I've seen this odd behavior right now that I could not define
function(){}
or
function(a){
console.log(a)
}
It throws an Uncaught SyntaxError.
But test = function(){} or (function(){}) did work.
The Safari dev tools have a better error report: It says
SyntaxError: Function statements must have a name.
Okay it makes no sense to define a function like that if you will never use it. But it's still odd. I guess I have already provided the answer in the question.
JavaScript has function declarations and function expressions. The former cannot be immediately invoked, while the latter can. It's a matter of the language grammar.
Function declaration:
function foo() {
// ...
}
Function expression (on the right-hand side of the =):
var foo = function() {
// ...
};
By adding parentheses around the function, you're forcing the function to be parsed as a function expression because it's not possible to put a function declaration within parentheses (where only expressions are expected and accepted):
(function foo() {
// ...
});
You can then immediately invoke it:
(function foo() {
// ...
})();
For a more detailed write-up, check out Function Definitions in JavaScript.
The brackets in (function(){}) mean that the function evaluates, the object is now contained inside the brackets, and this evaluation returns the function. It can be called, like this: (function(){})();.
function(){} does not evaluate. It makes sense for this to cause an error, because as you said, you will not be able to use it in any way.
function(){}()(if not used as argument) but you can do(function(){})()