http://www.bing.com:80/search?q=Undefined+Array+Key+User+IDSearch resultshttp://www.bing.com:80/s/a/rsslogo.gifhttp://www.bing.com:80/search?q=Undefined+Array+Key+User+IDCopyright © 2026 Microsoft. All rights reserved. These XML results may not be used, reproduced or transmitted in any manner or for any purpose other than rendering Bing results within an RSS aggregator for your personal, non-commercial use. Any other use of these results requires express written permission from Microsoft Corporation. By accessing this web page or using these results in any manner whatsoever, you agree to be bound by the foregoing restrictions.https://stackoverflow.com/questions/3390396/how-can-i-check-for-undefined-in-javascriptIf it is undefined, it will not be equal to a string that contains the characters "undefined", as the string is not undefined. You can check the type of the variable:Thu, 16 Apr 2026 03:06:00 GMThttps://stackoverflow.com/questions/5076944/what-is-the-difference-between-null-and-undefined-in-javascriptNaN. See for yourself. console.log (null-undefined). The difference between null and undefined is NaN. (Note that this is an attempt at humour, before you flame me for misunderstanding the question.)Wed, 25 Mar 2026 16:39:00 GMThttps://stackoverflow.com/questions/833661/what-is-the-difference-in-javascript-between-undefined-and-not-definedAssigning undefined to a variable though is probably confusing, since it's a bit of a paradox (you've defined the variable as undefined) and it's not possible to distinguish that variable from either variables that don't exist or uninitialised variables.Thu, 16 Apr 2026 11:41:00 GMThttps://stackoverflow.com/questions/2559318/how-can-i-check-for-an-undefined-or-null-variable-in-javascriptWhile literally using the keyword undefined, Boolean(undefined) works, trying that with an undefined variable doesn't work, and that is the whole point of doing the check for null or undefined.Thu, 26 Mar 2026 18:40:00 GMThttps://stackoverflow.com/questions/776950/javascript-undefined-undefinedThe biggest misconception in many of the answers here is that 'undefined' is a Javascript keyword. It's not a keyword at all, but a variable that (most of the time) happens to be undefined. So the only time "somevar === undefined" works is when the 'undefined' variable really hasn't been defined.Wed, 15 Apr 2026 02:53:00 GMThttps://stackoverflow.com/questions/20624566/understanding-undefined-in-javascript-how-it-works-how-to-safely-check-againThere are 3 undefined in javascript. The global variable undefined, the value undefined and the type undefined. Even if the global variable undefined is overridden, the value undefined still exists. There are several ways to get it one of which is an empty argument to a function, another is a variable declaration without assigning anything:Tue, 14 Apr 2026 16:30:00 GMThttps://es.stackoverflow.com/questions/49109/c%c3%b3mo-determinar-si-una-variable-es-undefined-o-null-en-javascriptDeseo determinar si una variable no está definida o si tiene el valor nulo. En el siguiente ejemplo, quiero que se ejecute la clásula correspondiente a la condición verdadera, es decir se imprima ...Wed, 08 Apr 2026 17:41:00 GMThttps://stackoverflow.com/questions/2703102/the-difference-between-typeof-x-undefined-and-x-nullI can't find any difference between typeof somevar == 'undefined' and typeof somevar === 'undefined', because typeof always returns string. For null it will return 'object'. Or could be that I am wrong?Thu, 16 Apr 2026 03:20:00 GMThttps://stackoverflow.com/questions/4725603/variable-undefined-vs-typeof-variable-undefined391 The jQuery Core Style Guidelines suggest two different ways to check whether a variable is defined. Global Variables: typeof variable === "undefined" Local Variables: variable === undefined Properties: object.prop === undefined Why does jQuery use one approach for global variables and another for locals and properties?Sat, 11 Apr 2026 12:59:00 GMThttps://stackoverflow.com/questions/1984721/how-to-handle-undefined-in-javascripttypeof 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).Mon, 13 Apr 2026 17:36:00 GMT