[wpears]

You mixed up your definitions of function declaration and function expression. In this case, if the '!' weren't there, it would indeed be a function declaration, but with improper syntax (removing the '!' would result in a syntax error).

What makes a function either an expression or declaration is simply whether the parser expects an expression or declaration in the given context. The grouping operator '()' and the NOT operator '!' both are parameterized by expressions, and so the Javascript parser treats functions passed to them as expressions. Unless located where an expression is expected, functions will default to declarations.

[rmrfrmrf]

You're right, my mistake. Brain cramp :)