Map
An operation that applies a function to values in a collection or computational context, producing corresponding results while preserving the collection or context’s structure.
Spring in Action, Sixth Edition
map is a Project Reactor operator that transforms each value emitted by a reactive publisher into another value while preserving the publisher type.
- How it is used
- Transforms emitted values inside Reactor pipelines for typed results in security lookup and reactive order aggregation.
- How deep it goes
- Defines synchronous per-item mapping, demonstrates typed transformations, and contrasts flatMap() when a transformation returns a publisher.
- Examples
- Tests basketball-player name conversion with StepVerifier and reassembles TacoOrder aggregates from repository-emitted tacos.
- What you can do after
- Apply it via Reactor's Flux and Mono to transform values into types required by downstream services.
Learn Haskell by Example
map is a standard Haskell function that applies a function independently to every element of a list and returns the resulting list.
- How it is used
- Applies a per-element transformation across strings and lists in cipher, input-processing, and formatting tasks.
- How deep it goes
- Defines ordered element-wise list transformation and demonstrates composition, while deferring any implementation of `map` itself.
- Examples
- Uses Caesar and ROT13 ciphers, uppercase console input, and nested `show` and `length` transformations for line numbering.
- What you can do after
- Apply it via Haskell's `map` to transform every element of a list while keeping element logic separate.
Clojure: The Essential Reference
map is a Clojure function that applies a function to corresponding elements of one or more collections and returns a lazy sequence of results.
- How it is used
- Transforms corresponding elements of sequentially iterable collections in functional data pipelines.
- How deep it goes
- Details three arity patterns, lazy results, shortest-input termination, and the caveat against relying on once-only side effects.
- Examples
- Demonstrates negating a range, combining strings from uneven collections, and assigning indices to keywords.
- What you can do after
- Apply it via Clojure’s map to transform one or several collections, or use its collection-free arity as a transducer.
D3.js in Action, Third Edition
The JavaScript map() method creates a new array by applying a callback to every element of an input array.
- How it is used
- Transforms loaded datasets into arrays of values needed for data wrangling and visualization.
- How deep it goes
- Defines callback-based array transformation with working results, but stops short of broader array and object manipulation techniques.
- Examples
- Uses employee names and dataset years as mapped arrays for labels, scale domains, and chart positioning.
- What you can do after
- Apply it via JavaScript to reshape array data for D3 scales and other visualization operations.
Secrets of the JavaScript Ninja, Second Edition
An Array method that creates a new array by applying a callback to each element.
- How it is used
- Uses map to transform array items into a new array as one common array operation.
- How deep it goes
- Defines callback-based transformation and demonstrates extracting object properties, without detailing further mechanics.
- Examples
- Extracts weapon names from an array of ninja objects and compares map with forEach and push.
- What you can do after
- Apply Array.map to transform an array of objects into an array of their weapon properties.
Read how Secrets of the JavaScript Ninja, Second Edition explains map →
F# in Action: A Revised Edition of Get Programming with F#
A map operation applies a function to a value inside a computational effect and rewraps the result in that effect.
- How it is used
- Transforms values inside List, Option, Result, and Task while preserving their surrounding effect.
- How deep it goes
- Explains map through its generic signature, list use, and effect-specific handling, while deferring implementations to open-source code.
- Examples
- Uses integer-to-string list conversion, a timesTwo formatter, customers’ orders, and chained division results.
- What you can do after
- Apply it via List.map, Option.map, Result.map, or Task.map to reuse ordinary transformations across effects.
Read how F# in Action: A Revised Edition of Get Programming with F# explains map →
Apache Kafka in Action
map is a stream-processing function that transforms each input record into a new record or value.
- How it is used
- Transforms records in real-time stream-processing topologies, especially when both message key and value must change.
- How deep it goes
- Defines input/output signatures, contrasts mapValues(), and explains repartitioning-related performance loss without detailing implementation internals.
- Examples
- Demonstrates dollar-to-cent conversion with mapValues() and key creation from an unkeyed Apple price record.
- What you can do after
- Apply it via Kafka Streams to transform records, create keys, and write the resulting stream to a topic.
Fabulous Adventures in Data Structures and Algorithms
A monadic map operation applies a function to values inside a monadic computational context while preserving that context's structure.
- How it is used
- Transforms ordinary functions into operations over monadic workflows while preserving capabilities such as laziness, nullability, asynchrony, and sequencing.
- How deep it goes
- Defines Map, implements it for Lazy<T>, relates it to Bind and Unit, and stops short of detailed implementations for every context.
- Examples
- Uses Lazy<int>, DivTwo, Nullable<T>, Task<T>, IEnumerable<T>, and the nested Lazy<Lazy<double>> problem.
- What you can do after
- Apply Map to lift ordinary functions into lazy, nullable, asynchronous, or sequence-based workflows and recognize when Bind is required.
Read how Fabulous Adventures in Data Structures and Algorithms explains map →
Related concepts
- arrayproduces · An array is an indexed, ordered collection of elements arranged in one or more dimensions, with elements accessed by position.
- JavaScriptused by · JavaScript is a high-level, dynamically typed, multi-paradigm programming language standardized as ECMAScript and used for web and general-purpose programming.
- functional, stateless philosophyuses · Functional programming is a programming paradigm that expresses computation through the evaluation and composition of functions, typically favoring immutable data and minimizing mutable state and side effects.
- pipe methodused by · The RxJS pipe method composes operators to transform or otherwise process values emitted by an Observable.
- map-indexedalternative to · map-indexed is a Clojure function that applies a function to each collection element together with its index and returns the results as a lazy sequence or transducer output.
- transducerproduces · A transducer is a composable transformation that processes inputs independently of the context in which the transformation is consumed.
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