exception - 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?

Answer 1

Update your code to assign your prototype to the Error.prototype and the instanceof and your asserts work.

function NotImplementedError(message = "") {
    this.name = "NotImplementedError";
    this.message = message;
}
NotImplementedError.prototype = Error.prototype;

However, I would just throw your own object and just check the name property.

throw {name : "NotImplementedError", message : "too lazy to implement"}; 

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:

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);
}