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- How can I check for undefined in JavaScript? - Stack Overflow
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:
- What is the difference in JavaScript between undefined and not . . .
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 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
- What is the difference between null and undefined in JavaScript?
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 the question )
- 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
- 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?
- 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
- javascript - variable === undefined vs. typeof . . . - Stack Overflow
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 === undefined Properties: object prop === undefined Why does jQuery use one approach for global variables and another for locals and properties?
- 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)
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