Array Methods: map, filter, find, reduce

Array methods communicate common collection operations without manual index management.

• map transforms each item and returns an array of the same length.
• filter returns items that pass a condition.
• find returns the first matching item or undefined.
• some checks whether at least one item matches; every checks whether all do.
• reduce combines items into one accumulated result.

const lessons = [
  { title: "Variables", minutes: 8, complete: true },
  { title: "Functions", minutes: 12, complete: false },
  { title: "Objects", minutes: 10, complete: true },
];

const titles = lessons.map(lesson => lesson.title);
const incomplete = lessons.filter(lesson => !lesson.complete);
const firstLongLesson = lessons.find(lesson => lesson.minutes > 10);
const totalMinutes = lessons.reduce((sum, lesson) => sum + lesson.minutes, 0);

Avoid side effects in transformations#

Keep map and filter callbacks focused on returning values. Mutating external state from inside a transformation can make behaviour harder to follow. If a regular loop better communicates your intent, use it.

Sorting#

sort mutates the original array. Use toSorted in modern environments when you want a sorted copy, or spread/slice before sorting for broader compatibility.

const scores = [10, 3, 21];
const ascending = [...scores].sort((a, b) => a - b);

Practice#

Given an array of products, return only products in stock, map them to names, and calculate the sum of their prices.