flatmaps

FlatMap is a higher-order function used in functional programming and many modern languages. It maps each element of a collection to another collection and then flattens the resulting nested structure by one level, yielding a single combined collection. This makes it convenient for operations where each input item can produce multiple output items.

Formally, if you have a collection A and a function f: A -> Collection[B], flatMap returns a Collection[B] by concatenating f(a) for every a in A. It is conceptually equivalent to first applying map to obtain a collection of collections and then flattening, but many implementations perform the two steps in one pass for efficiency.

FlatMap is used when the transformation yields multiple results per input, such as splitting strings into words, expanding entries by a nested list, or chaining optional computations that may fail. In monadic contexts it also serves to propagate absence; for example, mapping an element to an optional value and then flattening yields a single optional result. Many languages provide a built-in flatMap or an equivalent, including JavaScript (Array.prototype.flatMap), Scala collections, Java Streams, Kotlin collections, and Haskell (concatMap).

Performance characteristics depend on the implementation and the data structure; flatMap typically performs a single traversal and avoids creating an intermediate nested collection. However, it may allocate a new flat result proportional to the total number of inner elements, and it can affect memory usage for large inputs.

Related concepts include map, flatten, and concatMap; flatMap is a practical shorthand for the common pattern of mapping to a collection and then flattening.