# mapError(_:) Converts any failure from the upstream publisher into a new error. ``` func mapError(_ transform: @escaping (Self.Failure) -> E) -> Publishers.MapError where E : Error ``` ## Parameters `transform` A closure that takes the upstream failure as a parameter and returns a new error for the publisher to terminate with. ## Return Value A publisher that replaces any upstream failure with a new error produced by the `transform` closure. ## Discussion Use the [`mapError(_:)`](/documentation/Combine/Publisher/mapError(_:)) operator when you need to replace one error type with another, or where a downstream operator needs the error types of its inputs to match. The following example uses a [`tryMap(_:)`](/documentation/Combine/Publisher/tryMap(_:)) operator to divide `1` by each element produced by a sequence publisher. When the publisher produces a `0`, the [`tryMap(_:)`](/documentation/Combine/Publisher/tryMap(_:)) fails with a `DivisionByZeroError`. The [`mapError(_:)`](/documentation/Combine/Publisher/mapError(_:)) operator converts this into a `MyGenericError`. ``` struct DivisionByZeroError: Error {} struct MyGenericError: Error { var wrappedError: Error } func myDivide(_ dividend: Double, _ divisor: Double) throws -> Double { guard divisor != 0 else { throw DivisionByZeroError() } return dividend / divisor } let divisors: [Double] = [5, 4, 3, 2, 1, 0] divisors.publisher .tryMap { try myDivide(1, $0) } .mapError { MyGenericError(wrappedError: $0) } .sink( receiveCompletion: { print ("completion: \($0)") , receiveValue: { print ("value: \($0)", terminator: " ") } ) // Prints: "0.2 0.25 0.3333333333333333 0.5 1.0 completion: failure(MyGenericError(wrappedError: DivisionByZeroError()))" ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)