@loopback/docs
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Documentation for LoopBack 4
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lang: en
title: Error Handling
keywords: LoopBack 4.0
sidebar: lb4_sidebar
permalink: /doc/en/lb4/Error-handling.html
## Known Error Codes
In order to allow clients to reliably detect individual error causes, LoopBack
sets the error `code` property to a machine-readable string.
| Error code | Description |
| :------------------------- | :-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| ENTITY_NOT_FOUND | The entity (model) was not found. This error is returned for example by [`EntityCrudRepository.prototype.findById`](https://loopback.io/doc/en/lb4/apidocs.repository.entitycrudrepository.findbyid.html) |
| VALIDATION_FAILED | The data provided by the client is not a valid entity. |
| INVALID_PARAMETER_VALUE | The value provided for a parameter of a REST endpoint is not valid. For example, a string value was provided for a numeric parameter. |
| MISSING_REQUIRED_PARAMETER | No value was provided for a required parameter. |
Besides LoopBack-specific error codes, your application can encounter low-level
error codes from Node.js and the underlying operating system. For example, when
a connector is not able to reach the database, an error with code `ECONNREFUSED`
is returned.
See the following resources for a list of low-level error codes:
- [Common System Errors](https://nodejs.org/api/errors.html#errors_common_system_errors)
- [Node.js Error Codes](https://nodejs.org/api/errors.html#errors_node_js_error_codes)