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Bookmark or Share- How can I check for "undefined" in JavaScript? - Stack OverflowdefinedButNotInitialized in window: true definedAndInitialized in window: false someObject.firstProp in window: false someObject.secondProp in window: false someObject.undefinedProp in window: true notDefined in window: Exception--ReferenceError: notDefined is not defined "definedButNotInitialized" in window: false "definedAndInitialized" in ...
- What is the difference between null and undefined in JavaScript?undefined cannot be an empty wall if the holder represents the variable. In this representation, undefined is an empty holder while null is an empty holder with post-it note empty. (so the unfortunate user knows it's useless to ask the cleaning lady for toilet paper).
- Can I set variables to undefined or pass undefined as an argument?@Hortitude: That's point (4). I generally wouldn't typeof foo=='undefined'; it tends to be used for (4a) global-sniffing, in which case I'd prefer to be explicit and say 'foo' in window, or (4b) testing against the undefined-value itself, in which case I'd prefer to be readable and say foo===undefined.
- What is the difference in JavaScript between 'undefined' and 'not ...An undeclared variable (that is, one that doesn't exist) does not have a type - and so its type is undefined. I believe that the generally accepted way to test if something is undefined is . typeof var === 'undefined' rather than. var === undefined since if the variable does not exist, you cannot access it.
- How to check for an undefined or null variable in JavaScript?Strict equality checks (===) should be used in favor of ==. The only exception is when checking for undefined and null by way of null. // Check for both undefined and null values, for some important reason. undefOrNull == null; EDIT 2021-03: Nowadays most browsers support the Nullish coalescing operator (??
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