Wrong constructor of Error subclass from 2.1.1 #12660

Activity

1. Mo (marty-wang)

   changed the title [-]Wrong subclass constructor from 2.1.1[/-] [+]Wrong constructor of Error subclass from 2.1.1[/+] on Dec 5, 2016
2. HerringtonDarkholme commented on Dec 5, 2016

   

   Herrington Darkholme (HerringtonDarkholme)

   on Dec 5, 2016

   Contributor

   More actions

   #12123
3. Ryan Cavanaugh (RyanCavanaugh)

   added

   DuplicateAn existing issue was already createdAn existing issue was already created

   on Dec 5, 2016
4. RyanCavanaugh commented on Dec 5, 2016

   

   Ryan Cavanaugh (RyanCavanaugh)

   on Dec 5, 2016

   Last edited by RyanCavanaugh

   Member

   More actions

   A summary of what's going on based on a conversation with Ron Buckton (@rbuckton):

   1. This didn't "work" in any meaningful sense in 2.0 -- the base constructor didn't properly run due to how weird internal objects like Error get constructed. See footnote*
   2. This isn't possible to make work in regular ES5 because you need to do some prototype/proto hackery that can't be emulated without ES6 APIs
   3. Targeting ES6 this is fine because you'll just have native class emit anyway

   Possible fixes: Either don't use instanceof with custom classes, don't try to extend weird built-ins, or target ES6.

   * Code:

   class Foo extends Error {
       constructor() {
           super("oh no");
       }
   }
   let x = new Foo();
   // Expected: x.message === "oh no"
   // Actual: x.message === ""
   alert(x.message);

   See also http://stackoverflow.com/a/17891099/1704166

   👍React with 👍1Reacted by Aluan Haddad
5. Ryan Cavanaugh (RyanCavanaugh)

   closed this as completedon Dec 5, 2016
6. mhegazy commented on Dec 5, 2016

   

   Mohamed Hegazy (mhegazy)

   on Dec 5, 2016

   Contributor

   More actions

   My recommendation to get this working is to set the prototype explicitly on your instances. for instance:

   class MyError extends Error {
       sayHello() {
           return "hello " + this.message;
       }
       constructor(m) {
           super(m);
   
           // Set the prototype explictilly
           Object.setPrototypeOf(this, MyError.prototype);
       }
   }
   
   
   var e = new MyError("my error");
   
   console.log("e instanceof Error => " + (e instanceof Error)); // true
   console.log("e instanceof MyError => " + (e instanceof MyError)); // true
   console.log("e.message  => " + (e.message)); // "my error"
   console.log("e.sayHello()  => " + (e.sayHello()));  // "hello my error"

   The only change here from extending non-built-in classes is Object.setPrototypeOf(this, MyError.prototype);

   alternatively, you can just set this.__proto__ instead of using Object.setPrototypeOf`

   👍React with 👍1Reacted by Nikolay Matrosov
7. Yaojian commented on Jun 6, 2018

   

   Yaojian (Yaojian)

   on Jun 6, 2018

   Last edited by Yaojian

   More actions

   With support for stack trace
8. microsoft

   locked as resolved and limited conversation to collaborators on Oct 21, 2025