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:

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 …

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 …

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 …

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 …

Aug 29, 2011 · undefined is a variable, not a constant, and can be assigned a value. Because of this, one school of thought says the second path is safer, since you cannot be sure of the …

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

If you simply do var v;, it will automatically have an initial value of undefined, so if v gets some other non-falsey value, x will be assigned that value. But if v isn't assigned anything, or gets …

Dec 4, 2016 · But check the question: "How to determine if a variable is undefined or null", the ! operand used with an if will always return to true if the variable is null or undefined.