In JavaScript, what is the scope of a function declared within the condition of an if statement?

Question

I recently had an interview question where I was shown this block of code (without the answer) and asked what would be printed to the console:

var i = 1;

if(function f(){}) {
  i += typeof f;
}

console.log(i);

// prints "1undefined"

I understand that typeof returns a string. Implicit type conversions combines the string from the typeof evaluation and the number into '1undefined'. However, I thought it would print '1function'.

What I don't completely understand, is why f is not in scope?

Is this not even a scope issue? Is the function declaration solely a Logical expression that gets evaluated to true as part of the if statement, and then no longer exist in any scope?

Any clarification would be greatly appreciated.

The function is declared in the parentheses and is not passed down. Inside of the parentheses, you can use it. — Rojo, Jan 21 '21 at 14:51
The function name in that case is not supposed to be bound into the enclosing scope. In some old browsers, it was (erroneously), but I don't think that's been true for a lot of years. The function name is only bound *inside* the function, so outside there's no symbol `f`. — Pointy, Jan 21 '21 at 14:52
@Rojo no, it's not bound externally (to the function itself) at all. — Pointy, Jan 21 '21 at 14:52

Thank you · Answer 1 · 2021-01-21T15:00:03.333

f is not in scope because it is an inline named function expression. It is not a function declration. The differences are subtle but important -

// declaration
function f () { ... }

// named function expression
const foo = function f () { ... }

// named arrow expression
const f = () => { ... }

// unnamed arrow expression
() => { ... }

The semantics of if help us understand -

if (<expression>) {
  <consequent>
} else {
  <alternative>
}

As you can see, the conditional supplied to an if statement must be an expression. You cannot declare a variable in an expression. This includes function declarations.

score 1 · Answer 2 · answered Jan 21 '21 at 14:54

The function statement is inside parentheses. This makes it a function expression and not a function declaration.

The variable f only exists inside the function.

(function f(){ /* i.e. here */ })

Outside the function there is no f, so you get undefined as you would for any other variable that doesn't exist in the current scope.

console.log(typeof f);

In JavaScript, what is the scope of a function declared within the condition of an if statement?

2 Answers2