Understanding JavaScript currying

Functional programming often makes use of currying, which is a process that transforms a function that takes multiple arguments into a series of functions that each take a single argument. This makes code more flexible and reusable.

Currying a function

Creating a curry() function can be tricky in some cases. For the sake of simplicity, let's start with a function whose number of arguments is fixed. In that case, we will use Function.prototype.length to determine the number of arguments the function accepts.

Knowing the number of arguments needed to call the function, we will then use recursion to create a curried function that will call the original function once the number of arguments provided is sufficient. If not, it will return a new function that will accept the remaining arguments.

const curry = (fn) => {
  const curried = (...args) => (
    args.length >= fn.length
      ? fn(...args)
      : (...rest) => curried(...args, ...rest)
  );
  return curried;
};

const add = (x, y) => x + y;
const curriedAdd = curry(add);
curriedAdd(1)(2); // 3

Working with variadic functions

Variadic functions are functions that accept a variable number of arguments. For example, Math.min() is a variadic function that returns the smallest of zero or more numbers.

Our previous implementation will simply not work. Indeed, Function.prototype.length will return 0 for variadic functions. In order to circumvent this issue, we should be able to define the arity of the function we want to curry. This way, the curried function will know when to stop expecting further arguments and call the original function.

On top of this change, we can also expect some arguments to be provided upfront. This is useful when we want to partially apply a function. Instead of using the previous approach that relied on closures, we can use Function.prototype.bind() to return a curried function that expects the rest of the arguments.

Putting everything together, we arrive at a more robust and shorter, albeit a little less readable, implementation.

const curry = (fn, arity = fn.length, ...args) =>
  arity <= args.length ? fn(...args) : curry.bind(null, fn, arity, ...args);

curry(Math.pow)(2)(10); // 1024
curry(Math.min, 3)(10)(50)(2); // 2

Uncurrying a function

What about the opposite? How can we uncurry a function? We will still need to know the arity of the function, but this time, we will use Array.prototype.reduce() to call each subsequent curry level of the function. If the number of arguments isn't sufficient, we will throw an error. Otherwise, we will call the function with the proper amount of arguments, using Array.prototype.slice().

const uncurry = (fn, arity = 1) => (...args) => {
  const next = acc => args => args.reduce((x, y) => x(y), acc);
  if (arity> args.length) throw new RangeError('Arguments too few!');
  return next(fn)(args.slice(0, arity));
};

const add = x => y => z => x + y + z;
const uncurriedAdd = uncurry(add, 3);
uncurriedAdd(1, 2, 3); // 6