how do you throw a custom error class in Javascript?

Question

in JS, you can throw a "new Error(message)", but if you want to detect the type of the exception and do something different with the message, it is not so easy.

This post: http://www.nczonline.net/blog/2009/03/10/the-art-of-throwing-javascript-errors-part-2/

Is saying you can do it something like this:

function MyError(message){
  this.message=messsage;
  this.name="MyError";
  this.poo="poo";
}
MyError.prototype = new Error();

try{
  alert("hello hal");
  throw new MyError("wibble");
} catch (er) {
  alert (er.poo);   // undefined.
  alert (er instanceof MyError);  // false
      alert (er.name);  // ReferenceError.
}

But it does not work (get "undefined" and false)

Is this even possible?

BentOnCoding · Accepted Answer · 2013-01-31 15:45:07Z

5

Douglas Crockford recommends throwing errors like this:

throw{

    name: "SomeErrorName", 
    message: "This is the error message", 
    poo: "this is poo?"

}

And then you can easily say :

try {
    throw{

        name: "SomeErrorName", 
        message: "This is the error message", 
        poo: "this is poo?"

    }

}
catch(e){
    //prints "this is poo?"
    console.log(e.poo)
}

If you really want to use the MyError Function approach it should probably look like this :

function MyError(message){

    var message = message;
    var name = "MyError";
    var poo = "poo";

    return{

        message: message, 
        name: name, 
        poo: poo
    }

};

answered Jan 31, 2013 at 15:45

BentOnCoding

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2 Comments

wingnut Over a year ago

Brilliant. the throw of a direct object seems to work. The only issue I read is that browsers dont handle it so well. Whats the difference between my MyError and your MyError? Im guessing its the scoping. yours has no this, so is kind of private.

BentOnCoding Over a year ago

In your MyError function all the variables are scoped within that object.

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