Javascript Typeof Undefined Error

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Typeof Undefined Javascript

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Typeof Operator In Javascript

like you, helping each other. Join them; it only takes a minute: Sign up typeof !== “undefined” vs. != null up vote 318 down vote favorite 118 I often see JavaScript code which checks for undefined parameters typeof function in javascript etc. this way: if (typeof input !== "undefined") { // do stuff } This seems kind of wasteful, since it involves both a type lookup and a string comparison, not to mention its verbosity. It's needed because 'undefined' could be renamed, though. My question is: How is that code any better than this approach: if (null != input) { // do stuff } As far as I know, you can't redefine null, so it's not typeof in java going to break unexpectedly. And, because of the type-coercion of the != operator, this checks for both undefined and null... which is often exactly what you want (e.g. for optional function parameters). Yet this form does not seem widespread, and it even causes JSLint to yell at you for using the evil != operator. Why is this considered bad style? javascript coding-style share|improve this question edited Aug 23 '13 at 11:45 WoIIe 1,151622 asked Apr 24 '10 at 3:26 Derek Thurn 4,91372545 9 @ Marcel, there is not real difference, but there are two reasons to do it. One, is that for some it is clearer to read. And the second reason, is that it prevents accidental overwriting of a variable. Have you ever done this: if( foo = "value" ) when intending to do a comparison. If you get into the habit of reversing the variable, in the assignment/comparison operator, then you won't have that problem. –Layke Oct 11 '11 at 9:14 25 For some (including me) this is actually more difficult to read. Also, most IDEs warn you of accidental assignment. But I still use this form if the compared variable is very long. YMMV. –johndodo Apr 4 '12 at 7:05 13 @MarcelKorpel This is called "Yoda condition": umumble.com/blogs/Programming/321 –kol Apr 18 '12 at 14:37 49 It's

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Typeof In Jquery

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Undefined Javascript Check

Join the Stack Overflow Community Stack Overflow is a community of 4.7 million programmers, just like you, helping each other. Join them; it only takes a minute: Sign up How to check for http://stackoverflow.com/questions/2703102/typeof-undefined-vs-null “undefined” in JavaScript? [duplicate] up vote 1254 down vote favorite 373 This question already has an answer here: Detecting an undefined object property 33 answers What is the most appropriate way to test if a variable is undefined in JavaScript? I've seen several possible ways: if (window.myVariable) Or if (typeof(myVariable) != "undefined") Or if (myVariable) //This throws an error if undefined. Should this be in http://stackoverflow.com/questions/3390396/how-to-check-for-undefined-in-javascript Try/Catch? javascript undefined share|improve this question edited Nov 23 '14 at 13:29 Peter Mortensen 10.3k1369107 asked Aug 2 '10 at 17:53 LamonteCristo 15.9k25128302 marked as duplicate by James Lawruk, Josh Mein, Shivan Dragon, Lee Taylor, karthikr Oct 10 '14 at 13:59 This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question. 4 Do you want to check for only undefined, or null as well? –Nick Craver♦ Jun 6 '10 at 20:26 3 check this stackoverflow.com/questions/27509/… –Amr Badawy Aug 2 '10 at 17:59 4 @Robert - that question has an accepted answer that answers here have proven to be wrong –Daniel Schaffer Aug 2 '10 at 18:09 2 See: How to check for undefined in javascript?, and whether a variable is undefined and How to handle ‘undefined’ in javascript –Shog9♦ Aug 3 '10 at 8:47 1 Explanation for defined/undefined object properties see this: [stackoverflow.com/a/18135509/1823469][1] [1]: stackoverflow.com/a/18135509/1823469 –Konstantin Smolyanin Aug 8 '13 at 20:41 | show 1 more comment 17 Answers 17 active oldest votes up vote 1521 down vote accepted If you are interest

several ways of checking whether a variable has the value undefined. This blog post explains the differences. Checking via === Using strict equality [1] is the http://www.2ality.com/2013/04/check-undefined.html canonical way of checking for undefined: if (x === undefined) ... Changing undefined undefined is a property of the global object (and thus a global variable). Under ECMAScript 3, you could change its value. Under ECMAScript 5, you can't do that, any more: > undefined = 123 > undefined undefined You can, however, shadow it in a function, either typeof undefined via a parameter or via a local variable: > (function () { var undefined = 123; return undefined; }()) 123 From now on, undefined is used to refer to the identifier, while undefined is used to refer to the actual value. Because you could globally change the value of undefined under ECMAScript 3, two techniques were often used javascript typeof undefined to ensure that it had the correct value. If you are targeting older browsers, these techniques are still relevant. Technique 1: shadow undefined yourself. (function (undefined) { if (x === undefined) ... // safe now }()); // don't hand in a parameter Above, undefined is a parameter whose value is undefined, because it has not been provided by the caller. Technique 2: compare with void 0. The void operator [2] evaluates its operand, ignores the result and returns undefined. That means that void 0 will always evaluate to undefined. if (x === void 0) // always safe Checking via typeof You can also check for undefined via the typeof operator [3]: if (typeof x === 'undefined') ... This is more verbose and can be slower (though many engines optimize). It has two advantages: First, it is safe with regard to a changed undefined (not that important under ECMAScript 5). Second, it also works for unknown variables: > typeof iDontKnowThisVariable === 'undefined' true > iDontKnowThisVariable === undefined ReferenceError: iDontKnowThisVariable is not defined Checking for falsine