What is the instanceof operator in JavaScript

Question

The instanceof keyword in JavaScript can be quite confusing when it is first encountered  as people tend to think that JavaScript is not an object-oriented programming language    What is it  What problems does it solve  When is it appropriate and when not

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And you can use it for error handling and debugging  like this   try      somefunction       catch error       if  error instanceof TypeError               Handle type Error       else if  error instanceof ReferenceError               Handle ReferenceError       else              Handle all other error types

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There s an important facet to instanceof that does not seem to be covered in any of the comments thus far  inheritance   A variable being evaluated by use of instanceof could return true for multiple  types  due to prototypal inheritance   For example  let s define a type and a subtype   function Foo      a Foo constructor       assign some props     return this     function SubFoo      a SubFoo constructor     Foo call  this      inherit static props       assign some new props     return this     SubFoo prototype   Object create Foo prototype      Inherit prototype SubFoo prototype constructor   SubFoo    Now that we have a couple of  classes  lets make some instances  and find out what they re instances of   var      foo   new Foo       subfoo   new SubFoo      alert        Q  Is foo an instance of Foo         A        foo instanceof Foo          - gt  true  alert        Q  Is foo an instance of SubFoo          A        foo instanceof SubFoo          - gt  false  alert        Q  Is subfoo an instance of Foo         A        subfoo instanceof Foo          - gt  true  alert        Q  Is subfoo an instance of SubFoo         A        subfoo instanceof SubFoo          - gt  true  alert        Q  Is subfoo an instance of Object         A        subfoo instanceof Object          - gt  true   See that last line   All  new  calls to a function return an object that inherits from Object   This holds true even when using object creation shorthand   alert        Q  Is    an instance of Object         A           instanceof Object          - gt  true   And what about the  class  definitions themselves   What are they instances of   alert        Q  Is Foo an instance of Object         A       Foo instanceof Object         - gt  true  alert        Q  Is Foo an instance of Function         A       Foo instanceof Function         - gt  true   I feel that understanding that any object can be an instance of MULTIPLE types is important  since you my  incorrectly  assume that you could differentiate between  say and object and a function by use of instanceof   As this last example clearly shows a function is an object   This is also important if you are using any inheritance patterns and want to confirm the progeny of an object by methods other than duck-typing     Hope that helps anyone exploring instanceof

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What is it    Javascript is a prototypal language which means it uses prototypes for  inheritance   the instanceof operator tests if a constructor function s prototype propertype is present in the   proto   chain of an object  This means that it will do the following  assuming that testObj is a function object    obj instanceof testObj     Check if prototype of the object is equal to the prototype of the constructor  obj   proto       testObj prototype     if this is true instanceof will return true  Will climb up the prototype chain  For example  obj   proto     proto       testObj prototype     if this is true instanceof will return true  Will repeat step 2 until the full prototype of object is inspected  If nowhere on the prototype chain of the object is matched with testObj prototype then instanceof operator will return false    Example    x000D   x000D  function Person name    x000D    this name   name  x000D    x000D  var me   new Person  Willem    x000D   x000D  console log me instanceof Person      true x000D     because   me   proto       Person prototype     evaluates true x000D   x000D  console log me instanceof Object      true x000D     because   me   proto     proto       Object prototype     evaluates true x000D   x000D  console log me instanceof Array       false x000D     because  Array is nowhere on the prototype chain x000D   x000D   x000D       What problems does it solve     It solved the problem of conveniently checking if an object derives from a certain prototype  For example  when a function recieves an object which can have various prototypes  Then  before using methods from the prototype chain  we can use the instanceof operator to check whether the these methods are on the object   Example    x000D   x000D  function Person1  name    x000D    this name   name  x000D    x000D   x000D  function Person2  name    x000D    this name   name  x000D    x000D   x000D  Person1 prototype talkP1   function      x000D    console log  Person 1 talking    x000D    x000D   x000D  Person2 prototype talkP2   function      x000D    console log  Person 2 talking    x000D    x000D   x000D   x000D  function talk  person    x000D    if  person instanceof Person1    x000D      person talkP1    x000D      x000D     x000D    if  person instanceof Person2    x000D      person talkP2    x000D      x000D     x000D     x000D    x000D   x000D  const pers1   new Person1   p1    x000D  const pers2   new Person2   p2    x000D   x000D  talk pers1   x000D  talk pers2   x000D   x000D   x000D    Here in the talk   function first is checked if the prototype is located on the object  After this the appropriate method is picked to execute  Not doing this check could result in executing a method which doesn t exist and thus a reference error      When is it appropriate and when not    We kind of already went over this  Use it when you are in need of checking the prototype of an object before doing something with it

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instanceof  The Left Hand Side  LHS  operand is the actual object being tested to the Right Hand Side  RHS  operand which is the actual constructor of a class  The basic definition is   Checks the current object and returns true if the object is of the specified object type    Here are some good examples and here is an example taken directly from Mozilla s developer site   var color1   new String  green    color1 instanceof String     returns true var color2    coral     no type specified color2 instanceof String     returns false  color2 is not a String object    One thing worth mentioning is instanceof evaluates to true if the object inherits from the classe s prototype   var p   new Person  Jon    p instanceof Person   That is p instanceof Person is true since p inherits from Person prototype   Per the OP s request  I ve added a small example with some sample code and an explanation     When you declare a variable you give it a specific type     For instance   int i  float f  Customer c    The above show you some variables  namely i  f  and c  The types are integer  float and a user defined Customer data type  Types such as the above could be for any language  not just JavaScript   However  with JavaScript when you declare a variable you don t explicitly define a type  var x  x could be a number   string   a user defined data type  So what instanceof does is it checks the object to see if it is of the type specified so from above taking the Customer object we could do   var c   new Customer    c instanceof Customer    Returns true as c is just a customer c instanceof String    Returns false as c is not a string  it s a customer silly    Above we ve seen that c was declared with the type Customer  We ve new d it and checked whether it is of type Customer or not  Sure is  it returns true  Then still using the Customer object we check if it is a String  Nope  definitely not a String we newed a Customer object not a String object  In this case  it returns false   It really is that simple

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Vehicle is a function  But by naming conventions    first letter is uppercase   it is also an object   constructor function   class    function Vehicle numWheels        this numWheels   numWheels       We can create new instances and check their types  myRoadster   new Vehicle 4   alert myRoadster instanceof Vehicle

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On the question  When is it appropriate and when not    my 2 cents   instanceof is rarely useful in production code  but useful in tests where you want to assert that your code returns   creates objects of the correct types  By being explicit about the kinds of objects your code is returning   creating  your tests become more powerful as a tool for understanding and documenting your code

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instanceof is just syntactic sugar for isPrototypeOf   function Ctor      var o   new Ctor     o instanceof Ctor     true Ctor prototype isPrototypeOf o      true  o instanceof Ctor     Ctor prototype isPrototypeOf o      equivalent   instanceof just depends on the prototype of a constructor of an object   A constructor is just a normal function  Strictly speaking it is a function object  since everything is an object in Javascript  And this function object has a prototype  because every function has a prototype   A prototype is just a normal object  which is located within the prototype chain of another object  That means being in the prototype chain of another object makes an object to a prototype   function f          ordinary function var o          ordinary object  p   f prototype   o     oops  o is a prototype now p   new f       oops  f is a constructor now  o isPrototypeOf p      true p instanceof f     true   The instanceof operator should be avoided because it fakes classes  which do not exist in Javascript  Despite the class keyword not in ES2015 either  since class is again just syntactic sugar for   but that s another story

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I think it s worth noting that instanceof is defined by the use of the  new  keyword when declaring the object  In the example from JonH   var color1   new String  green    color1 instanceof String     returns true var color2    coral   color2 instanceof String     returns false  color2 is not a String object    What he didn t mention is this   var color1   String  green    color1 instanceof String     returns false   Specifying  new  actually copied the end state of the String constructor function into the color1 var  rather than just setting it to the return value  I think this better shows what the new keyword does   function Test name       this test   function            return  This will only work through the  new  keyword              return name     var test   new Test  test    test test       returns  This will only work through the  new  keyword   test    returns the instance object of the Test   function   var test   Test  test    test test       throws TypeError  Object   lt Test gt  has no method  test  test    returns  test    Using  new  assigns the value of  this  inside the function to the declared var  while not using it assigns the return value instead

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I just found a real-world application and will use it more often now  I think   If you use jQuery events  sometimes you want to write a more generic function which may also be called directly  without event   You can use instanceof to check if the first parameter of your function is an instance of jQuery Event and react appropriately   var myFunction   function  el                        if  el instanceof   Event              event specific code     else            generic code        button   click recalc         Will execute event specific code recalc  myParameter        Will execute generic code   In my case  the function needed to calculate something either for all  via click event on a button  or only one specific element  The code I used   var recalc   function  el         el    el    undefined    el instanceof   Event       span allItems       el          calculate

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The other answers here are correct  but they don t get into how instanceof actually works  which may be of interest to some language lawyers out there   Every object in JavaScript has a prototype  accessible through the   proto   property  Functions also have a prototype property  which is the initial   proto   for any objects created by them  When a function is created  it is given a unique object for prototype  The instanceof operator uses this uniqueness to give you an answer  Here s what instanceof might look like if you wrote it as a function   function instance of V  F      var O   F prototype    V   V   proto      while  true        if  V     null        return false      if  O     V        return true      V   V   proto            This is basically paraphrasing ECMA-262 edition 5 1  also known as ES5   section 15 3 5 3    Note that you can reassign any object to a function s prototype property  and you can reassign an object s   proto   property after it is constructed  This will give you some interesting results   function F       function G       var p       F prototype   p  G prototype   p  var f   new F    var g   new G     f instanceof F       returns true f instanceof G       returns true g instanceof F       returns true g instanceof G       returns true  F prototype       f instanceof F       returns false g   proto         g instanceof G       returns false

[javascript] What is the instanceof operator in JavaScript?

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