Strong-error-handler is an error handler for use in both development and production environments.
Page Contents


OpenSSF Best Practices OpenSSF Scorecard Continuous Integration CodeQL

This package is an error handler for use in both development (debug) and production environments.

In production mode, strong-error-handler omits details from error responses to prevent leaking sensitive information:

  • For 5xx errors, the output contains only the status code and the status name from the HTTP specification.
  • For 4xx errors, the output contains the full error message (error.message) and the contents of the details property (error.details) that ValidationError typically uses to provide machine-readable details about validation problems. It also includes error.code to allow a machine-readable error code to be passed through which could be used, for example, for translation.

In debug mode, strong-error-handler returns full error stack traces and internal details of any error objects to the client in the HTTP responses.

Supported versions

This module adopts the Module Long Term Support (LTS) policy, with the following End Of Life (EOL) dates:

Version Status Published EOL
4.x Current Oct 2020 Apr 2023 (minimum)
3.x Active LTS Jun 2018 Dec 2022
2.x End-of-life Mar 2017 Oct 2020

Learn more about our LTS plan in the LoopBack documentation.


$ npm install --save strong-error-handler


In an Express-based application:

var express = require('express');
var errorHandler = require('strong-error-handler');

var app = express();
// setup your routes
// `options` are set to default values. For more info, see `options` below.
// app.use(errorHandler({ /* options, see below */ }));
  debug: app.get('env') === 'development',
  log: true,


The module also exports writeErrorToResponse, a non-middleware flavor of the error handler:

const http = require('http');
const writeErrorToResponse = require('strong-error-handler')
const errHandlingOptions = {debug: process.env.NODE_ENV === 'development'}

  .createServer((req, res) => {
    if (errShouldBeThrown) {
        new Error('something went wrong'),

In LoopBack applications, add the following entry to server/middleware.json:

  "final:after": {
    "strong-error-handler": {
      "params": {
         "debug": false,
         "log": true

In general, strong-error-handler must be the last middleware function registered.

The above configuration will log errors to the server console, but not return stack traces in HTTP responses. For details on configuration options, see below.

Response format and content type

The strong-error-handler package supports JSON, HTML and XML responses:

  • When the object is a standard Error object, it returns the string provided by the stack property in HTML/text responses.
  • When the object is a non-Error object, it returns the result of util.inspect in HTML/text responses.
  • For JSON responses, the result is an object with all enumerable properties from the object in the response.

The content type of the response depends on the request’s Accepts header.

  • For Accepts header json or application/json, the response content type is JSON.
  • For Accepts header html or text/html, the response content type is HTML.
  • For Accepts header xml or text/xml, the response content type is XML.

There are plans to support other formats such as Plain-text.


Option Type Default Description
debug Boolean    false If true, HTTP responses include all error properties, including sensitive data such as file paths, URLs and stack traces. See Example output below.
log Boolean true If true, all errors are printed via console.error, including an array of fields (custom error properties) that are safe to include in response messages (both 4xx and 5xx).
If false, sends only the error back in the response.
safeFields [String] [] Specifies property names on errors that are allowed to be passed through in 4xx and 5xx responses. See Safe error fields below.
defaultType String "json" Specifies the default response content type to use when the client does not provide any Accepts header.
rootProperty String or false "error" Specifies the root property name for json or xml. If the value is set to false, no wrapper will be added to the json object. The false value is ignored by XML as a root element is always required.
negotiateContentType Boolean true Negotiate the response content type via Accepts request header. When disabled, strong-error-handler will always use the default content type when producing responses. Disabling content type negotiation is useful if you want to see JSON-formatted error responses in browsers, because browsers usually prefer HTML and XML over other content types.

Customizing log format


To use a different log format, add your own custom error-handling middleware then disable errorHandler.log. For example, in an Express application:

app.use(errorHandler({log: false}));

In general, add strong-error-handler as the last middleware function, just before calling app.listen().


For LoopBack applications, put custom error-logging middleware in a separate file; for example, server/middleware/error-logger.js:

module.exports = function(options) {
  return function logError(err, req, res, next) {
    console.log('unhandled error' ,err);

Then in server/middleware.json, specify your custom error logging function as follows:

  // ...
  "final:after": {
    "./middleware/error-logger": {},
    "strong-error-handler": {
      "params": {
        "log": false

The default middleware.development.json file explicitly enables logging in strong-error-handler params, so you will need to change that file too.

Safe error fields

By default, strong-error-handler will only pass through the name, message and details properties of an error. Additional error properties may be allowed through on 4xx and 5xx status code errors using the safeFields option to pass in an array of safe field names:

  "final:after": {
    "strong-error-handler": {
      "params": {
        "safeFields": ["errorCode"]

Using the above configuration, an error containing an errorCode property will produce the following response:

  "error": {
    "statusCode": 500,
    "message": "Internal Server Error",
    "errorCode": "INTERNAL_SERVER_ERROR"

Migration from old LoopBack error handler

NOTE: This is only required for applications scaffolded with old versions of the slc loopback tool.

To migrate a LoopBack 2.x application to use strong-error-handler:

  1. In package.json dependencies, remove "errorhandler": "^x.x.x”,
  2. Install the new error handler by entering the command:
    npm install --save strong-error-handler
  3. In server/config.json, remove:
     "remoting": {
       "errorHandler": {
         "disableStackTrace": false

    and replace it with:

     "remoting": {
       "rest": {
         "handleErrors": false
  4. In server/middleware.json, remove:
     "final:after": {
       "loopback#errorHandler": {}

    and replace it with:

     "final:after": {
       "strong-error-handler": {}
  5. Delete server/middleware.production.json.
  6. Create server/middleware.development.json containing:
     "final:after": {
       "strong-error-handler": {
         "params": {
           "debug": true,
           "log": true


For more information, see Migrating apps to LoopBack 3.0.


5xx error generated when debug: false :

{ error: { statusCode: 500, message: 'Internal Server Error' } }

The same error generated when debug: true :

{ error:
  { statusCode: 500,
  name: 'Error',
  message: 'a test error message',
  stack: 'Error: a test error message
  at Context.<anonymous> (User/strong-error-handler/test/handler.test.js:220:21)
  at callFnAsync (User/strong-error-handler/node_modules/mocha/lib/runnable.js:349:8)
  at (User/strong-error-handler/node_modules/mocha/lib/runnable.js:301:7)
  at Runner.runTest (User/strong-error-handler/node_modules/mocha/lib/runner.js:422:10)
  at User/strong-error-handler/node_modules/mocha/lib/runner.js:528:12
  at next (User/strong-error-handler/node_modules/mocha/lib/runner.js:342:14)
  at User/strong-error-handler/node_modules/mocha/lib/runner.js:352:7
  at next (User/strong-error-handler/node_modules/mocha/lib/runner.js:284:14)
  at Immediate._onImmediate (User/strong-error-handler/node_modules/mocha/lib/runner.js:320:5)
  at tryOnImmediate (timers.js:543:15)
  at processImmediate [as _immediateCallback] (timers.js:523:5)' }}

4xx error generated when debug: false :

{ error:
  { statusCode: 422,
  name: 'Unprocessable Entity',
  message: 'Missing required fields',