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 …

– Daniel Schaffer Aug 2, 2010 at 18:09 3 See: How to check for undefined in javascript?, and whether a variable is undefined and How to handle ‘undefined’ in javascript – Shog9 Aug 3, …

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 …

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 · The difference between null and undefined is: JavaScript will never set anything to null, that's usually what we do. While we can set variables to undefined, we prefer null …

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 …

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 …

string, number, boolean, null, undefined, and symbol. In JavaScript there are 7 primitive types: undefined, null, boolean, string, number, bigint and symbol. Everything else is an object. The …

May 6, 2010 · In JavaScript you can declare a variable and if it’s undefined, you can check variable == undefined; I know that, but how can you compare a value that you don’t know yet if …

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 …