Another useful feature built into jsonapi-serializer is providing JSON API-formatted error responses. We can update our /middleware/rest.js middleware to provide 404 error responses in our REST API that are formatted consistently to align with the JSON API specification.

...
var JSONAPIError = require('jsonapi-serializer').Error;

module.exports = function(resourceId, store, serialize, deserialize) {
  var serializer = new JSONAPISerializer(resourceId, serialize);
  var deserializer = new JSONAPIDeserializer(deserialize);

  var error = function(status, title, description) {
    return new JSONAPIError({
      status: status,
      title: title,
      detail: description
    });
  };

  var fileNotFound = function() {
    return error(404, 'Not found', 'Resource does not exist.');
  };
  ...
  return resource({
    id : resourceId,

    load : function(req, id, callback) {
      find(id, function(item) {
        if (!item) {
          callback(fileNotFound());
        } else {
          callback(null, item);
        }
      });
    },
    ...

Now, when our API can't find a resource, it will return the JSON API error response:

{
  "errors": [{
    "status": 404,
    "title": "Not found",
    "detail": "Resource does not exist."
  }]
}

There are many ways to customize these error responses, including providing error status codes, source pointers to identify request body format issues, and much more.