Why does accessing a property of indexOf still compile?












11















I made a typo in TypeScript which was picked up during code review.



I used someArray.indexOf[someObject] instead of someArray.indexOf(someObject).



I would expect an error from the IDE/Compiler. Instead, no errors were raised and the result was simply undefined.



Can anyone explain this?










share|improve this question




















  • 1





    What did you assign the result of? Because what you wrote is valid you are taking the someObject member of the indexOf method. Well, trying to. The only error would come from TypeScript compilation and only if you try to assign the result to something that doesn't match the expected type.

    – vlaz
    1 hour ago






  • 1





    Welcome to javascript, where everything is an object!

    – Jean-Baptiste Yunès
    1 hour ago






  • 2





    @DeWetvanAs I am actually curious about your problem - this seems like a genuine bug/problem with TypeScript see example here. It seems that if you are trying to assign to a variable of type number, the result of .indexOf[someObject] shouldn't be considered a number and thus the compilation would fail. That's the whole idea of TypeScript is - to enforce the types. The answers here focus on JS but ignore this.

    – vlaz
    21 mins ago






  • 1





    @DeWetvanAs You might want to not accept an answer yet if none of them explains why you are not getting a TypeScript error.

    – Bergi
    7 mins ago
















11















I made a typo in TypeScript which was picked up during code review.



I used someArray.indexOf[someObject] instead of someArray.indexOf(someObject).



I would expect an error from the IDE/Compiler. Instead, no errors were raised and the result was simply undefined.



Can anyone explain this?










share|improve this question




















  • 1





    What did you assign the result of? Because what you wrote is valid you are taking the someObject member of the indexOf method. Well, trying to. The only error would come from TypeScript compilation and only if you try to assign the result to something that doesn't match the expected type.

    – vlaz
    1 hour ago






  • 1





    Welcome to javascript, where everything is an object!

    – Jean-Baptiste Yunès
    1 hour ago






  • 2





    @DeWetvanAs I am actually curious about your problem - this seems like a genuine bug/problem with TypeScript see example here. It seems that if you are trying to assign to a variable of type number, the result of .indexOf[someObject] shouldn't be considered a number and thus the compilation would fail. That's the whole idea of TypeScript is - to enforce the types. The answers here focus on JS but ignore this.

    – vlaz
    21 mins ago






  • 1





    @DeWetvanAs You might want to not accept an answer yet if none of them explains why you are not getting a TypeScript error.

    – Bergi
    7 mins ago














11












11








11








I made a typo in TypeScript which was picked up during code review.



I used someArray.indexOf[someObject] instead of someArray.indexOf(someObject).



I would expect an error from the IDE/Compiler. Instead, no errors were raised and the result was simply undefined.



Can anyone explain this?










share|improve this question
















I made a typo in TypeScript which was picked up during code review.



I used someArray.indexOf[someObject] instead of someArray.indexOf(someObject).



I would expect an error from the IDE/Compiler. Instead, no errors were raised and the result was simply undefined.



Can anyone explain this?







typescript methods properties






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 10 mins ago









Bergi

366k58546872




366k58546872










asked 1 hour ago









De Wet van AsDe Wet van As

837




837








  • 1





    What did you assign the result of? Because what you wrote is valid you are taking the someObject member of the indexOf method. Well, trying to. The only error would come from TypeScript compilation and only if you try to assign the result to something that doesn't match the expected type.

    – vlaz
    1 hour ago






  • 1





    Welcome to javascript, where everything is an object!

    – Jean-Baptiste Yunès
    1 hour ago






  • 2





    @DeWetvanAs I am actually curious about your problem - this seems like a genuine bug/problem with TypeScript see example here. It seems that if you are trying to assign to a variable of type number, the result of .indexOf[someObject] shouldn't be considered a number and thus the compilation would fail. That's the whole idea of TypeScript is - to enforce the types. The answers here focus on JS but ignore this.

    – vlaz
    21 mins ago






  • 1





    @DeWetvanAs You might want to not accept an answer yet if none of them explains why you are not getting a TypeScript error.

    – Bergi
    7 mins ago














  • 1





    What did you assign the result of? Because what you wrote is valid you are taking the someObject member of the indexOf method. Well, trying to. The only error would come from TypeScript compilation and only if you try to assign the result to something that doesn't match the expected type.

    – vlaz
    1 hour ago






  • 1





    Welcome to javascript, where everything is an object!

    – Jean-Baptiste Yunès
    1 hour ago






  • 2





    @DeWetvanAs I am actually curious about your problem - this seems like a genuine bug/problem with TypeScript see example here. It seems that if you are trying to assign to a variable of type number, the result of .indexOf[someObject] shouldn't be considered a number and thus the compilation would fail. That's the whole idea of TypeScript is - to enforce the types. The answers here focus on JS but ignore this.

    – vlaz
    21 mins ago






  • 1





    @DeWetvanAs You might want to not accept an answer yet if none of them explains why you are not getting a TypeScript error.

    – Bergi
    7 mins ago








1




1





What did you assign the result of? Because what you wrote is valid you are taking the someObject member of the indexOf method. Well, trying to. The only error would come from TypeScript compilation and only if you try to assign the result to something that doesn't match the expected type.

– vlaz
1 hour ago





What did you assign the result of? Because what you wrote is valid you are taking the someObject member of the indexOf method. Well, trying to. The only error would come from TypeScript compilation and only if you try to assign the result to something that doesn't match the expected type.

– vlaz
1 hour ago




1




1





Welcome to javascript, where everything is an object!

– Jean-Baptiste Yunès
1 hour ago





Welcome to javascript, where everything is an object!

– Jean-Baptiste Yunès
1 hour ago




2




2





@DeWetvanAs I am actually curious about your problem - this seems like a genuine bug/problem with TypeScript see example here. It seems that if you are trying to assign to a variable of type number, the result of .indexOf[someObject] shouldn't be considered a number and thus the compilation would fail. That's the whole idea of TypeScript is - to enforce the types. The answers here focus on JS but ignore this.

– vlaz
21 mins ago





@DeWetvanAs I am actually curious about your problem - this seems like a genuine bug/problem with TypeScript see example here. It seems that if you are trying to assign to a variable of type number, the result of .indexOf[someObject] shouldn't be considered a number and thus the compilation would fail. That's the whole idea of TypeScript is - to enforce the types. The answers here focus on JS but ignore this.

– vlaz
21 mins ago




1




1





@DeWetvanAs You might want to not accept an answer yet if none of them explains why you are not getting a TypeScript error.

– Bergi
7 mins ago





@DeWetvanAs You might want to not accept an answer yet if none of them explains why you are not getting a TypeScript error.

– Bergi
7 mins ago












4 Answers
4






active

oldest

votes


















15














Quite easy.



someArray.indexOf you know that this is a function, which is also an object and can have properties.



By doing someArray.indexOf[someObject], you are trying to reach the property with the key valued to the value of someObject.



Of course, it is not defined on the indexOf function, so it returns undefined.



Quick example that illustrates the syntax and the fact that a function can have properties ;) :






const array = ;
array.indexOf['anyValue'] = 'test';
console.log(array.indexOf.anyValue);








share|improve this answer



















  • 3





    I think answers here are missing the TypeScript tag. It's entirely reasonable to expect a compilation error in TS. Then again, it depends if you have index: number and index = arr.indexOf[obj] then that should be a compilation error. But index: any wouldn't throw a compilation error.

    – vlaz
    1 hour ago











  • @vlaz +1. sjahan gives OP a quick explanation of the undefined result but the main question remains...

    – Florian
    50 mins ago



















3














array.indexOf is a function.



Functions are objects.



You were accessing the someObject property of the array.indexOf function.



You would have got undefined.






const array = [1, 2, 3]
const someObject = 'asdasd'

console.log(array.indexOf[someObject])
// undefined








share|improve this answer





















  • 1





    Array.indexOf is undefined, Array.prototype.indexOf, on the other hand, is a function.

    – Pavlo
    17 mins ago











  • You’re right! I fixed the typo in my answer. Thanks

    – 0xc14m1z
    10 mins ago



















0














In short: Because that is the way the language is designed.



JavaScript evaluates an attempt to access a property that doesn't exist as undefined.



It doesn't raise an exception.






share|improve this answer































    0














    Functions in JavaScript are first class objects.



    When you access function Array.indexOf() via bracket notation Array.indexOf['prop'] you actually trying to access a property which does not exist on indexOf so you get undefined.






    share|improve this answer



















    • 1





      What about TypeScript?

      – vlaz
      1 hour ago











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    4 Answers
    4






    active

    oldest

    votes








    4 Answers
    4






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    15














    Quite easy.



    someArray.indexOf you know that this is a function, which is also an object and can have properties.



    By doing someArray.indexOf[someObject], you are trying to reach the property with the key valued to the value of someObject.



    Of course, it is not defined on the indexOf function, so it returns undefined.



    Quick example that illustrates the syntax and the fact that a function can have properties ;) :






    const array = ;
    array.indexOf['anyValue'] = 'test';
    console.log(array.indexOf.anyValue);








    share|improve this answer



















    • 3





      I think answers here are missing the TypeScript tag. It's entirely reasonable to expect a compilation error in TS. Then again, it depends if you have index: number and index = arr.indexOf[obj] then that should be a compilation error. But index: any wouldn't throw a compilation error.

      – vlaz
      1 hour ago











    • @vlaz +1. sjahan gives OP a quick explanation of the undefined result but the main question remains...

      – Florian
      50 mins ago
















    15














    Quite easy.



    someArray.indexOf you know that this is a function, which is also an object and can have properties.



    By doing someArray.indexOf[someObject], you are trying to reach the property with the key valued to the value of someObject.



    Of course, it is not defined on the indexOf function, so it returns undefined.



    Quick example that illustrates the syntax and the fact that a function can have properties ;) :






    const array = ;
    array.indexOf['anyValue'] = 'test';
    console.log(array.indexOf.anyValue);








    share|improve this answer



















    • 3





      I think answers here are missing the TypeScript tag. It's entirely reasonable to expect a compilation error in TS. Then again, it depends if you have index: number and index = arr.indexOf[obj] then that should be a compilation error. But index: any wouldn't throw a compilation error.

      – vlaz
      1 hour ago











    • @vlaz +1. sjahan gives OP a quick explanation of the undefined result but the main question remains...

      – Florian
      50 mins ago














    15












    15








    15







    Quite easy.



    someArray.indexOf you know that this is a function, which is also an object and can have properties.



    By doing someArray.indexOf[someObject], you are trying to reach the property with the key valued to the value of someObject.



    Of course, it is not defined on the indexOf function, so it returns undefined.



    Quick example that illustrates the syntax and the fact that a function can have properties ;) :






    const array = ;
    array.indexOf['anyValue'] = 'test';
    console.log(array.indexOf.anyValue);








    share|improve this answer













    Quite easy.



    someArray.indexOf you know that this is a function, which is also an object and can have properties.



    By doing someArray.indexOf[someObject], you are trying to reach the property with the key valued to the value of someObject.



    Of course, it is not defined on the indexOf function, so it returns undefined.



    Quick example that illustrates the syntax and the fact that a function can have properties ;) :






    const array = ;
    array.indexOf['anyValue'] = 'test';
    console.log(array.indexOf.anyValue);








    const array = ;
    array.indexOf['anyValue'] = 'test';
    console.log(array.indexOf.anyValue);





    const array = ;
    array.indexOf['anyValue'] = 'test';
    console.log(array.indexOf.anyValue);






    share|improve this answer












    share|improve this answer



    share|improve this answer










    answered 1 hour ago









    sjahansjahan

    3,2051827




    3,2051827








    • 3





      I think answers here are missing the TypeScript tag. It's entirely reasonable to expect a compilation error in TS. Then again, it depends if you have index: number and index = arr.indexOf[obj] then that should be a compilation error. But index: any wouldn't throw a compilation error.

      – vlaz
      1 hour ago











    • @vlaz +1. sjahan gives OP a quick explanation of the undefined result but the main question remains...

      – Florian
      50 mins ago














    • 3





      I think answers here are missing the TypeScript tag. It's entirely reasonable to expect a compilation error in TS. Then again, it depends if you have index: number and index = arr.indexOf[obj] then that should be a compilation error. But index: any wouldn't throw a compilation error.

      – vlaz
      1 hour ago











    • @vlaz +1. sjahan gives OP a quick explanation of the undefined result but the main question remains...

      – Florian
      50 mins ago








    3




    3





    I think answers here are missing the TypeScript tag. It's entirely reasonable to expect a compilation error in TS. Then again, it depends if you have index: number and index = arr.indexOf[obj] then that should be a compilation error. But index: any wouldn't throw a compilation error.

    – vlaz
    1 hour ago





    I think answers here are missing the TypeScript tag. It's entirely reasonable to expect a compilation error in TS. Then again, it depends if you have index: number and index = arr.indexOf[obj] then that should be a compilation error. But index: any wouldn't throw a compilation error.

    – vlaz
    1 hour ago













    @vlaz +1. sjahan gives OP a quick explanation of the undefined result but the main question remains...

    – Florian
    50 mins ago





    @vlaz +1. sjahan gives OP a quick explanation of the undefined result but the main question remains...

    – Florian
    50 mins ago













    3














    array.indexOf is a function.



    Functions are objects.



    You were accessing the someObject property of the array.indexOf function.



    You would have got undefined.






    const array = [1, 2, 3]
    const someObject = 'asdasd'

    console.log(array.indexOf[someObject])
    // undefined








    share|improve this answer





















    • 1





      Array.indexOf is undefined, Array.prototype.indexOf, on the other hand, is a function.

      – Pavlo
      17 mins ago











    • You’re right! I fixed the typo in my answer. Thanks

      – 0xc14m1z
      10 mins ago
















    3














    array.indexOf is a function.



    Functions are objects.



    You were accessing the someObject property of the array.indexOf function.



    You would have got undefined.






    const array = [1, 2, 3]
    const someObject = 'asdasd'

    console.log(array.indexOf[someObject])
    // undefined








    share|improve this answer





















    • 1





      Array.indexOf is undefined, Array.prototype.indexOf, on the other hand, is a function.

      – Pavlo
      17 mins ago











    • You’re right! I fixed the typo in my answer. Thanks

      – 0xc14m1z
      10 mins ago














    3












    3








    3







    array.indexOf is a function.



    Functions are objects.



    You were accessing the someObject property of the array.indexOf function.



    You would have got undefined.






    const array = [1, 2, 3]
    const someObject = 'asdasd'

    console.log(array.indexOf[someObject])
    // undefined








    share|improve this answer















    array.indexOf is a function.



    Functions are objects.



    You were accessing the someObject property of the array.indexOf function.



    You would have got undefined.






    const array = [1, 2, 3]
    const someObject = 'asdasd'

    console.log(array.indexOf[someObject])
    // undefined








    const array = [1, 2, 3]
    const someObject = 'asdasd'

    console.log(array.indexOf[someObject])
    // undefined





    const array = [1, 2, 3]
    const someObject = 'asdasd'

    console.log(array.indexOf[someObject])
    // undefined






    share|improve this answer














    share|improve this answer



    share|improve this answer








    edited 11 mins ago

























    answered 1 hour ago









    0xc14m1z0xc14m1z

    1,409512




    1,409512








    • 1





      Array.indexOf is undefined, Array.prototype.indexOf, on the other hand, is a function.

      – Pavlo
      17 mins ago











    • You’re right! I fixed the typo in my answer. Thanks

      – 0xc14m1z
      10 mins ago














    • 1





      Array.indexOf is undefined, Array.prototype.indexOf, on the other hand, is a function.

      – Pavlo
      17 mins ago











    • You’re right! I fixed the typo in my answer. Thanks

      – 0xc14m1z
      10 mins ago








    1




    1





    Array.indexOf is undefined, Array.prototype.indexOf, on the other hand, is a function.

    – Pavlo
    17 mins ago





    Array.indexOf is undefined, Array.prototype.indexOf, on the other hand, is a function.

    – Pavlo
    17 mins ago













    You’re right! I fixed the typo in my answer. Thanks

    – 0xc14m1z
    10 mins ago





    You’re right! I fixed the typo in my answer. Thanks

    – 0xc14m1z
    10 mins ago











    0














    In short: Because that is the way the language is designed.



    JavaScript evaluates an attempt to access a property that doesn't exist as undefined.



    It doesn't raise an exception.






    share|improve this answer




























      0














      In short: Because that is the way the language is designed.



      JavaScript evaluates an attempt to access a property that doesn't exist as undefined.



      It doesn't raise an exception.






      share|improve this answer


























        0












        0








        0







        In short: Because that is the way the language is designed.



        JavaScript evaluates an attempt to access a property that doesn't exist as undefined.



        It doesn't raise an exception.






        share|improve this answer













        In short: Because that is the way the language is designed.



        JavaScript evaluates an attempt to access a property that doesn't exist as undefined.



        It doesn't raise an exception.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered 1 hour ago









        QuentinQuentin

        642k718661036




        642k718661036























            0














            Functions in JavaScript are first class objects.



            When you access function Array.indexOf() via bracket notation Array.indexOf['prop'] you actually trying to access a property which does not exist on indexOf so you get undefined.






            share|improve this answer



















            • 1





              What about TypeScript?

              – vlaz
              1 hour ago
















            0














            Functions in JavaScript are first class objects.



            When you access function Array.indexOf() via bracket notation Array.indexOf['prop'] you actually trying to access a property which does not exist on indexOf so you get undefined.






            share|improve this answer



















            • 1





              What about TypeScript?

              – vlaz
              1 hour ago














            0












            0








            0







            Functions in JavaScript are first class objects.



            When you access function Array.indexOf() via bracket notation Array.indexOf['prop'] you actually trying to access a property which does not exist on indexOf so you get undefined.






            share|improve this answer













            Functions in JavaScript are first class objects.



            When you access function Array.indexOf() via bracket notation Array.indexOf['prop'] you actually trying to access a property which does not exist on indexOf so you get undefined.







            share|improve this answer












            share|improve this answer



            share|improve this answer










            answered 1 hour ago









            GibboKGibboK

            34.3k107317542




            34.3k107317542








            • 1





              What about TypeScript?

              – vlaz
              1 hour ago














            • 1





              What about TypeScript?

              – vlaz
              1 hour ago








            1




            1





            What about TypeScript?

            – vlaz
            1 hour ago





            What about TypeScript?

            – vlaz
            1 hour ago


















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