Closures and higher-order functions

Treat functions as values, remember state with closures and process arrays with map, filter and reduce.

Beginner 14 min read Lesson 9 of 480 Challenge included

What you will be able to do

Return functions from functions
Explain what a closure keeps
Use map, filter and reduce

A closure is a function that keeps access to the scope where it was created. Higher-order functions take or return other functions, which makes small building blocks composable.

closures.jsJavaScript

function makeCounter() {
  let count = 0;
  return () => ++count;
}

const next = makeCounter();
console.log(next(), next(), next());

const scores = [88, 92, 79, 95];
const passed = scores.filter((score) => score >= 80);
const curved = scores.map((score) => Math.min(100, score + 3));
const total = scores.reduce((sum, score) => sum + score, 0);

console.log(passed);
console.log(curved);
console.log(total, Math.round(total / scores.length));

Closure: A function plus the variables it captured.
Callback: A function passed to another function.
Higher-order: A function that takes or returns a function.
Pure function: Same input, same output, no side effects.

Your turn

Average with reduce

Medium

Return the average score using reduce, and 0 for an empty list.

JAVASCRIPT · starter

function average(scores) {
  // your code here
}

Check your understanding before moving on.

Knowledge checkQuestion 1

What does filter return?

Select an answer to continue

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