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 …

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 …

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 …

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 …

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 …

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 …

Jan 18, 2011 · As @Struppi pointed out, jQuery's outermost function has an argument named undefined. Within jQuery, foo === undefined is checking against the local copy of undefined …