# map(_:) Transforms all elements from the upstream publisher with a provided closure. ``` func map(_ transform: @escaping (Self.Output) -> T) -> Publishers.Map ``` ## Parameters `transform` A closure that takes one element as its parameter and returns a new element. ## Return Value A publisher that uses the provided closure to map elements from the upstream publisher to new elements that it then publishes. ## Discussion Combine’s [`map(_:)`](/documentation/Combine/Publisher/map(_:)-99evh) operator performs a function similar to that of in the Swift standard library: it uses a closure to transform each element it receives from the upstream publisher. You use [`map(_:)`](/documentation/Combine/Publisher/map(_:)-99evh) to transform from one kind of element to another. The following example uses an array of numbers as the source for a collection based publisher. A [`map(_:)`](/documentation/Combine/Publisher/map(_:)-99evh) operator consumes each integer from the publisher and uses a dictionary to transform it from its Arabic numeral to a Roman equivalent, as a . If the [`map(_:)`](/documentation/Combine/Publisher/map(_:)-99evh)’s closure fails to look up a Roman numeral, it returns the string `(unknown)`. ``` let numbers = [5, 4, 3, 2, 1, 0] let romanNumeralDict: [Int : String] = [1:"I", 2:"II", 3:"III", 4:"IV", 5:"V"] cancellable = numbers.publisher .map { romanNumeralDict[$0] ?? "(unknown)" } .sink { print("\($0)", terminator: " ") } // Prints: "V IV III II I (unknown)" ``` If your closure can throw an error, use Combine’s [`tryMap(_:)`](/documentation/Combine/Publisher/tryMap(_:)) operator instead. --- 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)