Arrays and Objects

A single variable holds one value. Arrays and objects let you hold many values in an organised way.

Arrays

An array is an ordered list. Indexes start at zero.

const fruits = ["mango", "orange", "pawpaw"];

console.log(fruits[0]);      // mango
console.log(fruits.length);  // 3

fruits.push("banana");        // add to the end
fruits.unshift("apple");      // add to the start
console.log(fruits);

Array Methods

Methods such as map, filter and reduce transform arrays without writing manual loops.

const scores = [45, 72, 88, 61, 95];

const doubled = scores.map(score => score * 2);
const passed = scores.filter(score => score >= 70);
const total = scores.reduce((sum, score) => sum + score, 0);

console.log(doubled); // [90, 144, 176, 122, 190]
console.log(passed);  // [72, 88, 95]
console.log(total);   // 361

Objects

An object stores named properties. Use dot notation or bracket notation to read and update them.

const student = {
  name: "Amina",
  age: 16,
  subjects: ["Math", "English"],
  isPrefect: false,
};

console.log(student.name);       // Amina
student.age = 17;
student.subjects.push("Physics");
console.log(student.subjects);   // ["Math", "English", "Physics"]

Practice

1. Create an array of five subjects and print each one.
2. Filter an array of scores to keep only the passing ones.
3. Create an object describing yourself with at least four properties.

Hands-on exercise

Given the array [12, 5, 18, 7, 21, 3], write JavaScript that:
1. Keeps only numbers greater than 10.
2. Returns their total.
Print the result.

Recap

Arrays hold ordered lists and objects hold named properties. map, filter and reduce turn arrays into answers.