Can I set variables to undefined or pass undefined as an argument

Question

I   m a bit confused about JavaScript   s undefined and null values   What does if   testvar  actually do  Does it test for undefined and null or just undefined   Once a variable is defined can I clear it back to undefined  therefore deleting the variable    Can I pass undefined as a parameter  E g    function test var1  var2  var3        test  value1   undefined   value2

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The basic difference is that undefined and null represent different concepts   If only null was available  you would not be able to determine whether null was set intentionally as the value or whether the value has not been set yet unless you used cumbersome error catching  eg  var a   a    null     This is true a    undefined     This is true  a     undefined     This is true    However  if you intentionally set the value to null  strict equality with undefined fails  thereby allowing you to differentiate between null and undefined values   var b   null  b    null     This is true b    undefined     This is true  b     undefined     This is false    Check out the reference here instead of relying on people dismissively saying junk like  In summary  undefined is a JavaScript-specific mess  which confuses everyone   Just because you are confused  it does not mean that it is a mess   https   developer mozilla org en-US docs Web JavaScript Reference Global Objects undefined  This behaviour is also not specific to JavaScript and it completes the generalised concept that a boolean result can be true  false  unknown  null   no value  undefined   or something went wrong  error    http   en wikipedia org wiki Undefined value

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The best way to check for a null values is   if   testVar     null            do action here     and for undefined  if   testVar     undefined            do action here     You can assign a avariable with undefined   testVar   undefined    typeof testVar  will be equal to undefined

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I m a bit confused about Javascript undefined  amp  null   null generally behaves similarly to other scripting languages  concepts of the out-of-band    null        nil    or    None    objects  undefined  on the other hand  is a weird JavaScript quirk  It s a singleton object that represents out-of-band values  essentially a second similar-but-different null  It comes up   When you call a function with fewer arguments than the arguments list in the function statement lists  the unpassed arguments are set to undefined  You can test for that with eg   function dosomething arg1  arg2        if  arg2   undefined      arg2  DEFAULT VALUE FOR ARG2             With this method you can t tell the difference between dosomething 1  and dosomething 1  undefined   arg2 will be the same value in both  If you need to tell the difference you can look at arguments length  but doing optional arguments like that isn t generally very readable   When a function has no return value   it returns undefined  There s generally no need to use such a return result   When you declare a variable by having a var a statement in a block  but haven t yet assigned a value to it  it is undefined  Again  you shouldn t really ever need to rely on that   The spooky typeof operator returns  undefined  when its operand is a simple variable that does not exist  instead of throwing an error as would normally happen if you tried to refer to it   You can also give it a simple variable wrapped in parentheses  but not a full expression involving a non-existant variable   Not much use for that  either   This is the controversial one  When you access a property of an object which doesn t exist  you don t immediately get an error like in every other language  Instead you get an undefined object   And then when you try to use that undefined object later on in the script it ll go wrong in a weird way that s much more difficult to track down than if JavaScript had just thrown an error straight away   This is often used to check for the existence of properties  if  o prop   undefined     or often as truthiness test  if  o prop        do something     However  because you can assign undefined like any other value  o prop  undefined   that doesn t actually detect whether the property is there reliably  Better to use the in operator  which wasn t in the original Netscape version of JavaScript  but is available everywhere now  if   prop  in o             In summary  undefined is a JavaScript-specific mess  which confuses everyone  Apart from optional function arguments  where JS has no other more elegant mechanism  undefined should be avoided  It should never have been part of the language  null would have worked just fine for  2  and  3   and  4  is a misfeature that only exists because in the beginning JavaScript had no exceptions   what does if   testvar  actually do  Does it test for undefined and null or just undefined   Such a    truthiness    test checks against false  undefined  null  0  NaN and empty strings  But in this case  yes  it is really undefined it is concerned with  IMO  it should be more explicit about that and say if  testvar   undefined    once a variable is defined can I clear it back to undefined  therefore deleting the variable    You can certainly assign undefined to it  but that won t delete the variable  Only the delete object property operator really removes things  delete is really meant for properties rather than variables as such  Browsers will let you get away with straight delete variable  but it s not a good idea and won t work in ECMAScript Fifth Edition s strict mode  If you want to free up a reference to something so it can be garbage-collected  it would be more usual to say variable  null   can I pass undefined as a parameter   Yes

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To answer your first question  the not operator     will coerce whatever it is given into a boolean value  So null  0  false  NaN and     empty string  will all appear false

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JavaScript  how to set a variable to undefined on commandline   Set a variable to undefined in the js javascript command line terminal that comes with Java on Ubuntu 12 10   el defiant     js  js gt  typeof boo  undefined   js gt  boo typein 2  ReferenceError  boo is not defined  js gt  boo 5 5  js gt  typeof boo  number   js gt  delete boo  true  js gt  typeof boo  undefined   js gt  boo typein 7  ReferenceError  boo is not defined   If you set a variable to undefined in a javascript   Put this in myjs html    lt html gt   lt body gt       lt script type  text JavaScript  gt          document write  aliens      aliens           document write  typeof aliens       typeof aliens            var aliens    scramble the nimitz           document write  found some aliens       typeof aliens            document write  not sayings its aliens but        aliens           aliens   undefined          document write  aliens deleted            document write  typeof aliens       typeof aliens            document write  you sure they are gone      aliens        lt  script gt   lt  body gt   lt  html gt    It prints this   aliens  undefined typeof aliens  undefined found some aliens  string not sayings its aliens but    scramble the nimitz aliens deleted typeof aliens  undefined you sure they are gone  undefined   WARNING   When setting your variable to undefined you are setting your variable to another variable   If some sneaky person runs undefined    rm -rf     then whenever you set your variable to undefined  you will receive that value   You may be wondering how I can output the undefined value aliens at the start and have it still run   It s because of javascript hoisting  http   www adequatelygood com JavaScript-Scoping-and-Hoisting html

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Try this      found on UglifyJS variable   void 0

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Just for fun  here s a fairly safe way to assign  unassigned  to a variable  For this to have a collision would require someone to have added to the prototype for Object with exactly the same name as the randomly generated string  I m sure the random string generator could be improved  but I just took one from this question  Generate random string characters in JavaScript  This works by creating a new object and trying to access a property on it with a randomly generated name  which we are assuming wont exist and will hence have the value of undefined   function GenerateRandomString         var text           var possible    ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789        for  var i   0  i  lt  50  i            text    possible charAt Math floor Math random     possible length         return text     var myVar      GenerateRandomString

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YES  you can  because undefined is defined as undefined   console log       global   undefined     window  undefined    amp  amp       global   undefined      function         amp  amp     window  undefined         function            true   your case   test  value1   undefined   value2     you can also create your own undefined variable   Object defineProperty this   u    value   undefined    console log u     undefined

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The for if  something  and if   something  is commonly used to check if something is defined or not defined  For example   if  document getElementById    The identifier is converted to a boolean value  so undefined is interpreted as false  There are of course other values  like 0 and     that also are interpreted as false  but either the identifier should not reasonably have such a value or you are happy with treating such a value the same as undefined   Javascript has a delete operator that can be used to delete a member of an object  Depending on the scope of a variable  i e  if it s global or not  you can delete it to make it undefined   There is no undefined keyword that you can use as an undefined literal  You can omit parameters in a function call to make them undefined  but that can only be used by sending less paramters to the function  you can t omit a parameter in the middle

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You cannot  should not   define anything as undefined  as the variable would no longer be undefined  ndash  you just defined it to something   You cannot  should not   pass undefined to a function  If you want to pass an empty value  use null instead   The statement if  testvar  checks for boolean true false values  this particular one tests whether testvar evaluates to false  By definition  null and undefined shouldn t be evaluated neither as true or false  but JavaScript evaluates null as false  and gives an error if you try to evaluate an undefined variable   To properly test for undefined or null  use these   if typeof testvar       undefined            if testvar     null

[javascript] Can I set variables to undefined or pass undefined as an argument?

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