JuniorTheoryCommon

What does the program print?

JavaScript

console.log(typeof null, typeof undefined, typeof NaN);

object undefined number

Expecting null for typeof null: it gives "object". This is an old mistake in JavaScript that was never fixed, so it has to be remembered.
Expecting NaN to have its own type: NaN means “not a number”, yet it is a value of type number, so typeof NaN is "number".

JuniorTheoryCommon

JavaScript

console.log(typeof null, typeof undefined, typeof NaN);

object undefined number

Expecting null for typeof null: it gives "object". This is an old mistake in JavaScript that was never fixed, so it has to be remembered.
Expecting NaN to have its own type: NaN means “not a number”, yet it is a value of type number, so typeof NaN is "number".