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

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

function MyError(msg) {
    Error.call(this, msg);
    this.name = "MyError";
}
MyError.prototype.__proto__ = Error.prototype;

var error = new Error("message");
error.message; // "message"

var myError = new MyError("message");
myError instanceof Error; // true
myError.message; // "" !

Why does new Error("message") set the message property, while Error.call(this, msg); does not? Sure, I can just define this.message = msg in the MyError constructor, but I don't quite understand why it is not already set in the first place.

Solution 1:

A. Like, Raynos said, 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 set the result of the apply from the constructor on this, as well as setting the prototype in the usual complicated javascripty way:

function MyError() {
    var tmp = Error.apply(this, arguments)
    tmp.name = this.name = 'MyError'

    this.message = tmp.message
    // instead of this.stack = ..., a getter for more optimizy goodness
    Object.defineProperty(this, 'stack', {
        get: function () {
            return tmp.stack
        }
    })

    return this
}
var IntermediateInheritor = function () {}
IntermediateInheritor.prototype = Error.prototype
MyError.prototype = new IntermediateInheritor()

var myError = new MyError("message")
console.log("The message is: '"+myError.message+"'") // The message is: 'message'
console.log(myError instanceof Error)                    // true
console.log(myError instanceof MyError)                  // true
console.log(myError.toString())                          // MyError: message
console.log(myError.stack)                               // MyError: message \n 
                                                          // <stack trace ...>

The only problems with this way of doing it at this point (i've iteratted it a bit) are that

• properties other than stack and message aren't included in MyError, and
• the stacktrace has an additional line that isn't really necessary.

The first problem could be fixed by iterating through all the non-enumerable properties of error using the trick in this answer: Is it possible to get the non-enumerable inherited property names of an object?, but this isn't supported by ie<9. The second problem could be solved by tearing out that line in the stack trace, but I'm not sure how to safely do that (maybe just removing the second line of e.stack.toString() ??).

Update

I created an inheritance library that does this ^ https://github.com/fresheneesz/proto

Solution 2:

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 } 
});

Solution 3:

In Node.js you can create a custom error like this:

var util = require('util');

function MyError(message) {
  this.message = message;
  Error.captureStackTrace(this, MyError);
}

util.inherits(MyError, Error);

MyError.prototype.name = 'MyError';

See captureStackTrace in node docs