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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
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: