Hi @fernkul . I'd say that a closure is a function who has access to its outter scope. As each function define a new scope, closure have access to their outter scope :
function test(){
var message = "hello world!";
function write(){
console.log(message);
}();
}
The function "write" has access to its outter scope: it's a closure. You can relate on this article if you need more info.
Hope it helped :).