← ACARA Digital Technologies Years 9–10

Reading and changing an object's properties

Reads a property of an object, like rex.age. To change it, assign a new value: rex.age = 4. Each object keeps its own values.

Part of: ACARA Digital Technologies Years 9–10 — Modular and Object-Oriented Programming (JavaScript) — Module 57

Understanding Reading and changing an object's properties in ACARA Digital Technologies Years 9–10

What it does: Using object.property inside console.log reads the property and shows its value. Each object keeps its own values, so milo.age stays the same when rex.age is changed. A single = assigns a new value. == only compares two values and changes nothing. Assigning to a property replaces the old value, so rex.age is now 4. Properties are read with dot notation: object.property. Round brackets are for calling functions. const stops rex from being reassigned to a different object, but the properties inside the object can still be changed. The object name comes first, then a dot, then the property: rex.name. You must assign to rex.age. Writing just age creates or changes a separate variable, not the property of rex.

Common mistakes: Property names are case-sensitive. rex.Age does not exist, so it prints undefined. It should be rex.age. == compares two values and does not change anything. To change the property, use the assignment operator: rex.age = 4;.

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This module's key terms are also in a printable crossword (Modules 728–742) and word search (Modules 710–742).

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Learning

Duo

Dog class: const rex = new Dog('Rex', 3). Which line reads Rex's age?

console.log(Dog.age);console.log(rex.age);

Matches

rex.age == 4;

↓

== compares values instead of assigning. It should be rex.age = 4; to change the property.

Applying

Meanings

Term

object.property

Meaning

Reads a property of an object, like rex.age. To change it, assign a new value: rex.age = 4. Each object keeps its own values.

Practice

class Dog {

Testing

Gaps

console.log(.age);

Check Answer

Spots

1const rex = new Dog("Rex", 3);

2console.log(rex.Age);

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Quick Questions

1. Dog class: const rex = new Dog('Rex', 3). Which line reads Rex's age?

A. console.log(Dog.age);

B. console.log(rex.age); ✓

Use the object's name, a dot, then the property name. rex.age reads the age stored in the rex object.

2. Which line changes rex's age to 4?

A. rex.age = 4; ✓

B. rex.age == 4;

A single = assigns a new value. == only compares two values and changes nothing.

3. rex.age is 3. After rex.age = 4; runs, what does console.log(rex.age) print?

A. 4 ✓

B. 3

Assigning to a property replaces the old value, so rex.age is now 4.

4. const rex = new Dog('Rex', 3); const fido = new Dog('Fido', 5); rex.age = 4; What is fido.age?

A. 4

B. 5 ✓

Each object keeps its own values. Changing rex does not change fido.

5. Which is the correct way to read the name property of an object called fido?

A. fido.name ✓

B. fido(name)

Properties are read with dot notation: object.property. Round brackets are for calling functions.

Match the Pairs

rex.age → Reads the age property of the object stored in rex

rex.age = 4; → Changes rex's age property to the new value 4

Print the value of a property stored in an object called rex → console.log(rex.name);

Changing one object's property does not change another object's copy → rex.age = 4; console.log(milo.age);

rex.age == 4; → == compares values instead of assigning. It should be rex.age = 4; to change the property.

console.log(rex(age)); → Brackets are used instead of a dot. It should be rex.age to read the property.

Key Term

object.property

Reads a property of an object, like rex.age. To change it, assign a new value: rex.age = 4. Each object keeps its own values.

Fill in the Blanks

1. const rex = new Dog("Rex", 3); console.log(rex.age);

Hint: Type the name of the variable that holds the dog object whose age is being read.

2. const rex = new Dog("Rex", 3); rex.age = 4; console.log(rex.age);

Hint: Type the operator that stores a new value in the property.

3. const rex = new Dog("Rex", 3); const mia = new Dog("Mia", 5); mia.age = 6; // rex.age is still 3

Hint: Each object keeps its own values. Type the object variable that is changing, based on the comment.

4. const rex = new Dog("Rex", 3); rex.age = rex.age + 1; console.log(rex.age); // 4

Hint: Type the property name that is read to work out Rex's new value.

5. const rex = new Dog("Rex", 3); rex.age = 4; // Rex is now 4 years old console.log(rex.age);

Hint: Type the property name that stores how many years old the dog is.

Spot the Error

Question 1

A const rex = new Dog("Rex", 3);

B console.log(rex.Age);

C rex.age = 4;

Line B — Property names are case-sensitive. rex.Age does not exist, so it prints undefined. It should be rex.age.

Question 2

A const rex = new Dog("Rex", 3);

B console.log(rex.age);

C rex.age == 4;

D console.log(rex.age);

Line C — == compares two values and does not change anything. To change the property, use the assignment operator: rex.age = 4;

Question 3

A const mia = new Cat("Mia", 2);

B mia.age = age + 1;

C console.log(mia.age);

Line B — age on its own is not a variable, so this causes a ReferenceError. Read the property through the object: mia.age + 1.

Question 4

A let rex = Dog("Rex", 3);

B let zed = new Dog("Zed", 5);

C rex.age = 4;

D console.log(zed.age);

Line A — An object is created from a class with the new keyword. It should be let rex = new Dog("Rex", 3);

Question 5

A let rex = new Dog("Rex", 3);

B let zed = new Dog("Zed", 5);

C rex.age = 4;

D console.log(zed.age());

Line D — age is a property, not a method, so it is read without brackets. zed.age() causes a TypeError. Use zed.age, which prints 5.

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