How to unset a JavaScript variable

Question

I have a global variable in JavaScript  actually a window property  but I don t think it matters  which was already populated by a previous script but I don t want another script that will run later to see its value or that it was even defined   I ve put some var   undefined and it works for the purpose of testing typeof some var     undefined  but I really do not think it s the right way to go about it   What do you think

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Variables  in contrast with simple properties  have attribute   Configurable    meaning impossibility to remove a variable via the delete operator  However there is one execution context on which this rule does not affect  It is the eval context  there   Configurable   attribute is not set for variables

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TLDR  simple defined variables  without var  let  const  could be deleted with delete  If you use var  let  const - they could not be deleted neither with delete nor with Reflect deleteProperty   Chrome 55   simpleVar    1    1  delete simpleVar  true simpleVar  VM439 1 Uncaught ReferenceError  simpleVar is not defined     at  lt anonymous gt  1 1  anonymous    VM439 1 var varVar    1   undefined delete varVar  false varVar   1  let letVar    1   undefined delete letVar  true letVar   1  const constVar  1   undefined delete constVar  true constVar   1  Reflect deleteProperty  window   constVar    true constVar   1  Reflect deleteProperty  window   varVar    false varVar   1  Reflect deleteProperty  window   letVar    true letVar   1    FF Nightly 53 0a1 shows same behaviour

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The delete operator removes a property from an object  It cannot remove a variable  So the answer to the question depends on how the global variable or property is defined   1  If it is created with var  it cannot be deleted  For example  var g a   1    create with var  g a is a variable  delete g a    return false console log g a     g a is still 1   2  If it is created without var  it can be deleted  g b   1    create without var  g b is a property  delete g b    return true console log g b     error  g b is not defined  Technical Explanation 1  Using var In this case the reference g a is created in what the ECMAScript spec calls  quot VariableEnvironment quot  that is attached to the current scope - this may be the a function execution context in the case of using var inside a function  though it may be get a little more complicated when you consider let  or in the case of  quot global quot  code the VariableEnvironment is attached to the global object  often window   References in the VariableEnvironment are not normally deletable - the process detailed in ECMAScript 10 5 explains this in detail  but suffice it to say that unless your code is executed in an eval context  which most browser-based development consoles use   then variables declared with var cannot be deleted  2  Without Using var When trying to assign a value to a name without using the var keyword  Javascript tries to locate the named reference in what the ECMAScript spec calls  quot LexicalEnvironment quot   and the main difference is that LexicalEnvironments are nested - that is a LexicalEnvironment has a parent  what the ECMAScript spec calls  quot outer environment reference quot   and when Javascript fails to locate the reference in a LexicalEnvironment  it looks in the parent LexicalEnvironment  as detailed in 10 3 1 and 10 2 2 1   The top level LexicalEnvironment is the  quot global environment quot   and that is bound to the global object in that its references are the global object s properties  So if you try to access a name that was not declared using a var keyword in the current scope or any outer scopes  Javascript will eventually fetch a property of the window object to serve as that reference  As we ve learned before  properties on objects can be deleted  Notes  It is important to remember that var declarations are  quot hoisted quot  - i e  they are always considered to have happened in the beginning of the scope that they are in - though not the value initialization that may be done in a var statement - that is left where it is  So in the following code  a is a reference from the VariableEnvironment and not the window property and its value will be 10 at the end of the code  function test     a   5  var a   10      The above discussion is when  quot strict mode quot  is not enabled  Lookup rules are a bit different when using  quot strict mode quot  and lexical references that would have resolved to window properties without  quot strict mode quot  will raise  quot undeclared variable quot  errors under  quot strict mode quot   I didn t really understand where this is specified  but its how browsers behave

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Edited updated to clarify the various options  depending on what your desired intentions are  See  noah s answer for full details   Option A   set to null some var   null     Option B   set to undefined some var   undefined     Option C   remove delete the variable reference delete obj some var   if your variable was defined as a global  you ll need to   qualify the reference with  window  delete window some var   References  MDN delete API MDN SyntaxError when deleting an unqualified variable name in strict mode

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scunlife s answer will work  but technically it ought to be   delete window some var     delete is supposed to be a no-op when the target isn t an object property  e g     function        var foo   123     delete foo     wont do anything  foo is still 123    var bar     foo  123       delete bar foo     foo is gone         But since global variables are actually members of the window object  it works    When prototype chains are involved  using delete gets more complex because it only removes the property from the target object  and not the prototype  e g    function Foo      Foo prototype     bar  123    var foo   new Foo       foo bar is 123 foo bar   456     foo bar is now 456 delete foo bar     foo bar is 123 again    So be careful   EDIT  My answer is somewhat inaccurate  see  Misconceptions  at the end   The link explains all the gory details  but the summary is that there can be big differences between browsers and depending on the object you are deleting from  delete object someProp should generally be safe as long as object     window  I still wouldn t use it to delete variables declared with var although you can under the right circumstances

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I am bit confused  If all you are wanting is for a variables value to not pass to another script then there is no need to delete the variable from the scope  Simply nullify the variable then explicit check if it is or is not null  Why go through the trouble of deleting the variable from scope  What purpose does this server that nullifying can not   foo   null  if foo     null  or if foo     null

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The delete operator removes a property from an object   delete object property delete object  property     https   developer mozilla org en-US docs Web JavaScript Reference Operators delete  According to the question you need one of followings  delete some var  delete window some var  delete window  some var

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Note that delete returns true when it was successful  Update 2021  tested on Chrome 88 and Firefox 84  implicit global   1  delete implicit global     true  window explicit global   1  delete explicit global     true  const  object    property  1   delete  object property     true  function set   function       delete function set     true  function function declaration       delete function declaration     false   function          var  var   1      console log delete  var      false     console log  var      1        function          let  let   1      console log delete  let      false     console log  let      1        function          const  const   1      console log delete  const      false     console log  const      1       Original answer is no longer relevant  tested in 2015 on Chrome 52  implicit global   1  window explicit global   1  function set   function       function function dec        var declared variable   1   delete implicit global     true  tested on Chrome 52 delete window explicit global     true  tested on Chrome 52 delete function set     true  tested on Chrome 52 delete function dec     true  tested on Chrome 52 delete declared variable     true  tested on Chrome 52

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The accepted answer  and others  are outdated  TL DR  delete does not remove variables   It s only for removing a property from an object    The correct way to  quot unset quot  is to simply set the variable to null    source   This enables JavaScript s automatic processes to remove the variable from memory     Example  x   null     More info  Use of the delete operator on a variable is deprecated since 2012  when all browsers implemented  automatic  mark-and-sweep garbage-collection  The process works by automatically determining when objects variables become  quot unreachable quot   deciding whether or not the code still requires them   With JavaScript  in all modern browsers    Garbage collection is performed automatically  We cannot force or prevent it  Objects are retained in memory while they are reachable  Being referenced is not the same as being reachable  a pack of interlinked objects can become unreachable as a whole   source    The delete operator is only used to remove a property from an object  it does not remove variables   Unlike what common belief suggests  perhaps due to other programming languages like delete in C     the delete operator has nothing to do with directly freeing memory  Memory management is done indirectly via breaking references   source   When using strict mode   use strict    as opposed to regular  quot sloppy mode quot   an attempt to delete a variable will throw an error and is not allowed  Normal variables in JavaScript can t be deleted using the delete operator  source   or any other way  as of 2021             alas  the only solution  Freeing the contents of a variable To free the contents of a variable  you can simply set it to null   var x           x   null         x can now be garbage collected     source   Further Reading   Memory Management  MDN Docs  Garbage Collection    rebro University  The Very Basics of Garbage Collection  javascript info  Understanding JavaScript Memory Management using Garbage Collection Eradicating Memory Leaks In Javascript

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If you are implicitly declaring the variable without var  the proper way would be to use delete foo    However after you delete it  if you try to use this in an operation such as addition a ReferenceError will be thrown because you can t add a string to an undeclared  undefined identifier  Example   x   5  delete x alert  foo    x      ReferenceError  x is not defined   It may be safer in some situations to assign it to false  null  or undefined so it s declared and won t throw this type of error   foo   false   Note that in ECMAScript null  false  undefined  0  NaN  or    would all evaluate to false  Just make sure you dont use the     operator but instead    when type checking for booleans and you don t want identity checking  so null would    false and false    undefined    Also note that delete doesn t  delete  references but just properties directly on the object  e g    bah       foo       bah ref   foo   delete bah ref  alert   bah ref  foo          object Object   it deleted the property but not the reference to the other object    If you have declared a variable with var you can t delete it    function         var x   5      alert delete x         false         In Rhino   js gt  var x js gt  delete x false   Nor can you delete some predefined properties like Math PI   js gt  delete Math PI false   There are some odd exceptions to delete as with any language  if you care enough you should read    https   developer mozilla org en Core JavaScript 1 5 Reference Operators Special Operators delete Operator http   www ecma-international org publications files ECMA-ST ECMA-262 pdf

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You cannot delete a variable if you declared it  with var x   at the time of first use  However  if your variable x first appeared in the script without a declaration  then you can use the delete operator  delete x   and your variable will be deleted  very similar to deleting an element of an array or deleting a property of an object

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ECMAScript 2015 offers Reflect API  It is possible to delete object property with Reflect deleteProperty     Reflect deleteProperty myObject   myProp       it is equivalent to  delete myObject myProp  delete myObject  myProp      To delete property of global window object   Reflect deleteProperty window   some var      In some cases properties cannot be deleted  when the property is not configurable  and then this function returns false  as well as delete operator   In other cases returns true   Object defineProperty window   some var         configurable  false      writable  true      enumerable  true      value   some val       var frozen   Object freeze   myProperty   myValue      var regular     myProperty   myValue     var blank        console log Reflect deleteProperty window   some var        false console log window some var      some var  console log Reflect deleteProperty frozen   myProperty        false console log frozen myProperty      myValue  console log Reflect deleteProperty regular   myProperty        true console log regular myProperty      undefined  console log Reflect deleteProperty blank   notExistingProperty        true console log blank notExistingProperty      undefined   There is a difference between deleteProperty function and delete operator when run in strict mode    use strict   var frozen   Object freeze   myProperty   myValue       Reflect deleteProperty frozen   myProperty       false delete frozen myProperty     TypeError  property  myProperty  is non-configurable and can t be deleted

[javascript] How to unset a JavaScript variable?

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