definedButNotInitialized in window: true definedAndInitialized in window: false someObject.firstProp in window: false someObject.secondProp in window: false …

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 …

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 …

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 …

Feb 24, 2011 · 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 …

Dec 17, 2013 · 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 …

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 …

Aug 26, 2008 · 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 …

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 …

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 …