What is currying in JavaScript? When is it useful?

Currying transforms a function that takes multiple arguments into a sequence of functions, each taking one argument (or a fixed smaller batch).

// Uncurried
const add = (a, b, c) => a + b + c
add(1, 2, 3) // 6

// Curried
const curriedAdd = (a) => (b) => (c) => a + b + c
curriedAdd(1)(2)(3) // 6

// Partial application via currying
const addOne = curriedAdd(1)
const addOneAndTwo = addOne(2)
addOneAndTwo(3) // 6

When it's useful

• Partial application — fix some arguments and reuse the rest later.
• Composable pipelines — small unary functions chain cleanly.
• Configuration — e.g. const logError = logWithLevel('error').

Currying relies on closures to remember previously passed arguments. Libraries like Lodash provide _.curry() for n-ary functions automatically.

Named after mathematician Haskell Curry; distinct from partial application (which does not require unary steps) though they overlap in practice.