javascript: what kind of function is this?

Question

I had this question in my challenge and i didn‘ t recognice what kind of / type of function is this? Never seen before... 😇

(function(){

let a = 3;
}) ();

console.log(a);

//and i dont get a result of this function... i expected at least a „3“ as output, but there was nothing... 😩

Answer 1

its an anonymous function, that immediatelly executed. also it will only run once, and cannot be called.

let a = 3. only exist inside the scope of that function, thats why you dont get anything when call it from the outside.

Answer 2

The result was nothing because the function doesn't "return" anything. It just setup a variable "a" and nothing else.

The structure of this function is made to be automatically triggered.

If you write :

function(){}
The function is just defined but not used yet.

(function(){})()
The function is defined and immediately triggered.