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 · 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 …

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 …

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 …

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 …

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

typeof MyVariable == 'undefined' doesn't discern between an initialized variable with an undefined value and an undeclared variable unless the variable was initially declared and initialized to …

If you want to find the difference between an undefined property p of an object o, and a property p that has defined and set to undefined, you have to use the 'p' in o operator.

Nov 6, 2014 · undefined is a special type where it simply indicates that the variable language is not initialized or may be it's not yet defined. null in javascript simply indicates absence of a …

Limitations and warnings Depending on how Javascript is implemented. It is possible that undefined will be converted to null instead of just being removed. Nested Object, Array will be …