Partial Application

It is a process of converting a function with multiple parameters into a number of functions with one or more arguments already set. The function returns a new function by applying the argument in the returned function, thereby reducing the arity in the returned function. Partial application is also useful in the scenario when its required to supply some but not all arguments to a function.

In the example below, we have a function named greet that takes four arguments namely salutation, title, firstname and lastname. Using partial application we create a new function greetHello by fixing the first argument i.e salutation to Hello, reducing it's arity to three. We can use greetHello as a wrapper of greet with salutation set to Hello.

We then created one more function greetHelloToMrs by partially applygin the title argument in greetHello to Mrs, which was created on top of greet, thereby reducing the arity to two.

const greet = (salutation, title, firstname, lastname) => {
  return `${salutation}, ${title} ${firstname} ${lastname}.`;
};

greet("Hi", "Mr.", "John", "Doe"); // Hi, Mr. John Doe.

const greetHello = greet.bind(null, "Hello");

greetHello("Mr.", "John", "Doe"); // Hello, Mr. John Doe.

const greetHelloToMrs = greetHello.bind(null, "Mrs.");

greetHelloToMrs("Maria", "Doe"); // Hello, Mrs. Maria Doe

Partial application is also referred to as delaying the instantiation of some of the parameters of a function. It may be useful when we need to parameter some processes for a later use.

// without partial application
const drink = (verb, drinkName) => `I am ${verb} ${drinkName}`;
drink("brewing", "coffee");
drink("brewing", "beer");
drink("juicing", "orange");
drink("juicing", "apple");

// partially applying argument
const makeDrink = (verb) => (drinkName) => `I am ${verb} ${drinkName}`;
const makeBrew = makeDrink("brewing");
const makeJuice = makeDrink("juicing");

makeBrew("coffee");
makeBrew("beer");
makeJuice("orange");
makeJuice("apple");