Javascript tricky question: 0undefined

Question

var y = 0;
if (function f(){}) {
y += typeof f;
}
console.log(y);
// 0undefined

I found this snippet in a SL quiz but don't understand the output.

Function f() is an IIFE that runs without calling it and executes the third line.
Here y = 0 and typeof f = undefined.
However, 0 + undefined should be NaN.
So why is the output 0undefined????

Answer 1

Apparently you can concatenate numbers, strings, objects, null and undefined in JS.

Reference: https://masteringjs.io/tutorials/fundamentals/string-concat

Answer 2

It seems that a function declaration (treated as logical expression) is evaluated as truthy, hence, the `if` conditional body block gets processed.

But 'f' does not actually get declared as a function, because the "declaration" was only used as an evaluable expression (branch condition) for the `if` conditional. That's why it is 'undefined' inside the `if` conditional block.

'f' is not an IIFE either, verify by putting an instruction in its body e.g. an alert box, it won't be invoked.

That's how I see it ... could be wrong though.

Answer 3

0+undefined becomes a string

Answer 4

Alessandro Palazzolo 0 + undefined is evaluated as NaN, not as a string - try it!

Answer 5

Ipang Thank you for the clear explanation! My only question is why 0 + undefined is evaluated to 0undefined.

Answer 6

In the meantime, I figured out the solution: typeof always returns a string.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof