Inheriting from the Error object - where is the message property?

Question

I noticed a strange behavior while defining custom error objects in Javascript:

function MyError(msg) {
    Error.call(this, msg);
    this.name = \"MyError\         


        

Answer 1

function MyError(msg) {
    var err = Error.call(this, msg);
    err.name = "MyError";
    return err;
}

Error doesn't manipulate this, it creates a new error object which is returned. That's why Error("foo") works aswell without the new keyword.

Note this is implementation specific, v8 (chrome & node.js) behave like this.

Also MyError.prototype.__proto__ = Error.prototype; is a bad practice. Use

MyError.prototype = Object.create(Error.prototype, { 
  constructor: { value: MyError } 
});