JavaScript: undefined !== undefined? - Stack Overflow 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 the only time "somevar === undefined" works is when the 'undefined' variable really hasn't been defined
What is the difference in JavaScript between undefined and not . . . Assigning undefined to a variable though is probably confusing, since it's a bit of a paradox (you've defined the variable as undefined) and it's not possible to distinguish that variable from either variables that don't exist or uninitialised variables
The difference between `typeof x !== undefined` and `x != null` 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?
How can I check for an undefined or null variable in JavaScript? 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
How to handle undefined in JavaScript - Stack Overflow 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 if the global varible is defined, but not initialized (or explicitly set to undefined)
Javascript undefined condition - Stack Overflow 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 value of undefined There is another school of thought that says those who redefine undefined deserve exactly what they get