Handling specific errors in JavaScript (think exceptions)

Question

How would you implement different types of errors, so you'd be able to catch specific ones and let others bubble up..?

One way to achieve this is to modify the prototype of the Error object:

Error.prototype.sender = "";


function throwSpecificError()
{
    var e = new Error();

    e.sender = "specific";

    throw e;
}

Catch specific error:

try
{
    throwSpecificError();
}

catch (e)
{
    if (e.sender !== "specific") throw e;

    // handle specific error
}

Have you guys got any alternatives?

Answer 1

You can use 'conditional catch' blocks. e.g.:

try {
  ...
  throwSpecificError();
  ...
}
catch (e if e.sender === "specific") {
  specificHandler(e);
}
catch (e if e.sender === "unspecific") {
  unspecificHandler(e);
}
catch (e) {
  // don't know what to do
  throw e;
}

This gives something more akin to typed exception handling used in Java, at least syntactically.

Answer 2

To create custom exceptions, you can inherit from the Error object:

function SpecificError () {

}

SpecificError.prototype = new Error();

// ...
try {
  throw new SpecificError;
} catch (e) {
  if (e instanceof SpecificError) {
   // specific error
  }
}

A minimalistic approach, without inheriting from Error, could be throwing a simple object having a name and a message properties:

function throwSpecificError() {
  throw {
    name: 'SpecificError',
    message: 'SpecificError occurred!'
  };
}


// ...
try {
  throwSpecificError();
} catch (e) {
  if (e.name == 'SpecificError') {
   // specific error
  }
}