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What is the difference in JavaScript between undefined and not . . . An undeclared variable does not exist in Javascript memory whereas a declared one does and can be set to undefined The difference is caused by Javascript design confusion If a variable's value is undefined then it should be deleted Indeed falsy values like 0, "", null, and undefined, are all basically meaningless and identical A lot of time is wasted on them They are all nothing and non
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
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 - 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?
JavaScript || or operator with an undefined variable undefined, null, and 0 are all examples of falsy values that a variable can hold (yes, even the first one), and the operator (or if statement) acts accordingly