you to keep in mind is that Function Declaration no matter where declared is available throughout (because they are hoisted to top) whereas Function Expression can only run if it has been defined before or up somewhere for your later code. An example would be:
foo1(); // alerts "I am function declaration"
foo2(); // error, undefined function
function foo1() {
alert('I am function declaration');
}
var foo2 = function() {
alert('I am function expression');
};
foo1(); // alerts "I am function declaration"
foo2(); // alerts "I am function expression"
FYI, there are also self-invoking anonymous functions and self-inovking named functions:
// self invoking anonymous function
(function(){
// code
})();
// self invoking named function
(function foo(){
// code
})();
And function declarations converted into function expression using those chars:
! function(){
// code
})();
+ function foo(){
// code
})();
Notice that self-invoking functions are run as soon as parsed because of () at the end of their signature.

$.myFunction = function() {
//function body
};
This creates a function as a property of the $ object. If you're using jQuery then $ is the global jQuery object. You would call this function with $.myFunction()
var myFunction = function() {
//function body
};
This creates a function assigned to a variable called myFunction. If you do this inside another function, then myFunction will only be available in that scope.
$(myFunction = function() {
//function body
});
This one is doing two things, let's break it down into two steps to make it more clear:
myFunction = function() { // step 1
//function body
};
$(myFunction); // step 2
The first step is creating a function assigned to a global variable called myFunction. The variable is global because you didn't use the var keyword. In general, you probably don't want to use a global variable, and certainly not when its definition isn't very obvious.
The second step is (assuming you're using jQuery) passing myFunction to jQuery to be executed when the DOM has finished loading. See the jQuery documentation for more information.
It's more common to pass a function to jQuery without assigning it to any variable, i.e.
$(function() {
//function body
});

Well really only one of those is what I would call a function definition, the second one. The first one is extending the global jQuery object (or any other library that uses the dollar sign) with that function and third one is just weird.

Assuming the "$" stands for jQuery (which seems quite obvious here), the third syntax defines an anonymous function and pass it as an argument to jQuery, which is a shortcut for registering a callback for jQuery.DocumentReady.

What is this? (function(){ })() [duplicate]

This question already has answers here:
Closed 11 years ago.
Possible Duplicates:
What does this JavaScript snippet mean?
Location of parenthesis for auto-executing anonymous JavaScript functions?
(function(){
//something here...
})() <--//This part right here.
What exactly is this )()?
What if I change it to this ())?
(function(){
//something here...
}()) <--//Like this

This declares an anonymous function and calls it immediately.
The upside of doing this is that the variables the function uses internally are not added to the current scope, and you are also not adding a function name to the current scope either.
It is important to note that the parentheses surrounding the function declaration are not arbitrary. If you remove these, you will get an error.
Finally, you can actually pass arguments to the anonymous function using the extra parentheses, as in
(function (arg) {
//do something with arg
})(1);
See http://jsfiddle.net/eb4d4/

They are the same.
There has to be a parenthesis either around the function definition or around the function call to make it valid Javascript syntax, but it doesn't matter which you use.
To demonstrate what it does, using a named function it would be:
function something() {}
// parenthesis around the function reference:
(something)();
// parenthesis around the function call:
(something());

It's an anonymous function that gets called immediately the () calls the function and there are ( and ) wrapping the whole thing.
( // arbitrary wrapping
(function() { // begin anon function
}) // end anon function
() // call the anon function
) // end arbitrary wrapping

The first one just wraps the function in ( ) so that it can call the function immediately with the ()
(function(){
alert('Hi');
})();
Alerts Hi, while
function(){
alert('Hi');
}
Doesn't do anything since your function is never executed.

That's simply an anonymous function. The () parens call the function immediately, instead of waiting for it to be called elsewhere.

Its an immediately invoked anonymous function. ()) would not work because you need () around the function before you can call it with ().
Sorta equivalent to:
function a(){}
a();

This is declaring an anonymous function and then immediately executing it. This is common for creating scoped variables.

In JavaScript, function definition is a literal - which means, it's an expression with the value of the Function object.
If you put () right after it, you effectively call the function right after defining it.

Develop Reference

JavaScript is the programming language of the Web.

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