error-handler.md

@errorHandler Directive

General

This directive allows to do one or both of the following actions:

1. Catch error thrown by the field/object type resolver and return some value instead. The directives allows to catch only certain errors using catchError.condition argument.

2. Throw custom error instead of returning result of field/object type resolver. The directive allows to throw error only in certain cases using throwError.condition argument.

Definition

input CatchErrorInput {
  condition: String
  returnValue: JSON
}

input ThrowErrorInput {
  condition: String
  errorToThrow: String
}

directive @errorHandler(catchError: CatchErrorInput, throwError: ThrowErrorInput) on OBJECT | FIELD_DEFINITION

Arguments

• catchError: (object, optional, default: undefined) This argument has 2 properties and allows to configure what errors to catch and what value to return

  • condition: (string, optional, default: true) - this expression is evaluated using Argument injection mechanism. Additionally to the regular source, args, context and info variables, error variable is available. Its value is the error thrown by resolver.
  • returnValue (object, optional, default: null) - object or string that may be evaluated using Argument injection mechanism. This value will be resolved if the error will be caught.

• throwError (object, optional, default: undefined) - This argument has 2 properties and allows to configure what error will be thrown instead of resolver value.

  • condition: (string, optional, default: true) - this expression is evaluated using Argument injection mechanism. Additionally to the regular source, args, context and info variables, result variable is available. Its value is the resolver value or catchError section value. See above.
  • errorToThrow: (string, optional, default: empty string). String that may be evaluated using Argument injection mechanism. If the evaluation result is typeof Error it will be thrown. If the expression cannot be evaluated or is evaluated to string new Error(result) will be thrown.

Execution order

Examples

No arguments. The directive has no effect.

type Query {
  foo: String @errorHandler
}

Catches all errors and returns null instead

type Query {
  foo: Foo @errorHandler(catchError: {})
}

Catches only errors that has property status with value 404 and returns Not found instead

type Query {
  foo: String @errorHandler(catchError: { condition: "{ error.status === 404 }", returnValue: "Not found" })
}

Throws if the field resolver returns empty string

type Query {
  foo: String
    @errorHandler(
      throwError: { condition: "{ result.length === 0 }", errorToThrow: "{ new Error('404: Foo not found') }" }
    )
}

Catches authorization errors and throws 'not found' errors instead

type Query {
  foo: String
    @errorHandler(
      catchError: { condition: "{ error.message.includes('Unauthorized) }", returnValue: "Not found" }
      throwError: { condition: "{ result === 'Not found }", errorToThrow: "{ new Error('404: Foo not found') }" }
    )
}