Fun with Typescript and Higher Order Functions

Functional programming is fun and building higher order functions in regular JavaScript is straightforward. Let's build one such function: once - a function that accepts another function as parameter and returns a function that caches the result (so it is only invoked once):

function once(fn) {
  let isRun = false;
  let result;
  return function () {
    if (!isRun) {
      isRun = true;
      result = fn.apply(null, arguments);
    }

    return result;
  };
}

Looks decent. If you are worried about caching result per parameters, that is a topic for another day. And a great exercise for the reader :)

It becomes slightly prettier when using ES6 and spread operator:

function once(fn) {
  let isRun = false;
  let result;
  return function (...args) {
    if (!isRun) {
      isRun = true;
      result = fn(...args);
    }

    return result;
  };
}

Much better. Let's use it!

function sum(a, b) {
  return a + b;
}

const sumOnce = once(sum);

Ok, so let's port it to TypeScript and add types. What type will we use for the argument? It is a function, but with variable number of parameters. We could use (...args: any[]) => any, but that will hide the original types and make the sumOnce useless!

Luckily, we can use type inference and higher order types ReturnType<T> and Parameters<T>. But what is T?

We will pretend that our function type is generic and extends functions with arbitrary number of parameters:

function once<T extends (...args: any[]) => any>(fn: T);

Now we can type result using the higher order type:

let result: ReturnType<T>;

And instead of using ...args: any[] we can infer the original parameters:

  return function(...args: Parameters<T>) {

Which completes our typing and sumOnce has the original (number, number) -> number signature.

Final code is:

function once<T extends (...args: any[]) => any>(fn: T) {
  let isRun = false;
  let result: ReturnType<T>;
  return function (...args: Parameters<T>) {
    if (!isRun) {
      isRun = true;
      result = fn(...args);
    }

    return result;
  };
}