All JavaScript Function Types?

Question

While making several test projects for my small library of code I have come across many tutorials that go about making functions in many different ways.

For example:

1. Function Declarations

   • FunctionDeclaration : function Identifier ( FormalParameterList opt ){ FunctionBody }

2. Function Expressions

   • FunctionExpression : function Identifier opt ( FormalParameterList opt ){ FunctionBody }

3. Anonymous Self-Executing Functions

   • (function(msg){ alert(msg); })('SO'); // alerts "SO"

4. Anonymous Self-Executing Functions that I have seen in source code that I suspect may be incorrect but I want to make sure

   • (function(msg){ alert(msg); }('SO')); // alerts "SO", but the passed parameter appears inside the parenthesis that wrap the function, not after like I think is correct.

With all this said, I want to clear the air about a few questions I had regarding each of the types of functions.

A function declaration:

function display(){
    //this function is evaluated at parse time,
      can be called any time
}

Is it true that a function declaration (that is evaluated at parse time and does not require parenthesis after the brackets) can be overwritten by function expressions as this question seems to indicate and this source code seems to back up?:

var display = function(){
                 //this function will be called as opposed to the
                 //display() declaration (above) any time 
                 //display() is used after this variable assignment
              };

A function expression:

var display = function nameOptional(){

              };

Is the semi-colon after the bracket in function expressions that has been assigned to a variable necessary? And also, does that mean that this:

function name(){
    //...
};

is different than this:?

function name(){
    //...
}//no semi-colon

Is the first name() a function expression, and the second name() a function declaration? Or is the first name() a function expression and the second name() a function expression with an optional left-out semi-colon. Because I have read that in function expressions, the semi-colon is only recommended. What confuses me is that in a function declaration you must have an identifier, and a body. In an expression, the identifier is optional, but you must have a body. So if you have

function identifier(){
    //...body...
}

is this a declaration, or an expression since the semi-colon is not required on expressions or declarations. On expressions it is only recommended for optimum results in the case of variable assigning.

An Anonymous Self-Executing Function:

I understand how these work, but I have seen two ways of writing this, one I am almost certain is wrong. I just would like to clear this up:

(function(msg){
    alert(msg);
})("SO");

and

(function(msg){
    alert(msg);
}("SO"));

I have read that the reason an anonymous function executes is because it is wrapped in () and if you want any parameters to be passed to it, they must be added after those () and before a semi-colon tying the whole thing off, if this is true...the second anonymous function is written incorrectly...is this true?

I have read several questions that have been answered with this link and content containing this informatino

Answer 1

Only the first two types of functions you've identified here are "real". The other two are applications of function expressions.

Going through your questions in order:

Is it true that a function declaration (that is evaluated at parse time and does not require parenthesis after the brackets) can be overwritten by function expressions as this question seems to indicate and this source code seems to back up?

Yes. (Unfortunately.) Declaring a function a second time overwrites the first definition. The original function may still be accessible if a reference to it has been been assigned to a variable, though; for instance:

function a() { console.log(1); }
var a1 = a;
function a() { console.log(2); }

a1(); // outputs "1"
a();  // outputs "2"

Is the semi-colon after the bracket in function expressions that has been assigned to a variable necessary?

Generally, yes — it denotes the end of the statement, just like the semicolon in var a = 1;. It's sometimes optional, like most semicolons in Javascript, but omitting it is inadvisable.

And also, does that mean that [a function declaration with a semicolon] is different from [a function declaration without one]?

The semicolon is not required, or meaningful, at the end of a function declaration. It's actually interpreted as a standalone semicolon in between the declarations, which has no effect.

I have read that the reason an anonymous function executes is because it is wrapped in () and if you want any parameters to be passed to it, they must be added after those () and before a semi-colon tying the whole thing off, if this is true...the second anonymous function is written incorrectly...is this true?

They're both fine; it's a stylistic choice. The line has to begin with something other than function, so that it's not interpreted as a function definition, so an initial parenthesis is used, but the exact placement of the parentheses isn't significant. If we replace the whole function expression with a, the comparison is simply between (a)() and (a()); there's no real difference.