JavaScript undefined undefined

Question

NOTE  As per ECMAScript5 1  section 15 1 1 3  window undefined is read-only    Modern browsers implement this correctly  for example  Safari 5 1  Firefox 7  Chrome 20  etc  Undefined is still changeable in  Chrome 14        When I recently integrated Facebook Connect with Tersus  I initially received the error messages Invalid Enumeration Value and Handler already exists when trying to call Facebook API functions   It turned out that the cause of the problem was  object x     undefined   returning false when there is no property  x  in  object    I worked around the problem by replacing strict equality with regular equality in two Facebook functions   FB Sys isUndefined   function o    return o    undefined    FB Sys containsKey   function d  key    return d key     undefined      This made things work for me  but seems to hint at some sort of collision between Facebook s JavaScript code and my own   What could cause this   Hint   It is well documented that undefined    null while undefined     null  This is not the issue here  The question is how comes we get undefined     undefined

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A   I never have and never will trust any tool which purports to produce code without the user coding  which goes double where it s a graphical tool   B   I ve never had any problem with this with Facebook Connect  It s all still plain old JavaScript code running in a browser and undefined   undefined wherever you are   In short  you need to provide evidence that your object x really really was undefined and not null or otherwise  because I believe it is impossible for what you re describing to actually be the case - no offence    - I d put money on the problem existing in the Tersus code

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From - JQuery Core Style Guidelines      Global Variables  typeof variable      undefined     Local Variables  variable     undefined   Properties  object prop     undefined

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It turns out that you can set window undefined to whatever you want  and so get object x     undefined when object x is the real undefined   In my case I inadvertently set undefined to null   The easiest way to see this happen is   window undefined   null  alert window xyzw     undefined      shows false   Of course  this is not likely to happen   In my case the bug was a little more subtle  and was equivalent to the following scenario   var n   window someName     someName expected to be set but is actually undefined window n  null     I thought I was clearing the old value but was actually changing window undefined to null alert window xyzw     undefined      shows false

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var a   typeof a      undefined      true a     undefined     true typeof a     typeof undefined     true typeof a     typeof sdfuwehflj     true

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I d like to post some important information about undefined  which beginners might not know   Look at the following code            Consider there is no code above       The browser runs these lines only              var a          --- commented out to point that we ve forgotten to declare  a  variable      if   a     undefined            alert  Not defined         else          alert  Defined      a            alert  Doing important job below      If you run this code  where variable a HAS NEVER BEEN DECLARED using var  you will get an ERROR EXCEPTION and surprisingly see no alerts at all   Instead of  Doing important job below   your script will TERMINATE UNEXPECTEDLY  throwing unhandled exception on the very first line     Here is the only bulletproof way to check for undefined using typeof keyword  which was designed just for such purpose                Correct and safe way of checking for  undefined               if   typeof a      undefined             alert              The variable is not declared in this scope   n                or you are pointing to unexisting property   n                or no value has been set yet to the variable   n                 or the value set was  undefined    n                 two last cases are equivalent  don  t worry if it blows out your mind                                     Use  typeof  for checking things like that          This method works in all possible cases   The last argument to use it is that undefined can be potentially overwritten in earlier versions of Javascript             Trollface              undefined   2          Happy debuging         Hope I was clear enough

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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

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That s a bad practice to use the    equality operator instead of       undefined     undefined    true null    undefined    true null     undefined    false   The object x     undefined should return true if x is unknown property   In chapter Bad Parts of JavaScript  The Good Parts  Crockford writes the following      If you attempt to extract a value from   an object  and if the object does not   have a member with that name  it   returns the undefined value instead       In addition to undefined  JavaScript   has a similar value called null  They   are so similar that    thinks they are   equal  That confuses some programmers   into thinking that they are   interchangeable  leading to code like  value   myObject name   if  value    null        alert name     not found             It is comparing the wrong value with   the wrong operator  This code works   because it contains two errors that   cancel each other out  That is a crazy   way to program  It is better written   like this   value   myObject name   if  value     undefined        alert name     not found

[javascript] JavaScript: undefined !== undefined?

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