Question

When creating the function foo here, I reference a variable fromTheFuture the hasn’t been declared yet. This actually works as expected, but why? Is it considered dangerous or bad practice?

var foo = function(x) {
  return fromTheFuture * x;
};

var fromTheFuture = 5;

console.log(foo(10));

I can see this being very convenient though, if you have several functions that wants use each other in a cycle fashion – without having to declare them all with var in the beginning of the method.

Answer 1

By the time foo is called, fromTheFuture is defined. More accurately, due to hoisting, your code is essentially:

var foo, fromTheFuture;
foo = function(x) {return fromTheFuture*x;};
fromTheFuture = 5;
console.log(foo(10));

If you were to call foo(10) before fromTheFuture=5, you would get NaN.