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As a dependency of @loopback/rest, package strong-error-handler is an error handler for use in both development (debug) and production environments.

In LoopBack 4, the request handling process starts with the app’s sequence, a simple class with five injected helper methods in the constructor. It is the gatekeeper of all requests to the app. Errors are handled by one of the Sequence actions, reject, which calls strong-error-handler package to send back an HTTP response describing the error.

LoopBack 4 apps require 3.x versions of strong-error-handler.


In production mode (default), reject omits details from error responses to prevent leaking sensitive information.

In debug mode, reject also can return full error stack traces and internal details of any error objects to the client in the HTTP responses. Once the debug flag is set, strong-error-handler would handle such options.

app.bind(RestBindings.ERROR_WRITER_OPTIONS).to({debug: true});

Please check Sequence - Handling errors section for usages and examples of these two modes.

In general, reject is executed in try {} catch(err) {} block. strong-error-handler is the last utility to be used.

The above configuration will log errors to the server console, but not return stack traces in HTTP responses. The behaviors of the error handler are also configurable by setting other options to ERROR_WRITER_OPTIONS. For details on available options, see below.


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" Specify the default response content type to use when the client does not provide any Accepts header.
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.

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:

app.bind(RestBindings.ERROR_WRITER_OPTIONS).to({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"

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 text/plain.

Migration from LoopBack 3 error handler

Unlike LoopBack 3’s phase-based middleware routing system, LB4 uses a sequence handler that sits in front of a routing table and handles errors. To learn the differences and how strong-error-handler is being used in LB4, see LB3 vs LB4 request response cycle.


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',