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:

May 7, 2009 · An undeclared variable does not exist in Javascript memory whereas a declared one does and can be set to undefined. The difference is caused by Javascript design confusion. If a variable's …

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 misunderstanding …

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.

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 I am wrong?

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 be undefined. So …

undefined, null, and 0 are all examples of falsy values that a variable can hold (yes, even the first one), and the operator (or if statement) acts accordingly.

Jan 18, 2011 · 390 The jQuery Core Style Guidelines suggest two different ways to check whether a variable is defined. Global Variables: typeof variable === "undefined" Local Variables: variable === …

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 will return true …

Aug 26, 2008 · How do I check if an object property in JavaScript is undefined?