Summary - Functional programming

Build a price-calculation chain using compose and curry.

Summary - Functional programming

In this group you learned four core ideas. Pure functions have no side effects and return the same output for the same input. Immutability copies data instead of changing it. Composition combines small functions. Currying binds arguments one at a time.

Together, these four produce code that is easy to read, test, and reuse. Now we will combine all of them in a price-calculation chain.

Exercise Build a price-calculation chain

Build a price-calculation chain. First write a pure arrow function named addTax: price => price * 1.2. Then define a compose helper: (f, g) => x => f(g(x)). Write a double function: x => x * 2. Then call const total = compose(addTax, double); and console.log(total(100));.

Show example solution

const addTax = price => price * 1.2;
const compose = (f, g) => x => f(g(x));
const double = x => x * 2;
const total = compose(addTax, double);
console.log(total(100));

Test yourself

Match the functional concepts

pure function

no side effects
immutability

copy, don't mutate
compose

f(g(x))
curry

arguments one by one
Which one follows the functional style?
- prices.map(p => p * 1.2)
- user.age = 30;
- prices.push(100);
- for (let i = 0; i < arr.length; i++) { arr[i]++; }
Show answer

prices.map(p => p * 1.2) — Correct! map returns a new array and does not mutate the original; that is functional style.

Complete the final call of the chain

const addTax = price => price * 1.2;
const result = addTax(____);

Show answer

const addTax = price => price * 1.2;
const result = addTax(100);

Next lesson: What is a Proxy?

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