@financial-times/n-error

n-error

a convenient error creator with pure manipulation methods

patterns and tools for error parsing, descriptive error creation, and standardised handling

• quickstart
• install
• usage
  • constructor
  • manipulation
  • parse fetch error
• patterns
  • error sources
  • descriptitive error objects
  • universal error handler
  • reserved fields

quickstart

import nError from '@financial-times/n-error';

throw nError({ status: 404, message: 'sessionId not found', type: 'AUTH_FAILURE' });
throw nError.notFound({ message: 'sessionId not found', type: 'AUTH_FAILURE' });

catch (e) {
  throw e.extend({
    handler: 'REDIRECT_TO_INDEX',
    user: { message: 'Authentification Failed' },
  }).remove('message');
}

install

npm install @financial-times/n-error

usage

constructor

import nError from '@financial-times/n-error';

const e = nError({ status: 404 });
console.log(e instanceof nError); // true
console.log(e.stack);  // built-in .stack for stack tracing like Error

manipulation

use .extend() and .remove() to create new copy of the error that maintains the stack trace if you want the manipulation to be pure to avoid unclear behaviour, e.g. async failure logger attached on a lower level

throw e.extend({ handler: 'SOME_ACTION' }).remove('message');

parse fetch error

parse fetch error into NError object with Category for further error handling

Error or other objects would be thrown as it is

parseFetchError() returns a Promise, recommended to use await

/* api-service */
import { parseFetchError } from '@financial-times/n-error';

try{
  await fetch(url, options);
} catch (e) {
  throw await parseFetchError(e); // use `await`
}

/* controller/middleware */
import { CATEGORIES } from '@financial-times/n-error';

try {
  await APIService.call();
} catch (e) {
  // handle the error differently in case of network errors
  if(e.catogary === CATEGORIES.FETCH_NETWORK_ERROR){
    return next(e.extend({
      user: { message: `network error: ${e.code}` }
    }));
  }
  // handle fetch response error in grace 
  // parsed message according to content-type
  // stop `e.json() is not a function` error
  if(e.category === CATEGORIES.FETCH_RESPONSE_ERROR){
    const { errorCodes } = e.message;
    return next(e.extend({
      user: { message: errorCodesToUserMessage(errorCodes) }
    }));
  }
  return next(e);
}

patterns

error sources

• fetch response error
• fetch network error
• Error object
• other custom Error object

descriptitive error objects

const e = NError({
  status: 404,
  message: 'some type of message', // message from server to be logged
  handler: 'REDIRECT_TO_INDEX', // describe error handling behaviour
  user: { message: 'Authentification Failed' } // override the default message from the server for UI
});

universal error handler

function(e, req, res, next) {
  if(e.handler && e.handler === 'REDIRECT_TO_ORIGINAL'){
    return res.redirect(303, req.originalUrl);
  }
  return res.render('errors', message: e.user.message || e.message );
}

reserved fields

operation, action, category, result (if you use n-auto-logger) are reserved fields that can be overriden, be cautious if you really want to override the default. handler is recommended to specify the error handler behaviour, which would be filtered by n-auto-logger.