If it is undefined, it will not be equal to a string that contains the characters "undefined", as the string is not undefined. You can check the type of the variable:

Apr 22, 2009 · The 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 …

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 …

May 7, 2009 · Yes, 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 …

Dec 31, 2009 · typeof 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 …

Feb 22, 2011 · NaN. See for yourself. console.log (null-undefined). The difference between null and undefined is NaN. (Note that this is an attempt at humour, before you flame me for …

I can't find any difference between typeof somevar == 'undefined' and typeof somevar === 'undefined', because typeof always returns string. For null it will return 'object'. Or could be that …

Feb 24, 2011 · How do I check a variable if it's null or undefined and what is the difference between the null and undefined? What is the difference between == and === (it's hard to …

Deseo determinar si una variable no está definida o si tiene el valor nulo. En el siguiente ejemplo, quiero que se ejecute la clásula correspondiente a la condición verdadera, es decir se imprima ...

Diferença semântica Como já apontado nas respostas do Calebe Oliveira e do Alexandre Marcondes, existe uma diferença semântica entre null e undefined: o valor null é utilizado …