Explain why the following doesn't work as an IIFE: function foo(){}();. What needs to be changed to properly make it an IIFE?

The code function foo(){}(); doesn't work as an Immediately Invoked Function Expression (IIFE) because the JavaScript parser treats function foo(){} as a function declaration, not an expression. To make it an IIFE, you need to wrap the function in parentheses to turn it into a function expression: (function foo(){})();.