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- How can I 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 (??
- 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)
- Javascript undefined condition - Stack Overflow
So obj x === undefined and typeof obj x == "undefined" are usually equivalent However, in pre-ECMAScript 5 environments (which still acount for the majority of web requests, in general), undefined is a writable property of the global object, meaning that undefined may be used as variable name or the global property may be assigned a different
- How can I check for undefined in JavaScript? - Stack Overflow
definedButNotInitialized 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 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 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)
- Understanding “undefined” in Javascript: how it works, how to safely . . .
The statement "undefined is property on host object (window in browsers)" is sort of correct There is an undefined Type whose only value is the undefined value There is also an undefined property of the Global object, which is a built–in object In browsers, the window object (a host object) is effectively an alias for the Global object –
- The difference between `typeof x !== undefined` and `x != null`
function is_undefined(value) { var undefined_check; instantiate a new variable which gets initialized to the real undefined value return value === undefined_check; } This works because when someone writes undefined = "foo" he only lets the name undefined reference to a new value, but he doesn't change the actual value of undefined
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