Objects

A tour is not just a name. It has a name, a price, a difficulty level, and an availability flag. These values belong together. An object lets you group related values under a single variable with named properties.

What an object is

An object is a collection of key-value pairs, where each key is a string (the property name) and each value can be any type:

const tour = {
  name: 'Cascade Ridge Hike',
  price: 149,
  difficulty: 'moderate',
  available: true,
};

This is an object literal: curly braces containing comma-separated key: value pairs. The trailing comma after the last property is optional but conventional — it makes adding new properties easier.

Dot notation

Access a property’s value with a dot followed by the property name:

console.log(tour.name);        // 'Cascade Ridge Hike'
console.log(tour.price);       // 149
console.log(tour.difficulty);  // 'moderate'
console.log(tour.available);   // true

Bracket notation

Bracket notation accesses a property using a string in square brackets:

console.log(tour['name']);   // 'Cascade Ridge Hike'
console.log(tour['price']);  // 149

Bracket notation is useful when the property name is stored in a variable:

const field = 'price';
console.log(tour[field]);  // 149

Use dot notation by default. Switch to bracket notation when the property name is dynamic.

Adding properties

Assign to a property that does not exist yet and JavaScript creates it:

tour.maxGroupSize = 8;
console.log(tour.maxGroupSize);  // 8

Updating properties

Assign to an existing property to change its value:

tour.price = 169;
console.log(tour.price);  // 169

Like arrays declared with const, a const object’s properties can still be modified. const only prevents reassigning the variable itself.

Accessing a missing property

If you access a property that does not exist, JavaScript returns undefined — it does not throw an error:

console.log(tour.rating);  // undefined

This is why TypeError: Cannot read properties of undefined is a common bug: you access a property of an object, get undefined, then try to access a property of that undefined result.

console.log(tour.rating.average);  // TypeError — tour.rating is undefined

Arrays of objects

Objects and arrays work together to represent collections of structured records. This is the pattern you will use for tour listings throughout the rest of the course:

const tours = [
  { name: 'Cascade Ridge Hike', price: 149, difficulty: 'moderate', available: true },
  { name: 'Summit Loop Trek',   price: 199, difficulty: 'hard',     available: true },
  { name: 'Valley Floor Walk',  price: 99,  difficulty: 'easy',     available: false },
];

console.log(tours[0].name);        // 'Cascade Ridge Hike'
console.log(tours[1].price);       // 199
console.log(tours[2].available);   // false

Read this as: “an array of tour objects.” Each element is an object; each object has the same shape (same properties). This consistent shape is what makes them easy to loop over — which you will do in Module 03.

Exercise

const tour = {
  name: 'Cascade Ridge Hike',
  price: 149,
  difficulty: 'moderate',
  available: true,
};

// Access each property with dot notation
console.log(tour.name);
console.log(tour.price);
console.log(tour.difficulty);
console.log(tour.available);

// Update the price
tour.price = 169;
console.log(tour.price);  // 169

// Add a new property
tour.maxGroupSize = 8;
console.log(tour.maxGroupSize);  // 8

// Log the final object
console.log(tour);

Then create a second tour object for 'Summit Loop Trek' with price 199, difficulty 'hard', available true, and put both in an array. Access the second tour’s name via the array.

Recap

• An object groups related values under named properties using { key: value } syntax.
• Dot notation (object.property) is the standard way to read and write properties.
• Bracket notation (object['property']) is used when the property name is a variable.
• Assigning to a new property name creates it; assigning to an existing one updates it.
• Accessing a missing property returns undefined — it does not throw an error.
• Arrays of objects — a collection of same-shaped records — are the core data structure for tour listings and similar collections.