If it is undefined, it will not be equal to a string that contains the characters "undefined", as the string is not undefined. The difference between null and undefined is nan. While literally using the keyword undefined, boolean(undefined) works, trying that with an undefined variable doesn't work, and that is the whole point of doing the check for null or undefined.
May 7, 2009yes, variables can have a value of undefined and you can explicitly assign values to them. Assigning undefined to a variable though is probably confusing, since it's a bit of a paradox. Apr 22, 2009the biggest misconception in many of the answers here is that 'undefined' is a javascript keyword.
It's not a keyword at all, but a variable that (most of the time) happens to be undefined. Dec 17, 2013there are 3 undefined in javascript. The global variable undefined, the value undefined and the type undefined.
Even if the global variable undefined is overridden, the value undefined still. Jan 7, 2025cuál es la diferencia en javascript entre una variable undefined y una variable null? Cómo puedo saber si una variable está null, undefined o ambos?
I can't find any difference between typeof somevar == 'undefined' and typeof somevar === 'undefined', because typeof always returns string. Dec 31, 2009typeof foo !== 'undefined' window.foo !== undefined 'foo' in window the first two should be equivalent (as long as foo isn't shadowed by a local variable), whereas the last one will return true.
