Provide some examples of how currying and partial application can be used

TL;DR

Currying transforms a function with multiple arguments into a sequence of functions, each taking a single argument. Partial application fixes a few arguments of a function, producing another function with a smaller number of arguments. For example, currying a function add(a, b) would look like add(a)(b), while partial application of add(2, b) would fix the first argument to 2, resulting in a function that only needs the second argument.

Currying example:

const add = (a) => (b) => a + b;
const addTwo = add(2);
console.log(addTwo(3)); // 5

Partial application example:

const add = (a, b) => a + b;
const addTwo = add.bind(null, 2);
console.log(addTwo(3)); // 5

Currying and partial application

Currying

Currying is a technique where a function with multiple arguments is transformed into a sequence of functions, each taking a single argument. This allows for more flexible function composition and reuse.

Example

// Consider a simple function that adds two numbers:
function add(a, b) {
  return a + b;
}

// To curry this function, we transform it into a series of functions,
// each taking one argument:
const curriedAdd = (a) => (b) => a + b;

// Now, you can use the curried function like this:
const addTwo = curriedAdd(2);
console.log(addTwo(3)); // 5

Partial application

Partial application is a technique where you fix a few arguments of a function, producing another function with a smaller number of arguments. This is useful for creating specialized functions from more general ones.

Example

// Consider the same add function:
function add(a, b) {
  return a + b;
}

// To partially apply this function
// you can use the `bind` method to fix the first argument:
const addTwo = add.bind(null, 2);
console.log(addTwo(3)); // 5

// Alternatively, you can create a custom partial application function:
function partial(fn, ...fixedArgs) {
  return function (...remainingArgs) {
    return fn(...fixedArgs, ...remainingArgs);
  };
}

const addTwoCustom = partial(add, 2);
console.log(addTwoCustom(3)); // 5

Further reading

• JavaScript.info: Currying
• Functional programming in JavaScript

Exercises

const format = (currency) => (locale) => (amount) =>
  `${currency}:${locale}:${amount}`;

const usdForUs = format('USD')('en-US');
console.log(usdForUs(12), usdForUs(20));