Why You need Currying Function?

@ Pattern

What is Currying

Transforming a function that takes multiple parameters into a series of functions that take a single parameter

For example, there is a function to make a sandwich, which builds a sandwich by passing three parameters:

function makeSandwich(bread, meat, vegetable) {
  return `Sandwich: ${bread}, ${meat}, ${vegetable}`;
}


makeSandwich('rye bread', 'beef', 'onion'); // Sandwich: rye bread, beef, onion

Currying this function means breaking it down into three functions, each accepting a single parameter:

function makeSandwich(bread) {
  return function (meat) {
    return function (vegetable) {
      return `Sandwich: ${bread}, ${meat}, ${vegetable}`;
    };
  };
}


makeSandwich('rye bread')('beef')('onion'); // Sandwich: rye bread, beef, onion

This way of writing is still somewhat hard to read, so we can further simplify it using arrow functions:

const makeSandwich = (bread) => (meat) => (vegetable) => `Sandwich: ${bread}, ${meat}, ${vegetable}`;

This is currying. Now that you understand this cool pattern, what are the benefits of passing functions one by one instead of passing parameters?

Benefits of Currying

Higher Reusability

Using the sandwich example again, if not all parameters are fully passed, for instance, if only bread and meat are passed, it will return a function indicating that the vegetable parameter needs to be provided:

const ryeBeefSandwich = makeSandwich('rye bread')('beef');
ryeBeefSandwich(); // vegetable => `Sandwich: ${bread}, ${meat}, ${vegetable}`
ryeBeefSandwich('onion'); // 'Sandwich: rye bread, beef, onion'

This creates a partially applied function that can be used to build different sandwiches without passing all parameters each time. You must have already noticed that this awesome feature can be immediately applied in actual development, such as opening a sandwich shop to produce different types of sandwiches:

const whiteBreadBeefSandwich = makeSandwich('white bread')('beef');
const ryeChickenSandwich = makeSandwich('rye bread')('chicken');
// ... various sandwich templates


whiteBreadBeefSandwich('lettuce'); // 'Sandwich: white bread, beef, lettuce'
ryeChickenSandwich('onion'); // 'Sandwich: rye bread, chicken, onion'

References