Custom Javascript Error.toString()

Question

My custom Error class:

function MyError(message) {
  this.message = message || "";
}

MyError.prototype          = new Error();
MyError.prototype.name     = "MyError";
MyError.prototype.toString = function() {
  return "[" + this.name + "] " + this.message;
};

If I run throw new MyError("test") then FF/IE console shows a default message instead of the expected [MyError] test.

How do I get the JS engine to use my toString() method?

@fardjad nope [see here](http://stackoverflow.com/a/871646/1711500) — Bobby B, May 05 '13 at 17:30
@dystroy I guess that's true, but there are benefits to doing so (other standard and non-standard properties which are useful) — Bobby B, May 05 '13 at 17:32
@BobbyB I know, that's why I only made a comment, not an answer. — Denys Séguret, May 05 '13 at 17:41

score 3 · Answer 1 · answered May 05 '13 at 17:35

3

This is how I would inherit Error (tested and working on FF v20):

function MyError(message) {
    this.message = message || "";
}

MyError.prototype = Object.create(Error.prototype); // see the note
MyError.prototype.name = "MyError";
MyError.prototype.toString = function () { 
    return "[" + this.name + "] " + this.message;
}

console.log(new MyError("hello").toString()); // "[MyError] hello"

Note that old browsers may not support Object.create (ES5 syntax), you can use this shim to make it work.

answered May 05 '13 at 17:35

fardjad

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Unfortunately this doesn't exhibit the expected behavior when you do this: `throw new MyError("hello");` it still shows a default message. The way listed above is functionally equivalent to this, but a bit safer. I have a feeling this has to do with the browser's JS engine, and so I cannot change it. – Bobby B May 05 '13 at 17:40
Actually `throw new MyError('hello')` works fine for me in **FF**. – fardjad May 05 '13 at 17:43
1

@BobbyB You're probably right, since `Error` class is bound to native code, the behavior will depend on the JavaScript engine implementation. Anyways [here](http://i.stack.imgur.com/nWlv5.png)'s a screenshot of the code running on my browser. – fardjad May 05 '13 at 19:01

score 3 · Accepted Answer · answered May 05 '13 at 17:58

3

I may be mistaken, but I think the console output in this case is controlled by the JS engine, and so you cannot format it as I've done above.

answered May 05 '13 at 17:58

Bobby B

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Yes, that's probably true. But why would you want to change that anyway? – Bergi May 05 '13 at 18:19
Some other tool which is parsing the output. I'll need to change it. – Bobby B May 05 '13 at 18:23
Can you show us that tool? I'm positive that there is a better solution. – Bergi May 05 '13 at 18:34
You are right, not worth the effort. Besides, I am sure it displays differently in different browsers. I thought it would behave the same as overriding toString() on any other object, but seems the Error class is "special". – Bobby B May 05 '13 at 18:45
Even if it's not very satisfying, this is probably the right answer. I suggest you accept it. – Denys Séguret May 06 '13 at 14:54
@dystroy Must wait two days – Bobby B May 06 '13 at 17:29

Custom Javascript Error.toString()

2 Answers2