checking for typeof error in JS

Question

In JS it doesn t seem possible to check if an argument passed to a function is actually of the type  error  or an instance of Error   For example  this is not valid   typeof err      error   since there are only 6 possible types  in the form of strings    The typeof operator returns type information as a string  There are six possible values that typeof returns       number    string    boolean    object    function  and  undefined     MSDN  But what if I have a simple use case like this   function errorHandler err         if  typeof err      error             throw err            else           console error  Unexpectedly  no error was passed to error handler  But here is the message   err             so what is the best way to determine if an argument is an instance of Error   is the instanceof operator of any help

User · Accepted Answer

You can use the instanceof operator  but see caveat below     var myError   new Error  foo    myError instanceof Error    true var myString    Whatever   myString instanceof Error    false   The above won t work if the error was thrown in a different window frame iframe than where the check is happening  In that case  the instanceof Error check will return false  even for an Error object  In that case  the easiest approach is duck-typing   if  myError  amp  amp  myError stack  amp  amp  myError message         it s an error  probably     However  duck-typing may produce false positives if you have non-error objects that contain stack and message properties

User · Answer

Thanks  Trott for your code  I just used the same code and added with a real time working example for the benefit of others    x000D   x000D   lt html gt  x000D   lt body  gt  x000D   x000D   lt p gt The   instanceof   operator returns true if the specified object is an instance of the specified object  lt  p gt  x000D   x000D   x000D   x000D   lt script gt  x000D   var myError   new Error  TypeError  Cannot set property  innerHTML  of null       error type when element is not defined x000D   myError instanceof Error    true x000D    x000D    x000D    x000D    x000D   function test    x000D    x000D   var v1   document getElementById  myid   innerHTML   zunu      to change with this x000D    x000D   try   x000D      var v1   document getElementById  myidd   innerHTML   zunu      exception caught x000D         x000D       x000D   catch  e    x000D      if  e instanceof Error    x000D     console error e name          e message     error will be displayed at browser console x000D        x000D      x000D    finally  x000D    var v1   document getElementById  myid   innerHTML   Text Changed to Zunu      finally innerHTML changed to this  x000D     x000D    x000D     x000D    x000D   lt  script gt  x000D   lt p id  myid  gt This text will change lt  p gt  x000D   lt input type  button  onclick  test     gt  x000D   lt  body gt  x000D   lt  html gt  x000D   x000D   x000D

User · Answer

You can use obj constructor name to check the  class  of an object https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Function name Function names in classes  For Example  var error   new Error  ValidationError    console log error constructor name     The above line will log  Error  which is the class name of the object  This could be used with any classes in javascript  if the class is not using a property that goes by the name  name

User · Answer

You can use Object prototype toString to easily check if an object is an Error  which will work for different frames as well  function isError obj       return Object prototype toString call obj       quot  object Error  quot       x000D   x000D  function isError obj       return Object prototype toString call obj        object Error      console log  Error    isError new Error    console log  RangeError    isError new RangeError    console log  SyntaxError    isError new SyntaxError    console log  Object    isError       console log  Array    isError       x000D   x000D   x000D   This behavior is guaranteed by the ECMAScript Language Specification  Object prototype toString   When the toString method is called  the following steps are taken   If the this value is undefined  return  quot  object Undefined  quot   If the this value is null  return  quot  object Null  quot   Let O be the result of calling ToObject passing the this value as the argument  Let class be the value of the   Class   internal property of O  Return the String value that is the result of concatenating the three Strings  quot  object  quot   class  and  quot   quot     Properties of Error Instances   Error instances inherit properties from the Error prototype object and their   Class   internal property value is  quot Error quot   Error instances have no special properties

User · Answer

var myError   new Error  foo    myError instanceof Error    true var myString    Whatever   myString instanceof Error    false   Only problem with this is   myError instanceof Object    true   An alternative to this would be to use the constructor property   myError constructor     Object    false myError constructor     String    false myError constructor     Boolean    false myError constructor     Symbol    false myError constructor     Function    false myError constructor     Error    true   Although it should be noted that this match is very specific  for example   myError constructor     TypeError    false

User · Answer

I asked the original question -   Trott s answer is surely the best   However with JS being a dynamic language and with there being so many JS runtime environments  the instanceof operator can fail especially in front-end development when crossing boundaries such as iframes  See  https   github com mrdoob three js issues 5886  If you are ok with duck typing  this should be good   let isError   function e    return e  amp  amp  e stack  amp  amp  e message      I personally prefer statically typed languages  but if you are using a dynamic language  it s best to embrace a dynamic language for what it is  rather than force it to behave like a statically typed language   if you wanted to get a little more precise  you could do this      let isError   function e        return e  amp  amp  e stack  amp  amp  e message  amp  amp  typeof e stack      string                amp  amp  typeof e message      string

User · Answer

Or use this for different types of errors  function isError val      return    val  amp  amp  typeof val      object        amp  amp    Object prototype toString call val        object Error              typeof val message      string   amp  amp  typeof val name      string

[javascript] checking for typeof error in JS

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