← ACARA Digital Technologies Years 9–10

Common bugs in classes and objects

Common class bugs: forgetting new, leaving out this. in the constructor, swapping the arguments, or misspelling a property so it reads undefined.

Part of: ACARA Digital Technologies Years 9–10 — Modular and Object-Oriented Programming (JavaScript) — Module 67

Understanding Common bugs in classes and objects in ACARA Digital Technologies Years 9–10

What it does: this.name is the object's property and name is the parameter. Without this., the value is lost when the constructor finishes. Name comes first and age second. Writing new Pet(3, 'Rex') would swap the values and give the wrong properties. Calling a class without new throws: Class constructor Pet cannot be invoked without 'new'. Arguments are matched by position. Swapping them puts 3 in name and 'Rex' in age. nmae is a misspelt property name. JavaScript does not throw an error, it just gives undefined. The error message tells you the fix: add new before the class name. A class must be called with new. The fix is const p = new Pet('Rex', 3);.

Common mistakes: A class constructor cannot be called without new. Pet("Rex") throws "Class constructor Pet cannot be invoked without 'new'". It should be new Pet("Rex"). The word new is missing. Calling Dog("Bo") like a normal function throws an error. It should be new Dog("Bo") so a new object is created.

Class: _____________________ · Name: _____________________ · Date: _________

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This module's key terms are also in a printable crossword (Modules 743–781) and word search (Modules 743–809).

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Each concept is reinforced from every angle — learned, then applied, then tested.

Learning

Duo

Which line correctly creates a Pet object from a class?

const p = Pet('Rex', 3);const p = new Pet('Rex', 3);

Matches

const p = Pet('Rex', 3);

↓

The word new is missing, so JavaScript throws the error: Class constructor Pet cannot be invoked without 'new'.

Applying

Meanings

Term

new Pet(name, age)

Meaning

Common class bugs: forgetting new, leaving out this. in the constructor, swapping the arguments, or misspelling a property so it reads undefined.

Practice

class Pet {

Testing

Gaps

const pet = Pet("Rex", 3);

Check Answer

Spots

4const p = Pet("Rex");

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Quick Questions

1. Which line correctly creates a Pet object from a class?

A. const p = Pet('Rex', 3);

B. const p = new Pet('Rex', 3); ✓

Calling a class without new throws: Class constructor Pet cannot be invoked without 'new'.

2. Which constructor correctly saves the name on the object?

A. constructor(name) { this.name = name; } ✓

B. constructor(name) { name = name; }

Without this., the value is never stored on the object. name = name just assigns the parameter to itself.

3. The class has constructor(name, age). Which call gives the right values?

A. new Pet('Rex', 3) ✓

B. new Pet(3, 'Rex')

Arguments are matched by position. Swapping them puts 3 in name and 'Rex' in age.

4. The class sets this.name. What does console.log(p.nmae) print?

A. Rex

B. undefined ✓

nmae is a misspelt property name. JavaScript does not throw an error, it just gives undefined.

5. Which is the cause of this error: Class constructor Pet cannot be invoked without 'new'?

A. Calling Pet('Rex', 3) without new ✓

B. Using the wrong property name

The error message tells you the fix: add new before the class name.

Match the Pairs

const p = new Pet('Rex', 3); → Creates a Pet object with new, passing the name first and the age second, in the same order as the constructor

console.log(p.nme); → Prints undefined, because the property is called name and nme is a typo

Inside a constructor, this. saves a parameter as a property on the new object → this.name = name;

Arguments must be passed in the same order as the constructor parameters, for example constructor(name, age) → new Pet('Rex', 3)

const p = Pet('Rex', 3); → The word new is missing, so JavaScript throws the error: Class constructor Pet cannot be invoked without 'new'.

constructor(name) { name = name; } → The this. is missing, so the value is never stored on the object and p.name will be undefined.

Key Term

new Pet(name, age)

Common class bugs: forgetting new, leaving out this. in the constructor, swapping the arguments, or misspelling a property so it reads undefined.

Fill in the Blanks

1. const pet = new Pet("Rex", 3);

Hint: Type the keyword that must come before a class name to create an object, or JavaScript throws an error.

2. class Pet { constructor(name) { this.name = name; } }

Hint: Type the keyword that refers to the object being built, so the value is stored as a property.

3. // Pet constructor(name, age) const n = "Rex", a = 3; const p = new Pet(n, a);

Hint: Type the variable that matches the first constructor parameter, so the arguments are in the right order.

4. // Pet sets this.name and this.age const pet = new Pet("Rex", 3); console.log(pet.name);

Hint: Type the property name spelt exactly as the constructor sets it, to avoid undefined.

5. constructor(name, age) { this.name = name; this.age = age;

Hint: Type the constructor parameter that holds the value for this property, not a different name.

Spot the Error

Question 1

A class Pet {

B constructor(n) {this.n = n;}

C }

D const p = Pet("Rex");

E console.log(p.n);

Line D — A class constructor cannot be called without new. Pet("Rex") throws "Class constructor Pet cannot be invoked without 'new'". It should be new Pet("Rex").

Question 2

A const f = () => Dog("Bo");

B class Dog {

C constructor(name) {

D this.name = name;

E }

F }

Line A — The word new is missing. Calling Dog("Bo") like a normal function throws an error. It should be new Dog("Bo") so a new object is created.

Question 3

A class Car {

B construtor(make, year) {

C this.make = make;

D this.year = year;

E }

F }

Line B — The method name is misspelt. Only a method named exactly constructor runs when you use new, so make and year would never be set and would read as undefined. It should be constructor.

Question 4

A class Game {

B constructor(title) {

C this.title = title;

D this.level = level;

E }

F }

Line B — The constructor is missing the level parameter, so this.level = level has no level to read. It should be constructor(title, level).

Question 5

A class Fish {

B constructor(name) {

C this.nmae = name;

D }

E show() { return this.name; }

F }

Line C — The property name is misspelt. This creates this.nmae, so reading this.name later gives undefined. It should be this.name = name.

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