安裝中文字典英文字典辭典工具!
安裝中文字典英文字典辭典工具!
|
- 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 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)
- 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 (??
- JavaScript checking for null vs. undefined and difference between . . .
Undefined shows that a variable is declared but it is not assigned a value Null shows that a variable is deliberately set to null; Undefined happens most of the time in unintentional cases while null happens for intentional purposes
- Detecting an undefined object property - Stack Overflow
There is a difference between an undefined member and a defined member with an undefined value But unhappily typeof obj foo does not tell us which of the three cases we have However we can combine this with "foo" in obj to distinguish the cases | typeof obj x === 'undefined' | !("x" in obj) 1 { x:1 } | false | false 2
- 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 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)
|
|
|