// It does't add a semicolon in the case of a leading parenthesis, so....
// You will end up with a type error,
// since Javascript parser assumes that "5" is a function and "(function(){ console.log(x) })"
// is its parameter...

var x = 5 // JS parser does not add ; here
(function(){ console.log(x) })()

// the code above will get an error: Uncaught TypeError: 5 is not a function(…)
// because the interpeter read it like this
var x = 5(function(){ console.log(x) })()