freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/functional-programming/introduction-to-currying-and-partial-application.english.md

id, title, challengeType, isHidden, forumTopicId

id	title	challengeType	isHidden	forumTopicId
587d7dab367417b2b2512b70	Introduction to Currying and Partial Application	1	false	301232

Description

The arity of a function is the number of arguments it requires. Currying a function means to convert a function of N arity into N functions of arity 1. In other words, it restructures a function so it takes one argument, then returns another function that takes the next argument, and so on. Here's an example:

//Un-curried function
function unCurried(x, y) {
  return x + y;
}

//Curried function
function curried(x) {
  return function(y) {
    return x + y;
  }
}
//Alternative using ES6
const curried = x => y => x + y

curried(1)(2) // Returns 3

This is useful in your program if you can't supply all the arguments to a function at one time. You can save each function call into a variable, which will hold the returned function reference that takes the next argument when it's available. Here's an example using the curried function in the example above:

// Call a curried function in parts:
var funcForY = curried(1);
console.log(funcForY(2)); // Prints 3

Similarly, partial application can be described as applying a few arguments to a function at a time and returning another function that is applied to more arguments. Here's an example:

//Impartial function
function impartial(x, y, z) {
  return x + y + z;
}
var partialFn = impartial.bind(this, 1, 2);
partialFn(10); // Returns 13

Instructions

Fill in the body of the add function so it uses currying to add parameters x, y, and z.

Tests

tests:
  - text: <code>add(10)(20)(30)</code> should return <code>60</code>.
    testString: assert(add(10)(20)(30) === 60);
  - text: <code>add(1)(2)(3)</code> should return <code>6</code>.
    testString: assert(add(1)(2)(3) === 6);
  - text: <code>add(11)(22)(33)</code> should return <code>66</code>.
    testString: assert(add(11)(22)(33) === 66);
  - text: Your code should include a final statement that returns <code>x + y + z</code>.
    testString: assert(code.match(/[xyz]\s*?\+\s*?[xyz]\s*?\+\s*?[xyz]/g));

Challenge Seed

function add(x) {
  // Only change code below this line


  // Only change code above this line
}
add(10)(20)(30);

Solution

const add = x => y => z => x + y + z