Strings and Arrays

Mission 4, step 4. Most of what people see on a website is text. Today you shape text and keep many values in one list.

Quick check

Quick recall

10 XP

Tools for text

Every string has ready-made tools. .length is a property, so it has no brackets. The others are methods: they do something and need ().

Code	Result
"Hello world".length (spaces count too)	11
"Lena".toUpperCase(), toLowerCase() the opposite	"LENA"
"Learning JavaScript is fun.".indexOf("fun") (from 0; -1 = not found)	23
"2026-01-20".slice(0, 4) (from 0 up to 4, without 4)	"2026"
"apple apple".replace("apple", "pear") (first match only; replaceAll() changes all)	"pear apple"

On the slice() slide, console.log(year); stands inside the comment, so nothing prints. After //, the rest of the line is a comment: give every statement its own line. Some languages have their own capitals: for Turkish, use toLocaleUpperCase("tr-TR").

Matching

Code and result

Required 15 XP

Template literals

A template literal is text in backticks. Inside it, ${ } puts a value into the text, without + and many quotes. On a phone keyboard, the backtick is often on the second page of symbols.

let name = "Ayla";
let age = 25;
console.log("My name is " + name + " and I am " + age + ".");
console.log(`My name is ${name} and I am ${age}.`);   // the same text

Worked example: a welcome panel

The slide calls this number the length of "your user name", but it counts first name plus last name.

Practice

Welcome panel

Required 30 XP

The daily application from the course slides: tidy the data, then build the welcome message. Choose the right piece for every gap.

Arrays

An array keeps many values in one box, in square brackets. Each item has a number, its index. Counting starts at 0.

let fruits = ["Apple", "Pear", "Banana", "Strawberry"];
console.log(fruits[0]);       // Apple
fruits[1] = "Kiwi";           // Pear is gone, Kiwi is in its place
console.log(fruits.length);   // 4
let mixed = ["Ayla", 25, true, null];   // types can mix

Quick check

Counting from 0

Required 10 XP

Practice

Fruit list

Required 30 XP

Slides 45 to 48 of the course in one exercise: read, change and count the items of an array.

Method

A ready-made tool that does something, like toUpperCase().

Property

A value that belongs to something, like .length. No brackets.

Template literal

Text in backticks with values inside ${ }.

Array and index

A list in square brackets; the index is the number of an item, from 0.

In short

• .length counts, toUpperCase() changes letters, indexOf() searches, slice() cuts, replace() changes.
• A template literal puts values into text: `Hello, ${name}!`.
• fruits[0] is the first item; fruits.length is the number of items.

Sources

1. MDN Web Docs (2026). Useful string methods.
2. MDN Web Docs (2026). Template literals (Template strings).
3. MDN Web Docs (2026). Arrays.
4. MDN Web Docs (2026). String.prototype.toLocaleUpperCase().