How do I create a custom Error in JavaScript

Question

For some reason it looks like constructor delegation doesn t work in the following snippet   function NotImplementedError        Error apply this  arguments      NotImplementedError prototype   new Error     var nie   new NotImplementedError  some message    console log  The message is     nie message        Running this gives The message is      Any ideas as to why  or if there is a better way to create a new Error subclass  Is there a problem with applying to the native Error constructor that I don t know about

User · Answer

This is implemented nicely in the Cesium DeveloperError:

Docs
Source

In it's simplified form:

var NotImplementedError = function(message) {
    this.name = 'NotImplementedError';
    this.message = message;
    this.stack = (new Error()).stack;
}

// Later on...

throw new NotImplementedError();

User · Answer

class NotImplementedError extends Error     constructor message        super message       this message   message        NotImplementedError prototype name    NotImplementedError   module exports   NotImplementedError   and try     var e   new NotImplementedError  quot NotImplementedError message quot      throw e    catch  ex1      console log ex1 stack     console log  quot ex1 instanceof NotImplementedError    quot     ex1 instanceof NotImplementedError      console log  quot ex1 instanceof Error    quot     ex1 instanceof Error      console log  quot ex1 name    quot    ex1 name     console log  quot ex1 message    quot    ex1 message      It is just a class representation of this answer  output NotImplementedError  NotImplementedError message      stacktrace ex1 instanceof NotImplementedError   true ex1 instanceof Error   true ex1 name   NotImplementedError ex1 message   NotImplementedError message

User · Answer

This is fastest way to do it       let thisVar   false      if  thisVar     false                throw new Error  thisVar is false  It should be true

User · Answer

I like to do it like this    Make use of name so toString   throws   code    message   Return same thing to super so it appears the same in the stacktrace Attach code to error code as checking parsing a code is better in code than checking a message  which you might want to localize for example Attach message to error message as an alternative to error toString      x000D   x000D  class AppException extends Error   x000D    constructor code  message    x000D      const fullMsg   message      code     message     code  x000D      super fullMsg   x000D      this name   code  x000D      this code   code  x000D      this message   fullMsg  x000D      x000D     x000D    toString     x000D      return this message  x000D      x000D    x000D   x000D     Just a code x000D  try   x000D    throw new AppException  FORBIDDEN    x000D    catch e    x000D    console error e   x000D    console error e toString     x000D    console log e code      FORBIDDEN    x000D    x000D   x000D     A code and a message x000D  try   x000D    throw new AppException  FORBIDDEN    You don  t have access to this page    x000D    catch e    x000D    console error e   x000D    console error e toString     x000D    console log e code      FORBIDDEN    x000D    x000D   x000D   x000D

User · Answer

MDN has an excellent example   try     throw new Error  Whoops       catch  e      console log e name          e message

User · Answer

In ES2015  you can use class to do this cleanly    class NotImplemented extends Error     constructor message          args        super message     args       this message   message     has not yet been implemented            This does not modify the global Error prototype  allows you to customize message  name  and other attributes  and properly captures the stack  It s also pretty readable    Of course  you may need to use a tool like babel if your code will be running on older browsers

User · Answer

Try a new prototype object for each instance of the user defined error type  It allows instanceof checks to behave as usual plus type and message are correctly reported in Firefox and V8  Chome  nodejs    function NotImplementedError message       if NotImplementedError innercall   undefined           NotImplementedError innercall   true          NotImplementedError prototype   new Error message           NotImplementedError prototype name    NotImplementedError           NotImplementedError prototype constructor   NotImplementedError           return new NotImplementedError message             delete NotImplementedError innercall      Note that an additional entry will preceed the otherwise correct stack

User · Answer

All of the above answers are terrible awful - really  Even the one with 107 ups  The real answer is here guys    Inheriting from the Error object - where is the message property   TL DR   A  The reason message isn t being set is that Error is a function that returns a new Error object and does not manipulate this in any way   B  The way to do this right is to return the result of the apply from the constructor  as well as setting the prototype in the usual complicated javascripty way    x000D   x000D  function MyError     x000D      var temp   Error apply this  arguments   x000D      temp name   this name    MyError   x000D      this message   temp message  x000D      if Object defineProperty    x000D             getter for more optimizy goodness x000D            this stack     Object defineProperty this   stack      x000D              get  function     x000D                  return temp stack x000D                 x000D              configurable  true    so you can change it if you want x000D             x000D        else   x000D          this stack   temp stack x000D        x000D    x000D    inherit prototype using ECMAScript 5  IE 9   x000D  MyError prototype   Object create Error prototype    x000D      constructor    x000D          value  MyError  x000D          writable  true  x000D          configurable  true x000D        x000D      x000D   x000D  var myError   new MyError  message    x000D  console log  The message is       myError message            The message is   message  x000D  console log myError instanceof Error      true x000D  console log myError instanceof MyError      true x000D  console log myError toString        MyError  message x000D  console log myError stack      MyError  message  n  x000D      lt stack trace     gt  x000D   x000D   x000D    x000D    for EMCAScript 4 or ealier  IE 8 or ealier   inherit prototype this way instead of above code  x000D     x000D  var IntermediateInheritor   function       x000D  IntermediateInheritor prototype   Error prototype  x000D  MyError prototype   new IntermediateInheritor    x000D     x000D   x000D   x000D    You could probably do some trickery to enumerate through all the non-enumerable properties of the tmp Error to set them rather than explicitly setting only stack and message  but the trickery isn t supported in ie lt 9

User · Answer

Accoring to Joyent you shouldn   t mess with the stack property  which I see in lots of answers given here   because it will have a negative impact on performance  Here is what they say      stack  generally  don t mess with this  Don t even augment it  V8 only computes it if someone actually reads the property  which improves performance dramatically for handlable errors  If you read the property just to augment it  you ll end up paying the cost even if your caller doesn t need the stack    I like and would like to mention their idea of wrapping the original error which is a nice replacement for passing on the stack   So here is how I create a custom error  considering the above mentioned   es5 version    x000D   x000D  function RError options    x000D      options   options           eslint-disable-line no-param-reassign x000D      this name   options name  x000D      this message   options message  x000D      this cause   options cause  x000D   x000D         capture stack  this property is supposed to be treated as private  x000D      this  err   new Error    x000D   x000D         create an iterable chain x000D      this chain   this cause    this  concat this cause chain     this   x000D    x000D  RError prototype   Object create Error prototype    x000D      constructor    x000D          value  RError  x000D          writable  true  x000D          configurable  true x000D        x000D      x000D   x000D  Object defineProperty RError prototype   stack     x000D      get  function stack     x000D          return this name          this message     n    this  err stack split   n   slice 2  join   n    x000D        x000D      x000D   x000D  Object defineProperty RError prototype   why     x000D      get  function why     x000D          var  why   this name          this message  x000D          for  var i   1  i  lt  this chain length  i      x000D              var e   this chain i   x000D               why       lt -     e name          e message  x000D            x000D          return  why  x000D        x000D      x000D   x000D     usage x000D   x000D  function fail     x000D      throw new RError   x000D          name   BAR   x000D          message   I messed up   x000D          x000D    x000D   x000D  function failFurther     x000D      try   x000D          fail    x000D        catch  err    x000D          throw new RError   x000D              name   FOO   x000D              message   Something went wrong    x000D              cause  err x000D              x000D        x000D    x000D   x000D  try   x000D      failFurther    x000D    catch  err    x000D      console error err why   x000D      console error err stack   x000D      console error err cause stack   x000D    x000D   x000D   x000D    es6 version    x000D   x000D  class RError extends Error   x000D      constructor  name  message  cause     x000D          super    x000D          this name   name  x000D          this message   message  x000D          this cause   cause  x000D        x000D       Symbol iterator      x000D          let current   this  x000D          let done   false  x000D          const iterator     x000D              next     x000D                  const val   current  x000D                  if  done    x000D                      return   value  val  done  true    x000D                    x000D                  current   current cause  x000D                  if   val cause    x000D                      done   true  x000D                    x000D                  return   value  val  done  false    x000D                x000D             x000D          return iterator  x000D        x000D      get why     x000D          let  why       x000D          for  const e of this    x000D               why        why length      lt -          e name     e message    x000D            x000D          return  why  x000D        x000D    x000D   x000D     usage x000D   x000D  function fail     x000D      throw new RError   x000D          name   BAR   x000D          message   I messed up   x000D          x000D    x000D   x000D  function failFurther     x000D      try   x000D          fail    x000D        catch  err    x000D          throw new RError   x000D              name   FOO   x000D              message   Something went wrong    x000D              cause  err x000D              x000D        x000D    x000D   x000D  try   x000D      failFurther    x000D    catch  err    x000D      console error err why   x000D      console error err stack   x000D      console error err cause stack   x000D    x000D   x000D   x000D    I   ve put my solution into a module  here it is  https   www npmjs com package rerror

User · Answer

This is my implementation   class HttpError extends Error     constructor message  code   null  status   null  stack   null  name   null        super        this message   message      this status   500       this name   name    this constructor name      this code   code     E   this name toUpperCase          this stack   stack    null         static fromObject error        if  error instanceof HttpError          return error            else         const   message  code  status  stack     error        return new ServerError message  code  status  stack  error constructor name                expose         if  this instanceof ClientError          return      this              else         return           name  this name          code  this code          status  this status                       class ServerError extends HttpError     class ClientError extends HttpError      class IncorrectCredentials extends ClientError     constructor    args        super    args       this status   400         class ResourceNotFound extends ClientError     constructor    args        super    args       this status   404          Example usage  1   app use  req  res  next    gt      try       invalidFunction          catch  err        const error   HttpError fromObject err       return res status error status  send error expose               Example usage  2   router post   api auth   async  req  res    gt      try       const isLogged   await User logIn req body username  req body password        if   isLogged          throw new IncorrectCredentials  Incorrect username or password              else         return res status 200  send           token                        catch  err        const error   HttpError fromObject err       return res status error status  send error expose

User · Answer

The constructor needs to be like a factory method and return what you want  If you need additional methods properties  you can add them to the object before returning it   function NotImplementedError message    return new Error  Not implemented   message      x   new NotImplementedError      Though I m not sure why you d need to do this  Why not just use new Error      Custom exceptions don t really add much in JavaScript  or probably any untyped language

User · Answer

I had a similar issue to this  My error needs to be an instanceof both Error and NotImplemented  and it also needs to produce a coherent backtrace in the console   My solution   var NotImplemented    function       var NotImplemented  err    NotImplemented    function         function NotImplemented message          var err        err   new Error message         err name    NotImplemented         this message   err message        if  err stack  this stack   err stack            return NotImplemented            err   new Error      err name    NotImplemented     NotImplemented prototype   err     return NotImplemented     call this       TEST  console log  instanceof Error       new NotImplemented   instanceof Error    console log  instanceof NotImplemented       new NotImplemented   instanceofNotImplemented    console log  message     new NotImplemented  I was too busy   message    throw new NotImplemented  just didn t feel like it      Result of running with node js   instanceof Error  true instanceof NotImplemented  true message  I was too busy   private tmp t js 24 throw new NotImplemented  just didn t feel like it            NotImplemented  just didn t feel like it     at Error NotImplemented   Users colin projects gems jax t js 6 13      at Object  lt anonymous gt    Users colin projects gems jax t js 24 7      at Module  compile  module js 449 26      at Object Module  extensions  js  module js 467 10      at Module load  module js 356 32      at Function Module  load  module js 312 12      at Module runMain  module js 487 10      at process startup processNextTick process  tickCallback  node js 244 9    The error passes all 3 of my criteria  and although the stack property is nonstandard  it is supported in most newer browsers which is acceptable in my case

User · Answer

If anyone is curious on how to create a custom error and get the stack trace   function CustomError message      this name    CustomError     this message   message          var error   new Error this message     error name   this name    this stack   error stack    CustomError prototype   Object create Error prototype    try     throw new CustomError  foobar      catch  e      console log  name    e name     console log  message    e message     console log  stack    e stack

User · Answer

If you are using Node Chrome  The following snippet will get you extension which meets the following requirements    err instanceof Error err instanceof CustomErrorType console log   returns  CustomErrorType  when created with a message console log   returns  CustomErrorType  message  when created without a message throw stack provides the information at the point the error was created  Works optimally in Node JS  and Chrome  Will pass instanceof checks in Chrome  Safari  Firefox and IE 8   but will not have a valid stack outside of Chrome Safari  I m OK with that because I can debug in chrome  but code which requires specific error types will still function cross browser  If you need Node only you can easily remove the if statements and you re good to go    Snippet  var CustomErrorType   function message        if  Object defineProperty            Object defineProperty this   message                 value   message                    enumerable   false                   else           this message   message             if  Error captureStackTrace            Error captureStackTrace this  CustomErrorType            CustomErrorType prototype   new Error    CustomErrorType prototype name    CustomErrorType     Usage  var err   new CustomErrorType  foo      Output  var err   new CustomErrorType  foo    console log err   console log err stack     CustomErrorType  foo  CustomErrorType  foo     at Object  lt anonymous gt    errorTest js 27 12      at Module  compile  module js 456 26      at Object Module  extensions  js  module js 474 10      at Module load  module js 356 32      at Function Module  load  module js 312 12      at Function Module runMain  module js 497 10      at startup  node js 119 16      at node js 906 3   errorTest js 30         throw err                  CustomErrorType  foo     at Object  lt anonymous gt    errorTest js 27 12      at Module  compile  module js 456 26      at Object Module  extensions  js  module js 474 10      at Module load  module js 356 32      at Function Module  load  module js 312 12      at Function Module runMain  module js 497 10      at startup  node js 119 16      at node js 906 3

User · Answer

I just had to implement something like this and found that the stack was lost in my own error implementation  What I had to do was create a dummy error and retrieve the stack from that   My Error   function  message  innerException        var err   new Error        this stack   err stack     IMPORTANT      this name    Error       this message   message      this innerException   innerException    My Error prototype   new Error    My Error prototype constructor   My Error  My Error prototype toString   function  includeStackTrace        var msg   this message      var e   this innerException      while  e            msg      The details are  n    e message          e   e innerException            if  includeStackTrace            msg      n nStack Trace  n n    this stack            return msg

User · Answer

I used the Constructor Pattern to create the new error object  I defined the prototype chain such as an Error instance  See the MDN Error constructor reference   You can check this snippet on this gist   IMPLEMENTATION     Creates user-defined exceptions var CustomError    function        use strict        constructor   function CustomError           enforces  new  instance     if    this instanceof CustomError           return new CustomError arguments             var error          handles the arguments object when is passed by enforcing a  new  instance       args   Array apply null  typeof arguments 0       object    arguments 0    arguments         message   args shift       An exception has occurred          builds the message with multiple arguments     if   message indexOf               args forEach function arg  i            message   message replace RegExp         i         g    arg                          gets the exception stack     error   new Error message         access to CustomError prototype name     error name   this name         set the properties of the instance       in order to resemble an Error instance     Object defineProperties this          stack            enumerable  false          get  function     return error stack                   message            enumerable  false          value  message                           Creates the prototype and prevents the direct reference to Error prototype       Not used new Error   here because an exception would be raised here       but we need to raise the exception when CustomError instance is created    CustomError prototype   Object create Error prototype          fixes the link to the constructor  ES5      constructor  setDescriptor CustomError       name  setDescriptor  JSU Error            function setDescriptor value        return         configurable  false        enumerable  false        writable  false        value  value                 returns the constructor   return CustomError          USAGE  The CustomError constructor can receive many arguments to build the message  e g    var err1   new CustomError  The url of file is required        err2   new CustomError  Invalid Date   0      date        err3   new CustomError  The length must be greater than  0    4       err4   new CustomError  Properties   0  and   1  don t exist    p1    p2     throw err4    And this is how the custom error looks

User · Answer

easier way  You could make your object inherit from the Error object   Example   function NotImplementError message        this message   message      Error call        Error call message        what we are doing is using the function call   which call the constructor of the Error class so is basicly the same thing as implementing a class inheritance in other object oriented languages

User · Answer

Another alternative   might not work in all enviroments Atleast assured it works in nodejs 0 8 This approach uses a non standard way of modifying the internal proto prop  function myError msg          var e   new Error msg           this   this          this   proto     proto     e

User · Answer

Update your code to assign your prototype to the Error prototype and the instanceof and your asserts work  function NotImplementedError message    quot  quot         this name    quot NotImplementedError quot       this message   message    NotImplementedError prototype   Error prototype   However  I would just throw your own object and just check the name property  throw  name    quot NotImplementedError quot   message    quot too lazy to implement quot      Edit based on comments After looking at the comments and trying to remember why I would assign prototype to Error prototype instead of new Error   like Nicholas Zakas did in his article  I created a jsFiddle with the code below   x000D   x000D  function NotImplementedError message           this name    NotImplementedError     this message   message    NotImplementedError prototype   Error prototype   function NotImplementedError2 message           this message   message    NotImplementedError2 prototype   new Error     try     var e   new NotImplementedError  NotImplementedError message      throw e    catch  ex1      console log ex1 stack     console log  ex1 instanceof NotImplementedError        ex1 instanceof NotImplementedError      console log  ex1 instanceof Error        ex1 instanceof Error      console log  ex1 name       ex1 name     console log  ex1 message       ex1 message      try     var e   new NotImplementedError2  NotImplementedError2 message      throw e    catch  ex1      console log ex1 stack     console log  ex1 instanceof NotImplementedError2        ex1 instanceof NotImplementedError2      console log  ex1 instanceof Error        ex1 instanceof Error      console log  ex1 name       ex1 name     console log  ex1 message       ex1 message     x000D   x000D   x000D   The console output was this  undefined ex1 instanceof NotImplementedError   true ex1 instanceof Error   true ex1 name   NotImplementedError ex1 message   NotImplementedError message Error     at window onload  http   fiddle jshell net MwMEJ show  29 34  ex1 instanceof NotImplementedError2   true ex1 instanceof Error   true ex1 name   Error ex1 message   NotImplementedError2 message  This confirmes the  quot problem quot  I ran into was the stack property of the error was the line number where new Error   was created  and not where the throw e occurred   However  that may be better that having the side effect of a NotImplementedError prototype name    quot NotImplementedError quot  line affecting the Error object  Also  notice with NotImplementedError2  when I don t set the  name explicitly  it is equal to  quot Error quot    However  as mentioned in the comments  because that version sets prototype to new Error    I could set NotImplementedError2 prototype name    quot NotImplementedError2 quot  and be OK

User · Answer

At the expense of not being able to use instanceof  the following preserves the original stack trace and doesn t use any non-standard tricks      the function itself var fixError   function err  name        err name   name      return err        using the function try       throw fixError new Error  custom error message     CustomError      catch  e        if  e name     CustomError           console log  Wee  Custom Error  Msg    e message       else         throw e     unhandled  let it propagate upwards the call stack

User · Answer

The following worked for me taken from the official Mozilla documentation Error   function NotImplementedError message        var instance   new Error message       instance name    NotImplementedError        Object setPrototypeOf instance  Object getPrototypeOf this        if  Error captureStackTrace            Error captureStackTrace instance  NotImplementedError             return instance     NotImplementedError prototype   Object create Error prototype        constructor            value  Error          enumerable  false          writable  true          configurable  true

User · Answer

function InvalidValueError value  type        this message    Expected      type name           value      var error   new Error this message       this stack   error stack    InvalidValueError prototype   new Error    InvalidValueError prototype name   InvalidValueError name  InvalidValueError prototype constructor   InvalidValueError

User · Answer

This section of the standard may explain why the Error apply call doesn t initialize the object      15 11 1 The Error Constructor Called as a Function      When Error is called as a function rather than as a constructor  it creates and   initialises a new Error object  Thus the function call Error      is   equivalent to the object creation expression new Error      with the   same arguments    In this case the Error function probably determines that it s not being called as a constructor  so it returns a new Error instance rather than initializing the this object   Testing with the following code seems to demonstrate that this is in fact what s happening   function NotImplementedError         var returned   Error apply this  arguments      console log  returned message        returned message            console log  this message        this message           NotImplementedError prototype   new Error     var nie   new NotImplementedError  some message      The following output is generated when this is run   returned message    some message  this message

[javascript] How do I create a custom Error in JavaScript?

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