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 ...

May 7, 2009 · 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.

Feb 22, 2011 · 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).

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 (??

Aug 29, 2011 · 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 ...

Dec 31, 2009 · 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).

Apr 22, 2009 · The problem is that undefined compared to null using == gives true. The common check for undefined is therefore done like this: typeof x == "undefined" this ensures the type of the variable is really undefined.

This 2ality article mentions different recommended ways of checking for undefined, including interpreting it as falsy in boolean contexts. – martieva Commented Jun 20, 2017 at 13:44

string, number, boolean, null, undefined, and symbol. In JavaScript there are 7 primitive types: undefined, null, boolean, string, number, bigint and symbol. Everything else is an object. The primitive types boolean, string and number can be wrapped by their object counterparts.

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. In contrast, all objects and arrays (not including null) are "truthy" values, which allows for such things as this (used in the Google Analytics tracker code):