What is function composition in JavaScript?

Function composition is the technique of combining multiple functions where the output of one function becomes the input of the next. It creates a pipeline of transformations. Manual composition: const result = format(capitalize(trim(userInput))) — executes right to left. Compose helper: const compose = (...fns) => x => fns.reduceRight((acc, fn) => fn(acc), x). const process = compose(format, capitalize, trim). Pipe is the same but left-to-right (usually more readable): const pipe = (...fns) => x => fns.reduce((acc, fn) => fn(acc), x). Example: const getActiveUserEmails = pipe(filterActive, mapToEmail, sortAlphabetically). Composition works best with: pure functions (no side effects), curried functions (single input, single output), and data transformation pipelines. Libraries: Lodash/fp _.compose(), Ramda R.pipe(). Point-free style: define functions in terms of other functions without mentioning data arguments.