What are the error conventions?

In Node.js, it is considered standard practice to handle errors in asynchronous functions by returning them as the first argument to the current function's callback. If there is an error, the first parameter is passed an Error object with all the details. Otherwise, the first parameter is null.

It's simpler than it sounds; let's demonstrate.

var isTrue = function(value, callback) {
  if (value === true) {
    callback(null, "Value was true.");
  }
  else {
    callback(new Error("Value is not true!"));
  }
}

var callback = function (error, retval) {
  if (error) {
    console.log(error);
    return;
  }
  console.log(retval);
}

// Note: when calling the same asynchronous function twice like this, you are in a race condition.
// You have no way of knowing for certain which callback will be called first when calling the functions in this manner.

isTrue(false, callback);
isTrue(true, callback);

{ stack: [Getter/Setter],
  arguments: undefined,
  type: undefined,
  message: 'Value is not true!' }
Value was true.

As you can see from the example, the callback is called with null as its first argument if there is no error. However, if there is an error, you create an Error object, which then becomes the callback's only parameter.

The callback function shows the reason for this: it allows a user to easily know whether or not an error occurred. If null was not the first argument passed on success, the user would need to check the object being returned and determine themselves whether or not the object constituted an error - a much more complex and less user-friendly approach.

So to wrap it all up, when using callbacks, if an error comes up, then pass it as the first argument. Otherwise, pass null first, and then your return arguments. On the receiving end, inside the callback function, check if the first parameter is non-null. If it is non-null, then handle it as an error.