Question

Why I can write

 var foo = function(){}();

But can not

 function(){}();

Are there are any design reasons?

Answer 1

The first example is an assignment: the right-hand side is an expression, and the immediate execution of an anonymous function makes sense.

The second example is a declaration: once the closing "}" is hit the declaration has ended. Parens on their own don’t make sense–they must contain an expression. The trailing ")" is an error.

Standalone declarations must be turned into expressions:

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

The first makes the declaration an expression, then executes the result. The second turns both declaration and execution into an expression.

See also When do I use parenthesis and when do I not?