Why does instanceof return false for some literals

Question

quot foo quot  instanceof String     gt  false  quot foo quot  instanceof Object     gt  false  true instanceof Boolean     gt  false true instanceof Object     gt  false false instanceof Boolean     gt  false false instanceof Object     gt  false  12 21 instanceof Number     gt  false  foo  instanceof RegExp     gt  true     the tests against Object really don t make sense  Array literals and Object literals match     0 1  instanceof Array     gt  true  0 1  instanceof Object     gt  true  Why don t all of them  Or  why don t they all not  And  what are they an instance of  then  It s the same in FF3  IE7  Opera  and Chrome  So  at least it s consistent

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You can use constructor property    foo  constructor    String    returns true true constructor    Boolean    returns true

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For me the confusion caused by    str    proto       1   gt  String   So  str  istanceof String should return true because how istanceof works as below    str    proto      String prototype     2   gt  true   Results of expression  1 and  2 conflict each other  so there should be one of them wrong    1 is wrong  I figure out that it caused by the   proto   is non standard property  so use the standard one Object getPrototypeOf  Object getPrototypeOf  str       3   gt  TypeError  Object getPrototypeOf called on non-object   Now there s no confusion between expression  2 and  3

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This is defined in the ECMAScript specification Section 7 3 19 Step 3  If Type O  is not Object  return false   In other word  if the Obj in Obj instanceof Callable is not an object  the instanceof will short-circuit to false directly

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I use   function isString s        return typeof s       string     s instanceof String      Because in JavaScript strings can be literals or objects

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I use   function isString s        return typeof s       string     s instanceof String      Because in JavaScript strings can be literals or objects

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In JavaScript everything is an object  or may at least be treated as an object   except primitives  booleans  null  numbers  strings and the value undefined  and symbol in ES6     console log typeof true                boolean console log typeof 0                   number console log typeof                     string console log typeof undefined           undefined console log typeof null                object console log typeof                     object console log typeof                     object console log typeof function            function   As you can see objects  arrays and the value null are all considered objects  null is a reference to an object which doesn t exist   Functions are distinguished because they are a special type of callable objects  However they are still objects   On the other hand the literals true  0     and undefined are not objects  They are primitive values in JavaScript  However booleans  numbers and strings also have constructors Boolean  Number and String respectively which wrap their respective primitives to provide added functionality   console log typeof new Boolean true       object console log typeof new Number 0           object console log typeof new String             object   As you can see when primitive values are wrapped within the Boolean  Number and String constructors respectively they become objects  The instanceof operator only works for objects  which is why it returns false for primitive values    console log true instanceof Boolean                   false console log 0 instanceof Number                       false console log    instanceof String                      false console log new Boolean true  instanceof Boolean      true console log new Number 0  instanceof Number           true console log new String     instanceof String          true   As you can see both typeof and instanceof are insufficient to test whether a value is a boolean  a number or a string - typeof only works for primitive booleans  numbers and strings  and instanceof doesn t work for primitive booleans  numbers and strings   Fortunately there s a simple solution to this problem  The default implementation of toString  i e  as it s natively defined on Object prototype toString  returns the internal   Class   property of both primitive values and objects   function classOf value        return Object prototype toString call value      console log classOf true                     object Boolean  console log classOf 0                        object Number  console log classOf                          object String  console log classOf new Boolean true         object Boolean  console log classOf new Number 0             object Number  console log classOf new String               object String    The internal   Class   property of a value is much more useful than the typeof the value  We can use Object prototype toString to create our own  more useful  version of the typeof operator as follows   function typeOf value        return Object prototype toString call value  slice 8  -1      console log typeOf true                    Boolean console log typeOf 0                       Number console log typeOf                         String console log typeOf new Boolean true        Boolean console log typeOf new Number 0            Number console log typeOf new String              String   Hope this article helped  To know more about the differences between primitives and wrapped objects read the following blog post  The Secret Life of JavaScript  Primitives

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https   www npmjs com package typeof  Returns a string-representation of instanceof  the constructors name   function instanceOf object      var type   typeof object    if  type      undefined         return  undefined         if  object        type   object constructor name     else if  type      object         type   Object prototype toString call object  slice 8  -1         return type toLowerCase      instanceOf false                       boolean  instanceOf new Promise      gt            promise  instanceOf null                        null  instanceOf undefined                   undefined  instanceOf 1                           number  instanceOf      gt                        function  instanceOf                             array

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I believe I have come up with a viable solution   Object getPrototypeOf  test       String prototype      true Object getPrototypeOf 1      String prototype           false

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typeof text       string     text instanceof String     you can use this  it will work for both case as    var text  foo       typeof will work String text  new String  foo        instanceof will work

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Primitives are a different kind of type than objects created from within Javascript  From the Mozilla API docs   var color1   new String  green    color1 instanceof String     returns true var color2    coral   color2 instanceof String     returns false  color2 is not a String object    I can t find any way to construct primitive types with code  perhaps it s not possible  This is probably why people use typeof  foo       string  instead of instanceof   An easy way to remember things like this is asking yourself  I wonder what would be sane and easy to learn   Whatever the answer is  Javascript does the other thing

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For me the confusion caused by    str    proto       1   gt  String   So  str  istanceof String should return true because how istanceof works as below    str    proto      String prototype     2   gt  true   Results of expression  1 and  2 conflict each other  so there should be one of them wrong    1 is wrong  I figure out that it caused by the   proto   is non standard property  so use the standard one Object getPrototypeOf  Object getPrototypeOf  str       3   gt  TypeError  Object getPrototypeOf called on non-object   Now there s no confusion between expression  2 and  3

User · Answer

Primitives are a different kind of type than objects created from within Javascript  From the Mozilla API docs   var color1   new String  green    color1 instanceof String     returns true var color2    coral   color2 instanceof String     returns false  color2 is not a String object    I can t find any way to construct primitive types with code  perhaps it s not possible  This is probably why people use typeof  foo       string  instead of instanceof   An easy way to remember things like this is asking yourself  I wonder what would be sane and easy to learn   Whatever the answer is  Javascript does the other thing

User · Answer

In JavaScript everything is an object  or may at least be treated as an object   except primitives  booleans  null  numbers  strings and the value undefined  and symbol in ES6     console log typeof true                boolean console log typeof 0                   number console log typeof                     string console log typeof undefined           undefined console log typeof null                object console log typeof                     object console log typeof                     object console log typeof function            function   As you can see objects  arrays and the value null are all considered objects  null is a reference to an object which doesn t exist   Functions are distinguished because they are a special type of callable objects  However they are still objects   On the other hand the literals true  0     and undefined are not objects  They are primitive values in JavaScript  However booleans  numbers and strings also have constructors Boolean  Number and String respectively which wrap their respective primitives to provide added functionality   console log typeof new Boolean true       object console log typeof new Number 0           object console log typeof new String             object   As you can see when primitive values are wrapped within the Boolean  Number and String constructors respectively they become objects  The instanceof operator only works for objects  which is why it returns false for primitive values    console log true instanceof Boolean                   false console log 0 instanceof Number                       false console log    instanceof String                      false console log new Boolean true  instanceof Boolean      true console log new Number 0  instanceof Number           true console log new String     instanceof String          true   As you can see both typeof and instanceof are insufficient to test whether a value is a boolean  a number or a string - typeof only works for primitive booleans  numbers and strings  and instanceof doesn t work for primitive booleans  numbers and strings   Fortunately there s a simple solution to this problem  The default implementation of toString  i e  as it s natively defined on Object prototype toString  returns the internal   Class   property of both primitive values and objects   function classOf value        return Object prototype toString call value      console log classOf true                     object Boolean  console log classOf 0                        object Number  console log classOf                          object String  console log classOf new Boolean true         object Boolean  console log classOf new Number 0             object Number  console log classOf new String               object String    The internal   Class   property of a value is much more useful than the typeof the value  We can use Object prototype toString to create our own  more useful  version of the typeof operator as follows   function typeOf value        return Object prototype toString call value  slice 8  -1      console log typeOf true                    Boolean console log typeOf 0                       Number console log typeOf                         String console log typeOf new Boolean true        Boolean console log typeOf new Number 0            Number console log typeOf new String              String   Hope this article helped  To know more about the differences between primitives and wrapped objects read the following blog post  The Secret Life of JavaScript  Primitives

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Primitives are a different kind of type than objects created from within Javascript  From the Mozilla API docs   var color1   new String  green    color1 instanceof String     returns true var color2    coral   color2 instanceof String     returns false  color2 is not a String object    I can t find any way to construct primitive types with code  perhaps it s not possible  This is probably why people use typeof  foo       string  instead of instanceof   An easy way to remember things like this is asking yourself  I wonder what would be sane and easy to learn   Whatever the answer is  Javascript does the other thing

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The primitive wrapper types are reference types that are automatically created behind the scenes whenever strings  num  bers  or Booleans are read For example   var name    quot foo quot   var firstChar   name charAt 0   console log firstChar    This is what happens behind the scenes     what the JavaScript engine does var name    quot foo quot   var temp   new String name   var firstChar   temp charAt 0   temp   null  console log firstChar    Because the second line uses a string  a primitive  like an object  the JavaScript engine creates an instance of String so that charAt 0  will work The String object exists only for one statement before it   s destroyed check this The instanceof operator returns false because a temporary object is created only when a value is read  Because instanceof doesn   t actually read anything  no temporary objects are created  and it tells us the   values aren   t instances of primitive wrapper types  You can create primitive wrapper types manually

User · Answer

Primitives are a different kind of type than objects created from within Javascript  From the Mozilla API docs   var color1   new String  green    color1 instanceof String     returns true var color2    coral   color2 instanceof String     returns false  color2 is not a String object    I can t find any way to construct primitive types with code  perhaps it s not possible  This is probably why people use typeof  foo       string  instead of instanceof   An easy way to remember things like this is asking yourself  I wonder what would be sane and easy to learn   Whatever the answer is  Javascript does the other thing

User · Answer

This is defined in the ECMAScript specification Section 7 3 19 Step 3  If Type O  is not Object  return false   In other word  if the Obj in Obj instanceof Callable is not an object  the instanceof will short-circuit to false directly

User · Answer

Or you can just make your own function like so   function isInstanceOf obj  clazz     return  obj instanceof eval     clazz           typeof obj    clazz toLowerCase          usage   isInstanceOf     String    isInstanceOf new String     String      These should both return true

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https   www npmjs com package typeof  Returns a string-representation of instanceof  the constructors name   function instanceOf object      var type   typeof object    if  type      undefined         return  undefined         if  object        type   object constructor name     else if  type      object         type   Object prototype toString call object  slice 8  -1         return type toLowerCase      instanceOf false                       boolean  instanceOf new Promise      gt            promise  instanceOf null                        null  instanceOf undefined                   undefined  instanceOf 1                           number  instanceOf      gt                        function  instanceOf                             array

User · Answer

I believe I have come up with a viable solution   Object getPrototypeOf  test       String prototype      true Object getPrototypeOf 1      String prototype           false

User · Answer

Or you can just make your own function like so   function isInstanceOf obj  clazz     return  obj instanceof eval     clazz           typeof obj    clazz toLowerCase          usage   isInstanceOf     String    isInstanceOf new String     String      These should both return true

User · Answer

typeof text       string     text instanceof String     you can use this  it will work for both case as    var text  foo       typeof will work String text  new String  foo        instanceof will work

User · Answer

The primitive wrapper types are reference types that are automatically created behind the scenes whenever strings  num  bers  or Booleans are read For example   var name    quot foo quot   var firstChar   name charAt 0   console log firstChar    This is what happens behind the scenes     what the JavaScript engine does var name    quot foo quot   var temp   new String name   var firstChar   temp charAt 0   temp   null  console log firstChar    Because the second line uses a string  a primitive  like an object  the JavaScript engine creates an instance of String so that charAt 0  will work The String object exists only for one statement before it   s destroyed check this The instanceof operator returns false because a temporary object is created only when a value is read  Because instanceof doesn   t actually read anything  no temporary objects are created  and it tells us the   values aren   t instances of primitive wrapper types  You can create primitive wrapper types manually

User · Answer

You can use constructor property    foo  constructor    String    returns true true constructor    Boolean    returns true

[javascript] Why does instanceof return false for some literals?

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