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Why does isPrototypeOf() return false?


__proto__ VS. prototype in JavaScript[[Prototype]] vs prototype: ..what is the difference? (MyCons.__proto__ === MyCons.prototype) equals FALSEHow does JavaScript .prototype work?What does “use strict” do in JavaScript, and what is the reasoning behind it?event.preventDefault() vs. return falseWhat is JSONP, and why was it created?Why does Google prepend while(1); to their JSON responses?Why does ++[[]][+[]]+[+[]] return the string “10”?How does data binding work in AngularJS?Detecting a mobile browserIs Safari on iOS 6 caching $.ajax results?How do I return the response from an asynchronous call?






.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty height:90px;width:728px;box-sizing:border-box;








8















I have the below constructors and SubType prototype pointing to an instance of SuperType. When I do x.isPrototypeOf(SubType.prototype) it returns false. I am confused as I have explicitly set x as a prototype for SubType. Can someone tell me why this is happening?






function SuperType()

function SubType()

x = new SuperType();

SubType.prototype = x;
SubType.prototype.constructor = SubType;

console.log(x.isPrototypeOf(SubType)) // returns false
console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true












share|improve this question



















  • 2





    Not sure to get my head all clear here, but x === SubType.prototype how do you expect it to be its own prototype?

    – Kaiido
    Apr 24 at 2:21











  • Updated my question, sorry about that type

    – Gautam
    Apr 24 at 2:23











  • try console.log(x.isPrototypeOf(new SubType)) for example of how it's used.

    – dandavis
    Apr 24 at 2:29











  • Notice that you better should be using x = Object.create(SuperType.prototype)

    – Bergi
    Apr 24 at 7:12











  • Possible duplicate of [[Prototype]] vs prototype: ..what is the difference?

    – Bergi
    Apr 24 at 7:13


















8















I have the below constructors and SubType prototype pointing to an instance of SuperType. When I do x.isPrototypeOf(SubType.prototype) it returns false. I am confused as I have explicitly set x as a prototype for SubType. Can someone tell me why this is happening?






function SuperType()

function SubType()

x = new SuperType();

SubType.prototype = x;
SubType.prototype.constructor = SubType;

console.log(x.isPrototypeOf(SubType)) // returns false
console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true












share|improve this question



















  • 2





    Not sure to get my head all clear here, but x === SubType.prototype how do you expect it to be its own prototype?

    – Kaiido
    Apr 24 at 2:21











  • Updated my question, sorry about that type

    – Gautam
    Apr 24 at 2:23











  • try console.log(x.isPrototypeOf(new SubType)) for example of how it's used.

    – dandavis
    Apr 24 at 2:29











  • Notice that you better should be using x = Object.create(SuperType.prototype)

    – Bergi
    Apr 24 at 7:12











  • Possible duplicate of [[Prototype]] vs prototype: ..what is the difference?

    – Bergi
    Apr 24 at 7:13














8












8








8


2






I have the below constructors and SubType prototype pointing to an instance of SuperType. When I do x.isPrototypeOf(SubType.prototype) it returns false. I am confused as I have explicitly set x as a prototype for SubType. Can someone tell me why this is happening?






function SuperType()

function SubType()

x = new SuperType();

SubType.prototype = x;
SubType.prototype.constructor = SubType;

console.log(x.isPrototypeOf(SubType)) // returns false
console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true












share|improve this question
















I have the below constructors and SubType prototype pointing to an instance of SuperType. When I do x.isPrototypeOf(SubType.prototype) it returns false. I am confused as I have explicitly set x as a prototype for SubType. Can someone tell me why this is happening?






function SuperType()

function SubType()

x = new SuperType();

SubType.prototype = x;
SubType.prototype.constructor = SubType;

console.log(x.isPrototypeOf(SubType)) // returns false
console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true








function SuperType()

function SubType()

x = new SuperType();

SubType.prototype = x;
SubType.prototype.constructor = SubType;

console.log(x.isPrototypeOf(SubType)) // returns false
console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true





function SuperType()

function SubType()

x = new SuperType();

SubType.prototype = x;
SubType.prototype.constructor = SubType;

console.log(x.isPrototypeOf(SubType)) // returns false
console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true






javascript






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Apr 24 at 12:58









Boann

37.7k1291123




37.7k1291123










asked Apr 24 at 1:56









GautamGautam

610413




610413







  • 2





    Not sure to get my head all clear here, but x === SubType.prototype how do you expect it to be its own prototype?

    – Kaiido
    Apr 24 at 2:21











  • Updated my question, sorry about that type

    – Gautam
    Apr 24 at 2:23











  • try console.log(x.isPrototypeOf(new SubType)) for example of how it's used.

    – dandavis
    Apr 24 at 2:29











  • Notice that you better should be using x = Object.create(SuperType.prototype)

    – Bergi
    Apr 24 at 7:12











  • Possible duplicate of [[Prototype]] vs prototype: ..what is the difference?

    – Bergi
    Apr 24 at 7:13













  • 2





    Not sure to get my head all clear here, but x === SubType.prototype how do you expect it to be its own prototype?

    – Kaiido
    Apr 24 at 2:21











  • Updated my question, sorry about that type

    – Gautam
    Apr 24 at 2:23











  • try console.log(x.isPrototypeOf(new SubType)) for example of how it's used.

    – dandavis
    Apr 24 at 2:29











  • Notice that you better should be using x = Object.create(SuperType.prototype)

    – Bergi
    Apr 24 at 7:12











  • Possible duplicate of [[Prototype]] vs prototype: ..what is the difference?

    – Bergi
    Apr 24 at 7:13








2




2





Not sure to get my head all clear here, but x === SubType.prototype how do you expect it to be its own prototype?

– Kaiido
Apr 24 at 2:21





Not sure to get my head all clear here, but x === SubType.prototype how do you expect it to be its own prototype?

– Kaiido
Apr 24 at 2:21













Updated my question, sorry about that type

– Gautam
Apr 24 at 2:23





Updated my question, sorry about that type

– Gautam
Apr 24 at 2:23













try console.log(x.isPrototypeOf(new SubType)) for example of how it's used.

– dandavis
Apr 24 at 2:29





try console.log(x.isPrototypeOf(new SubType)) for example of how it's used.

– dandavis
Apr 24 at 2:29













Notice that you better should be using x = Object.create(SuperType.prototype)

– Bergi
Apr 24 at 7:12





Notice that you better should be using x = Object.create(SuperType.prototype)

– Bergi
Apr 24 at 7:12













Possible duplicate of [[Prototype]] vs prototype: ..what is the difference?

– Bergi
Apr 24 at 7:13






Possible duplicate of [[Prototype]] vs prototype: ..what is the difference?

– Bergi
Apr 24 at 7:13













3 Answers
3






active

oldest

votes


















7














SubType is a function. What you probably want to check is if an instance of SubType would inherit from x:






function SuperType()

function SubType()

x = new SuperType();

SubType.prototype = x;
SubType.prototype.constructor = SubType;

const instance = new SubType();
console.log(x.isPrototypeOf(instance)) // returns true
console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true








share|improve this answer






























    3














    It helps to add properties to the objects to see what's happening. I fixed a little of your code. You can run this in the console:



    function SuperType(foo) this.foo = foo ;
    function SubType(bar) this.bar = bar ;

    var x = new SubType("bar");

    SuperType.prototype = x;
    SuperType.prototype.constructor = SubType;


    Now, you asked x.isPrototypeOf(SuperType) and it returns false, because x is not a property of the class SuperType. But when you instantiate a SuperType, x is a property of that new object:



    var y = new SuperType("foo");
    console.log(x.isPrototypeOf(y)) // returns true


    In your example that is true, SubType.prototype is a prototype of SuperType.prototype and returns true.



    console.log(SubType.prototype.isPrototypeOf(SuperType.prototype)) // returns true





    share|improve this answer








    New contributor




    David Klinge is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
    Check out our Code of Conduct.



























      0














      I think that this is misunderstanding of prototype and __proto__ properties.



      Lets modify example above a little



      function SuperType()

      function SubType()

      x = new SuperType();

      SubType.prototype = x;
      console.log(x.isPrototypeOf(SubType)) // returns false.

      // Now most interesting piece
      SubType.__proto__ = x;
      console.log(x.isPrototypeOf(SubType)) // Now true.


      Demo



      In place of SubType.__proto__ = x we may use Object.setPrototypeOf(SubType, x) which will yeilds the same result



      The trik is __proto__ holds real object which is prototype. prototype is used only in constructor fucntions for defining prototype for constructed objects. (See here)



      So if we again modify first example



      function SuperType()

      function SubType()

      x = new SuperType();

      SubType.prototype = x;

      console.log(x.isPrototypeOf(SubType)) // returns false

      var x2 = new SubType();
      console.log(x.isPrototypeOf(x2)) // returns true


      Demo






      share|improve this answer























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        3 Answers
        3






        active

        oldest

        votes








        3 Answers
        3






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        7














        SubType is a function. What you probably want to check is if an instance of SubType would inherit from x:






        function SuperType()

        function SubType()

        x = new SuperType();

        SubType.prototype = x;
        SubType.prototype.constructor = SubType;

        const instance = new SubType();
        console.log(x.isPrototypeOf(instance)) // returns true
        console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true








        share|improve this answer



























          7














          SubType is a function. What you probably want to check is if an instance of SubType would inherit from x:






          function SuperType()

          function SubType()

          x = new SuperType();

          SubType.prototype = x;
          SubType.prototype.constructor = SubType;

          const instance = new SubType();
          console.log(x.isPrototypeOf(instance)) // returns true
          console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true








          share|improve this answer

























            7












            7








            7







            SubType is a function. What you probably want to check is if an instance of SubType would inherit from x:






            function SuperType()

            function SubType()

            x = new SuperType();

            SubType.prototype = x;
            SubType.prototype.constructor = SubType;

            const instance = new SubType();
            console.log(x.isPrototypeOf(instance)) // returns true
            console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true








            share|improve this answer













            SubType is a function. What you probably want to check is if an instance of SubType would inherit from x:






            function SuperType()

            function SubType()

            x = new SuperType();

            SubType.prototype = x;
            SubType.prototype.constructor = SubType;

            const instance = new SubType();
            console.log(x.isPrototypeOf(instance)) // returns true
            console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true








            function SuperType()

            function SubType()

            x = new SuperType();

            SubType.prototype = x;
            SubType.prototype.constructor = SubType;

            const instance = new SubType();
            console.log(x.isPrototypeOf(instance)) // returns true
            console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true





            function SuperType()

            function SubType()

            x = new SuperType();

            SubType.prototype = x;
            SubType.prototype.constructor = SubType;

            const instance = new SubType();
            console.log(x.isPrototypeOf(instance)) // returns true
            console.log(SuperType.prototype.isPrototypeOf(SubType.prototype)) // returns true






            share|improve this answer












            share|improve this answer



            share|improve this answer










            answered Apr 24 at 2:29









            KaiidoKaiido

            46.6k469110




            46.6k469110























                3














                It helps to add properties to the objects to see what's happening. I fixed a little of your code. You can run this in the console:



                function SuperType(foo) this.foo = foo ;
                function SubType(bar) this.bar = bar ;

                var x = new SubType("bar");

                SuperType.prototype = x;
                SuperType.prototype.constructor = SubType;


                Now, you asked x.isPrototypeOf(SuperType) and it returns false, because x is not a property of the class SuperType. But when you instantiate a SuperType, x is a property of that new object:



                var y = new SuperType("foo");
                console.log(x.isPrototypeOf(y)) // returns true


                In your example that is true, SubType.prototype is a prototype of SuperType.prototype and returns true.



                console.log(SubType.prototype.isPrototypeOf(SuperType.prototype)) // returns true





                share|improve this answer








                New contributor




                David Klinge is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                Check out our Code of Conduct.
























                  3














                  It helps to add properties to the objects to see what's happening. I fixed a little of your code. You can run this in the console:



                  function SuperType(foo) this.foo = foo ;
                  function SubType(bar) this.bar = bar ;

                  var x = new SubType("bar");

                  SuperType.prototype = x;
                  SuperType.prototype.constructor = SubType;


                  Now, you asked x.isPrototypeOf(SuperType) and it returns false, because x is not a property of the class SuperType. But when you instantiate a SuperType, x is a property of that new object:



                  var y = new SuperType("foo");
                  console.log(x.isPrototypeOf(y)) // returns true


                  In your example that is true, SubType.prototype is a prototype of SuperType.prototype and returns true.



                  console.log(SubType.prototype.isPrototypeOf(SuperType.prototype)) // returns true





                  share|improve this answer








                  New contributor




                  David Klinge is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                  Check out our Code of Conduct.






















                    3












                    3








                    3







                    It helps to add properties to the objects to see what's happening. I fixed a little of your code. You can run this in the console:



                    function SuperType(foo) this.foo = foo ;
                    function SubType(bar) this.bar = bar ;

                    var x = new SubType("bar");

                    SuperType.prototype = x;
                    SuperType.prototype.constructor = SubType;


                    Now, you asked x.isPrototypeOf(SuperType) and it returns false, because x is not a property of the class SuperType. But when you instantiate a SuperType, x is a property of that new object:



                    var y = new SuperType("foo");
                    console.log(x.isPrototypeOf(y)) // returns true


                    In your example that is true, SubType.prototype is a prototype of SuperType.prototype and returns true.



                    console.log(SubType.prototype.isPrototypeOf(SuperType.prototype)) // returns true





                    share|improve this answer








                    New contributor




                    David Klinge is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                    Check out our Code of Conduct.










                    It helps to add properties to the objects to see what's happening. I fixed a little of your code. You can run this in the console:



                    function SuperType(foo) this.foo = foo ;
                    function SubType(bar) this.bar = bar ;

                    var x = new SubType("bar");

                    SuperType.prototype = x;
                    SuperType.prototype.constructor = SubType;


                    Now, you asked x.isPrototypeOf(SuperType) and it returns false, because x is not a property of the class SuperType. But when you instantiate a SuperType, x is a property of that new object:



                    var y = new SuperType("foo");
                    console.log(x.isPrototypeOf(y)) // returns true


                    In your example that is true, SubType.prototype is a prototype of SuperType.prototype and returns true.



                    console.log(SubType.prototype.isPrototypeOf(SuperType.prototype)) // returns true






                    share|improve this answer








                    New contributor




                    David Klinge is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                    Check out our Code of Conduct.









                    share|improve this answer



                    share|improve this answer






                    New contributor




                    David Klinge is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                    Check out our Code of Conduct.









                    answered Apr 24 at 2:43









                    David KlingeDavid Klinge

                    3346




                    3346




                    New contributor




                    David Klinge is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                    Check out our Code of Conduct.





                    New contributor





                    David Klinge is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                    Check out our Code of Conduct.






                    David Klinge is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
                    Check out our Code of Conduct.





















                        0














                        I think that this is misunderstanding of prototype and __proto__ properties.



                        Lets modify example above a little



                        function SuperType()

                        function SubType()

                        x = new SuperType();

                        SubType.prototype = x;
                        console.log(x.isPrototypeOf(SubType)) // returns false.

                        // Now most interesting piece
                        SubType.__proto__ = x;
                        console.log(x.isPrototypeOf(SubType)) // Now true.


                        Demo



                        In place of SubType.__proto__ = x we may use Object.setPrototypeOf(SubType, x) which will yeilds the same result



                        The trik is __proto__ holds real object which is prototype. prototype is used only in constructor fucntions for defining prototype for constructed objects. (See here)



                        So if we again modify first example



                        function SuperType()

                        function SubType()

                        x = new SuperType();

                        SubType.prototype = x;

                        console.log(x.isPrototypeOf(SubType)) // returns false

                        var x2 = new SubType();
                        console.log(x.isPrototypeOf(x2)) // returns true


                        Demo






                        share|improve this answer



























                          0














                          I think that this is misunderstanding of prototype and __proto__ properties.



                          Lets modify example above a little



                          function SuperType()

                          function SubType()

                          x = new SuperType();

                          SubType.prototype = x;
                          console.log(x.isPrototypeOf(SubType)) // returns false.

                          // Now most interesting piece
                          SubType.__proto__ = x;
                          console.log(x.isPrototypeOf(SubType)) // Now true.


                          Demo



                          In place of SubType.__proto__ = x we may use Object.setPrototypeOf(SubType, x) which will yeilds the same result



                          The trik is __proto__ holds real object which is prototype. prototype is used only in constructor fucntions for defining prototype for constructed objects. (See here)



                          So if we again modify first example



                          function SuperType()

                          function SubType()

                          x = new SuperType();

                          SubType.prototype = x;

                          console.log(x.isPrototypeOf(SubType)) // returns false

                          var x2 = new SubType();
                          console.log(x.isPrototypeOf(x2)) // returns true


                          Demo






                          share|improve this answer

























                            0












                            0








                            0







                            I think that this is misunderstanding of prototype and __proto__ properties.



                            Lets modify example above a little



                            function SuperType()

                            function SubType()

                            x = new SuperType();

                            SubType.prototype = x;
                            console.log(x.isPrototypeOf(SubType)) // returns false.

                            // Now most interesting piece
                            SubType.__proto__ = x;
                            console.log(x.isPrototypeOf(SubType)) // Now true.


                            Demo



                            In place of SubType.__proto__ = x we may use Object.setPrototypeOf(SubType, x) which will yeilds the same result



                            The trik is __proto__ holds real object which is prototype. prototype is used only in constructor fucntions for defining prototype for constructed objects. (See here)



                            So if we again modify first example



                            function SuperType()

                            function SubType()

                            x = new SuperType();

                            SubType.prototype = x;

                            console.log(x.isPrototypeOf(SubType)) // returns false

                            var x2 = new SubType();
                            console.log(x.isPrototypeOf(x2)) // returns true


                            Demo






                            share|improve this answer













                            I think that this is misunderstanding of prototype and __proto__ properties.



                            Lets modify example above a little



                            function SuperType()

                            function SubType()

                            x = new SuperType();

                            SubType.prototype = x;
                            console.log(x.isPrototypeOf(SubType)) // returns false.

                            // Now most interesting piece
                            SubType.__proto__ = x;
                            console.log(x.isPrototypeOf(SubType)) // Now true.


                            Demo



                            In place of SubType.__proto__ = x we may use Object.setPrototypeOf(SubType, x) which will yeilds the same result



                            The trik is __proto__ holds real object which is prototype. prototype is used only in constructor fucntions for defining prototype for constructed objects. (See here)



                            So if we again modify first example



                            function SuperType()

                            function SubType()

                            x = new SuperType();

                            SubType.prototype = x;

                            console.log(x.isPrototypeOf(SubType)) // returns false

                            var x2 = new SubType();
                            console.log(x.isPrototypeOf(x2)) // returns true


                            Demo







                            share|improve this answer












                            share|improve this answer



                            share|improve this answer










                            answered Apr 24 at 8:22









                            FyodorFyodor

                            1708




                            1708



























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