Arrays and objects

Model real data and transform it with map, filter and reduce.

Beginner 14 min read Lesson 3 of 480 Challenge included

What you will be able to do

Create and access arrays and objects
Transform data immutably
Destructure values

Arrays hold ordered lists. Objects hold named properties. Most application logic is transforming one shape into another.

arrays.jsJavaScript

const lessons = [
  { title: "Variables", done: true, minutes: 10 },
  { title: "Functions", done: true, minutes: 12 },
  { title: "Arrays", done: false, minutes: 14 },
];

const finished = lessons.filter((lesson) => lesson.done);
const titles = lessons.map((lesson) => lesson.title);
const totalMinutes = lessons.reduce((sum, lesson) => sum + lesson.minutes, 0);

const { title } = lessons[2];
console.log(titles, totalMinutes, title);

Method	Use it to
map	Transform every item into a new array.
filter	Keep only items that match a condition.
reduce	Fold an array into a single value.
find	Get the first matching item.
some / every	Test whether any or all items match.

Your turn

Calculate total minutes

Medium

Given the lessons array, use reduce to calculate total minutes across all lessons.

JAVASCRIPT · starter

const lessons = [
  { minutes: 10 },
  { minutes: 12 },
  { minutes: 14 },
];
// your code here

Check your understanding before moving on.

Knowledge checkQuestion 1

Which method creates a new array by transforming each item?

Select an answer to continue

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Next: Operators and conditionals

Lesson 3 of 480 · 0% of course