Custom exception type

Question

Can I define custom types for user-defined exceptions in JavaScript   If so  how would I do it

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function MyError message     this message   message     MyError prototype   new Error    This allows for usage like    try     something       catch e      if e instanceof MyError     doSomethingElse      else if e instanceof Error     andNowForSomethingCompletelyDifferent

User · Answer

Yes  You can throw anything you want  integers  strings  objects  whatever  If you want to throw an object  then simply create a new object  just as you would create one under other circumstances  and then throw it  Mozilla s Javascript reference has several examples

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From WebReference   throw      name          System Error      level         Show Stopper      message       Error detected  Please contact the system administrator       htmlMessage   Error detected  Please contact the  lt a href   mailto sysadmin acme-widgets com   gt system administrator lt  a gt       toString     function   return this name          this message

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create error object var error   new Object    error reason  some reason       business function function exception        try          throw error       catch err           err reason            Now we set add the reason or whatever properties we want to the error object and retrieve it  By making the error more reasonable

User · Answer

Yes  You can throw anything you want  integers  strings  objects  whatever  If you want to throw an object  then simply create a new object  just as you would create one under other circumstances  and then throw it  Mozilla s Javascript reference has several examples

User · Answer

Use the throw statement   JavaScript doesn t care what the exception type is  as Java does   JavaScript just notices  there s an exception and when you catch it  you can  look  what the exception  says    If you have different exception types you have to throw  I d suggest to use variables which contain the string object of the exception i e  message  Where you need it use  throw myException  and in the catch  compare the caught exception to myException

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From WebReference   throw      name          System Error      level         Show Stopper      message       Error detected  Please contact the system administrator       htmlMessage   Error detected  Please contact the  lt a href   mailto sysadmin acme-widgets com   gt system administrator lt  a gt       toString     function   return this name          this message

User · Answer

You should create a custom exception that prototypically inherits from Error   For example   function InvalidArgumentException message        this message   message         Use V8 s native method if available  otherwise fallback     if   captureStackTrace  in Error          Error captureStackTrace this  InvalidArgumentException       else         this stack    new Error    stack     InvalidArgumentException prototype   Object create Error prototype   InvalidArgumentException prototype name    InvalidArgumentException   InvalidArgumentException prototype constructor   InvalidArgumentException    This is basically a simplified version of what disfated posted above with the enhancement that stack traces work on Firefox and other browsers   It satisfies the same tests that he posted   Usage   throw new InvalidArgumentException    var err   new InvalidArgumentException  Not yet         And it will behave is expected   err instanceof InvalidArgumentException             - gt  true err instanceof Error                                - gt  true InvalidArgumentException prototype isPrototypeOf err     - gt  true Error prototype isPrototypeOf err                   - gt  true err constructor name                                - gt  InvalidArgumentException err name                                            - gt  InvalidArgumentException err message                                         - gt  Not yet    err toString                                        - gt  InvalidArgumentException  Not yet    err stack                                           - gt  works fine

User · Answer

Use the throw statement   JavaScript doesn t care what the exception type is  as Java does   JavaScript just notices  there s an exception and when you catch it  you can  look  what the exception  says    If you have different exception types you have to throw  I d suggest to use variables which contain the string object of the exception i e  message  Where you need it use  throw myException  and in the catch  compare the caught exception to myException

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In short    If you are using ES6 without transpilers   class CustomError extends Error                See Extending Error in Javascript with ES6 syntax for what s the current best practice If you are using Babel transpiler    Option 1  use babel-plugin-transform-builtin-extend  Option 2  do it yourself  inspired from that same library       function CustomError    args          const instance   Reflect construct Error  args         Reflect setPrototypeOf instance  Reflect getPrototypeOf this          return instance            CustomError prototype   Object create Error prototype          constructor            value  Error          enumerable  false          writable  true          configurable  true                     Reflect setPrototypeOf CustomError  Error      If you are using pure ES5   function CustomError message  fileName  lineNumber      const instance   new Error message  fileName  lineNumber     Object setPrototypeOf instance  Object getPrototypeOf this      return instance    CustomError prototype   Object create Error prototype      constructor        value  Error      enumerable  false      writable  true      configurable  true         if  Object setPrototypeOf       Object setPrototypeOf CustomError  Error     else       CustomError   proto     Error     Alternative  use Classtrophobic framework   Explanation   Why extending the Error class using ES6 and Babel is a problem   Because an instance of CustomError is not anymore recognized as such   class CustomError extends Error    console log new CustomError  test   instanceof Error     true console log new CustomError  test   instanceof CustomError     false   In fact  from the official documentation of Babel  you cannot extend any built-in JavaScript classes such as Date  Array  DOM or Error   The issue is described here    Native extends breaks HTMLELement  Array  and others an object of The class which is extends by base type like Array Number Object String or Error is not instanceof this class   What about the other SO answers   All the given answers fix the instanceof issue but you lose the regular error console log   console log new CustomError  test        output     CustomError  name   MyError   message   test   stack   Error     at CustomError   lt anonymous gt  4 19      at  lt anonymous gt  1 5     Whereas using the method mentioned above  not only you fix the instanceof issue but you also keep the regular error console log   console log new CustomError  test        output     Error  test        at CustomError   lt anonymous gt  2 32         at  lt anonymous gt  1 5

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See this example in the MDN   If you need to define multiple Errors  test the code here     function createErrorType name  initFunction        function E message            this message   message          if  Error captureStackTrace              Error captureStackTrace this  this constructor           else             this stack    new Error    stack          initFunction  amp  amp  initFunction apply this  arguments             E prototype   Object create Error prototype       E prototype name   name      E prototype constructor   E      return E    var InvalidStateError   createErrorType       InvalidStateError       function  invalidState  acceptedStates            this message    The state     invalidState     is invalid  Expected     acceptedStates                 var error   new InvalidStateError  foo    bar or baz    function assert condition    if   condition  throw new Error      assert error message   assert error instanceof InvalidStateError     assert error instanceof Error    assert error name     InvalidStateError    assert error stack   error message    Code is mostly copied from  What  39 s a good way to extend Error in JavaScript

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You could implement your own exceptions and their handling for example like here      define exceptions  classes   function NotNumberException      function NotPositiveNumberException          try some code try          some function code that can throw     if  isNaN value           throw new NotNumberException        else     if  value  lt  0          throw new NotPositiveNumberException      catch  e        if  e instanceof NotNumberException            alert  not a number              else     if  e instanceof NotPositiveNumberException            alert  not a positive number              There is another syntax for catching a typed exception  although this won t work in every browser  for example not in IE       define exceptions  classes   function NotNumberException      function NotPositiveNumberException          try some code try          some function code that can throw     if  isNaN value           throw new NotNumberException        else     if  value  lt  0          throw new NotPositiveNumberException      catch  e if e instanceof NotNumberException        alert  not a number      catch  e if e instanceof NotPositiveNumberException        alert  not a positive number

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An alternative to the answer of asselin for use with ES2015 classes  class InvalidArgumentException extends Error       constructor message            super            Error captureStackTrace this  this constructor           this name    InvalidArgumentException           this message   message

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Here is how you can create custom errors with completely identical to native Error s behaviour  This technique works only in Chrome and node js for now  I also wouldn t recommend to use it if you don t understand what it does   Error createCustromConstructor    function          function define obj  prop  value            Object defineProperty obj  prop                value  value              configurable  true              enumerable  false              writable  true                        return function name  init  proto            var CustomError          proto   proto                function build message                var self   this instanceof CustomError                   this                   Object create CustomError prototype               Error apply self  arguments               Error captureStackTrace self  CustomError               if  message    undefined                    define self   message   String message                              define self   arguments   undefined               define self   type   undefined               if  typeof init     function                     init apply self  arguments                             return self                    eval  CustomError   function     name                         return build apply this  arguments                CustomError prototype   Object create Error prototype           define CustomError prototype   constructor   CustomError           for  var key in proto                define CustomError prototype  key  proto key                      Object defineProperty CustomError prototype   name     value  name             return CustomError                 As a reasult we get         name   The name of the constructor name    init   User-defined initialization function    proto  It s enumerable members will be added to            prototype of created constructor      Error createCustromConstructor   function name  init  proto    Then you can use it like this   var NotImplementedError   Error createCustromConstructor  NotImplementedError      And use NotImplementedError as you would Error   throw new NotImplementedError    var err   new NotImplementedError    var err   NotImplementedError  Not yet         And it will behave is expected   err instanceof NotImplementedError                  - gt  true err instanceof Error                                - gt  true NotImplementedError prototype isPrototypeOf err     - gt  true Error prototype isPrototypeOf err                   - gt  true err constructor name                                - gt  NotImplementedError err name                                            - gt  NotImplementedError err message                                         - gt  Not yet    err toString                                        - gt  NotImplementedError  Not yet    err stack                                           - gt  works fine    Note  that error stack works absolutle correct and won t include NotImplementedError constructor call  thanks to v8 s Error captureStackTrace      Note  There is ugly eval    The only reason it is used is to get correct err constructor name  If you don t need it  you can a bit simplify everything

User · Answer

You could implement your own exceptions and their handling for example like here      define exceptions  classes   function NotNumberException      function NotPositiveNumberException          try some code try          some function code that can throw     if  isNaN value           throw new NotNumberException        else     if  value  lt  0          throw new NotPositiveNumberException      catch  e        if  e instanceof NotNumberException            alert  not a number              else     if  e instanceof NotPositiveNumberException            alert  not a positive number              There is another syntax for catching a typed exception  although this won t work in every browser  for example not in IE       define exceptions  classes   function NotNumberException      function NotPositiveNumberException          try some code try          some function code that can throw     if  isNaN value           throw new NotNumberException        else     if  value  lt  0          throw new NotPositiveNumberException      catch  e if e instanceof NotNumberException        alert  not a number      catch  e if e instanceof NotPositiveNumberException        alert  not a positive number

User · Answer

function MyError message     this message   message     MyError prototype   new Error    This allows for usage like    try     something       catch e      if e instanceof MyError     doSomethingElse      else if e instanceof Error     andNowForSomethingCompletelyDifferent

User · Answer

In short    If you are using ES6 without transpilers   class CustomError extends Error                See Extending Error in Javascript with ES6 syntax for what s the current best practice If you are using Babel transpiler    Option 1  use babel-plugin-transform-builtin-extend  Option 2  do it yourself  inspired from that same library       function CustomError    args          const instance   Reflect construct Error  args         Reflect setPrototypeOf instance  Reflect getPrototypeOf this          return instance            CustomError prototype   Object create Error prototype          constructor            value  Error          enumerable  false          writable  true          configurable  true                     Reflect setPrototypeOf CustomError  Error      If you are using pure ES5   function CustomError message  fileName  lineNumber      const instance   new Error message  fileName  lineNumber     Object setPrototypeOf instance  Object getPrototypeOf this      return instance    CustomError prototype   Object create Error prototype      constructor        value  Error      enumerable  false      writable  true      configurable  true         if  Object setPrototypeOf       Object setPrototypeOf CustomError  Error     else       CustomError   proto     Error     Alternative  use Classtrophobic framework   Explanation   Why extending the Error class using ES6 and Babel is a problem   Because an instance of CustomError is not anymore recognized as such   class CustomError extends Error    console log new CustomError  test   instanceof Error     true console log new CustomError  test   instanceof CustomError     false   In fact  from the official documentation of Babel  you cannot extend any built-in JavaScript classes such as Date  Array  DOM or Error   The issue is described here    Native extends breaks HTMLELement  Array  and others an object of The class which is extends by base type like Array Number Object String or Error is not instanceof this class   What about the other SO answers   All the given answers fix the instanceof issue but you lose the regular error console log   console log new CustomError  test        output     CustomError  name   MyError   message   test   stack   Error     at CustomError   lt anonymous gt  4 19      at  lt anonymous gt  1 5     Whereas using the method mentioned above  not only you fix the instanceof issue but you also keep the regular error console log   console log new CustomError  test        output     Error  test        at CustomError   lt anonymous gt  2 32         at  lt anonymous gt  1 5

User · Answer

You could implement your own exceptions and their handling for example like here      define exceptions  classes   function NotNumberException      function NotPositiveNumberException          try some code try          some function code that can throw     if  isNaN value           throw new NotNumberException        else     if  value  lt  0          throw new NotPositiveNumberException      catch  e        if  e instanceof NotNumberException            alert  not a number              else     if  e instanceof NotPositiveNumberException            alert  not a positive number              There is another syntax for catching a typed exception  although this won t work in every browser  for example not in IE       define exceptions  classes   function NotNumberException      function NotPositiveNumberException          try some code try          some function code that can throw     if  isNaN value           throw new NotNumberException        else     if  value  lt  0          throw new NotPositiveNumberException      catch  e if e instanceof NotNumberException        alert  not a number      catch  e if e instanceof NotPositiveNumberException        alert  not a positive number

User · Answer

create error object var error   new Object    error reason  some reason       business function function exception        try          throw error       catch err           err reason            Now we set add the reason or whatever properties we want to the error object and retrieve it  By making the error more reasonable

User · Answer

Use the throw statement   JavaScript doesn t care what the exception type is  as Java does   JavaScript just notices  there s an exception and when you catch it  you can  look  what the exception  says    If you have different exception types you have to throw  I d suggest to use variables which contain the string object of the exception i e  message  Where you need it use  throw myException  and in the catch  compare the caught exception to myException

User · Answer

Use the throw statement   JavaScript doesn t care what the exception type is  as Java does   JavaScript just notices  there s an exception and when you catch it  you can  look  what the exception  says    If you have different exception types you have to throw  I d suggest to use variables which contain the string object of the exception i e  message  Where you need it use  throw myException  and in the catch  compare the caught exception to myException

User · Answer

ES6  With the new class and extend keywords it   s now much easier   class CustomError extends Error     constructor message        super message         something

User · Answer

You could implement your own exceptions and their handling for example like here      define exceptions  classes   function NotNumberException      function NotPositiveNumberException          try some code try          some function code that can throw     if  isNaN value           throw new NotNumberException        else     if  value  lt  0          throw new NotPositiveNumberException      catch  e        if  e instanceof NotNumberException            alert  not a number              else     if  e instanceof NotPositiveNumberException            alert  not a positive number              There is another syntax for catching a typed exception  although this won t work in every browser  for example not in IE       define exceptions  classes   function NotNumberException      function NotPositiveNumberException          try some code try          some function code that can throw     if  isNaN value           throw new NotNumberException        else     if  value  lt  0          throw new NotPositiveNumberException      catch  e if e instanceof NotNumberException        alert  not a number      catch  e if e instanceof NotPositiveNumberException        alert  not a positive number

User · Answer

An alternative to the answer of asselin for use with ES2015 classes  class InvalidArgumentException extends Error       constructor message            super            Error captureStackTrace this  this constructor           this name    InvalidArgumentException           this message   message

User · Answer

I often use an approach with prototypal inheritance  Overriding toString   gives you the advantage that tools like Firebug will log the actual information instead of  object Object  to the console for uncaught exceptions   Use instanceof to determine the type of exception   main js     just an exemplary namespace var ns   ns            include JavaScript of the following    source files here  e g  by concatenation   var someId   42  throw new ns DuplicateIdException  Another item with ID         someId     has been created       Firebug console     uncaught exception   Duplicate ID  Another item with ID 42 has been created   Exception js  ns Exception   function               Form a string of relevant information        When providing this method  tools like Firebug show the returned     string instead of  object Object  for uncaught exceptions         return  String  information about the exception     ns Exception prototype toString   function         var name   this name     unknown       var message   this message     no description       return       name          message       DuplicateIdException js  ns DuplicateIdException   function message        this name    Duplicate ID       this message   message      ns DuplicateIdException prototype   new ns Exception

User · Answer

You should create a custom exception that prototypically inherits from Error   For example   function InvalidArgumentException message        this message   message         Use V8 s native method if available  otherwise fallback     if   captureStackTrace  in Error          Error captureStackTrace this  InvalidArgumentException       else         this stack    new Error    stack     InvalidArgumentException prototype   Object create Error prototype   InvalidArgumentException prototype name    InvalidArgumentException   InvalidArgumentException prototype constructor   InvalidArgumentException    This is basically a simplified version of what disfated posted above with the enhancement that stack traces work on Firefox and other browsers   It satisfies the same tests that he posted   Usage   throw new InvalidArgumentException    var err   new InvalidArgumentException  Not yet         And it will behave is expected   err instanceof InvalidArgumentException             - gt  true err instanceof Error                                - gt  true InvalidArgumentException prototype isPrototypeOf err     - gt  true Error prototype isPrototypeOf err                   - gt  true err constructor name                                - gt  InvalidArgumentException err name                                            - gt  InvalidArgumentException err message                                         - gt  Not yet    err toString                                        - gt  InvalidArgumentException  Not yet    err stack                                           - gt  works fine

User · Answer

See this example in the MDN   If you need to define multiple Errors  test the code here     function createErrorType name  initFunction        function E message            this message   message          if  Error captureStackTrace              Error captureStackTrace this  this constructor           else             this stack    new Error    stack          initFunction  amp  amp  initFunction apply this  arguments             E prototype   Object create Error prototype       E prototype name   name      E prototype constructor   E      return E    var InvalidStateError   createErrorType       InvalidStateError       function  invalidState  acceptedStates            this message    The state     invalidState     is invalid  Expected     acceptedStates                 var error   new InvalidStateError  foo    bar or baz    function assert condition    if   condition  throw new Error      assert error message   assert error instanceof InvalidStateError     assert error instanceof Error    assert error name     InvalidStateError    assert error stack   error message    Code is mostly copied from  What  39 s a good way to extend Error in JavaScript

User · Answer

Here is how you can create custom errors with completely identical to native Error s behaviour  This technique works only in Chrome and node js for now  I also wouldn t recommend to use it if you don t understand what it does   Error createCustromConstructor    function          function define obj  prop  value            Object defineProperty obj  prop                value  value              configurable  true              enumerable  false              writable  true                        return function name  init  proto            var CustomError          proto   proto                function build message                var self   this instanceof CustomError                   this                   Object create CustomError prototype               Error apply self  arguments               Error captureStackTrace self  CustomError               if  message    undefined                    define self   message   String message                              define self   arguments   undefined               define self   type   undefined               if  typeof init     function                     init apply self  arguments                             return self                    eval  CustomError   function     name                         return build apply this  arguments                CustomError prototype   Object create Error prototype           define CustomError prototype   constructor   CustomError           for  var key in proto                define CustomError prototype  key  proto key                      Object defineProperty CustomError prototype   name     value  name             return CustomError                 As a reasult we get         name   The name of the constructor name    init   User-defined initialization function    proto  It s enumerable members will be added to            prototype of created constructor      Error createCustromConstructor   function name  init  proto    Then you can use it like this   var NotImplementedError   Error createCustromConstructor  NotImplementedError      And use NotImplementedError as you would Error   throw new NotImplementedError    var err   new NotImplementedError    var err   NotImplementedError  Not yet         And it will behave is expected   err instanceof NotImplementedError                  - gt  true err instanceof Error                                - gt  true NotImplementedError prototype isPrototypeOf err     - gt  true Error prototype isPrototypeOf err                   - gt  true err constructor name                                - gt  NotImplementedError err name                                            - gt  NotImplementedError err message                                         - gt  Not yet    err toString                                        - gt  NotImplementedError  Not yet    err stack                                           - gt  works fine    Note  that error stack works absolutle correct and won t include NotImplementedError constructor call  thanks to v8 s Error captureStackTrace      Note  There is ugly eval    The only reason it is used is to get correct err constructor name  If you don t need it  you can a bit simplify everything

User · Answer

Yes  You can throw anything you want  integers  strings  objects  whatever  If you want to throw an object  then simply create a new object  just as you would create one under other circumstances  and then throw it  Mozilla s Javascript reference has several examples

User · Answer

I often use an approach with prototypal inheritance  Overriding toString   gives you the advantage that tools like Firebug will log the actual information instead of  object Object  to the console for uncaught exceptions   Use instanceof to determine the type of exception   main js     just an exemplary namespace var ns   ns            include JavaScript of the following    source files here  e g  by concatenation   var someId   42  throw new ns DuplicateIdException  Another item with ID         someId     has been created       Firebug console     uncaught exception   Duplicate ID  Another item with ID 42 has been created   Exception js  ns Exception   function               Form a string of relevant information        When providing this method  tools like Firebug show the returned     string instead of  object Object  for uncaught exceptions         return  String  information about the exception     ns Exception prototype toString   function         var name   this name     unknown       var message   this message     no description       return       name          message       DuplicateIdException js  ns DuplicateIdException   function message        this name    Duplicate ID       this message   message      ns DuplicateIdException prototype   new ns Exception

User · Answer

Yes  You can throw anything you want  integers  strings  objects  whatever  If you want to throw an object  then simply create a new object  just as you would create one under other circumstances  and then throw it  Mozilla s Javascript reference has several examples

User · Answer

ES6  With the new class and extend keywords it   s now much easier   class CustomError extends Error     constructor message        super message         something

[javascript] Custom exception type

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