Explain the encapsulated anonymous function syntax

Question

Summary

Can you explain the reasoning behind the syntax for encapsulated anonymous functions in JavaScript? Why does this work: (function(){})(); but

Answer 1

Great answers have already being posted. But I want to note that function declarations return an empty completion record:

14.1.20 - Runtime Semantics: Evaluation

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody }

1. Return NormalCompletion(empty).

This fact is not easy to observe, because most ways of attempting to get the returned value will convert the function declaration to a function expression. However, eval shows it:

var r = eval("function f(){}");
console.log(r); // undefined

Calling an empty completion record makes no sense. That's why function f(){}() can't work. In fact the JS engine does not even attempt to call it, the parentheses are considered part of another statement.

But if you wrap the function in parentheses, it becomes a function expression:

var r = eval("(function f(){})");
console.log(r); // function f(){}

Function expressions return a function object. And therefore you can call it: (function f(){})().