Instance methods in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of instance method call

What is the output of this JavaScript code?

Javascript

class Car {
  constructor(make) {
    this.make = make;
  }
  getMake() {
    return `This car is a ${this.make}`;
  }
}
const myCar = new Car('Toyota');
console.log(myCar.getMake());

A undefined

B "This car is a myCar"

C "This car is a Toyota"

D TypeError

💡 Hint

Remember that instance methods use 'this' to access properties of the object.

The method getMake() returns a string using the 'make' property of the instance. Since 'myCar' was created with 'Toyota', the output is 'This car is a Toyota'.

❓ Predict Output

intermediate

Value of property after method call

What is the value of 'counter.count' after running this code?

Javascript

class Counter {
  constructor() {
    this.count = 0;
  }
  increment() {
    this.count += 1;
  }
}
const counter = new Counter();
counter.increment();
counter.increment();

A 0

B NaN

C undefined

D 2

💡 Hint

Each call to increment adds 1 to the count property.

The constructor sets count to 0. Each increment call adds 1, so after two calls, count is 2.

❓ Predict Output

advanced

Output with method using arrow function

What is the output of this code?

Javascript

class Person {
  constructor(name) {
    this.name = name;
  }
  greet = () => {
    return `Hello, my name is ${this.name}`;
  }
}
const p = new Person('Alice');
console.log(p.greet());

A "Hello, my name is undefined"

B "Hello, my name is Alice"

C SyntaxError

D TypeError

💡 Hint

Arrow functions keep the 'this' from their surrounding context.

The greet method is an arrow function property, so 'this.name' correctly refers to the instance property 'name'. The output is 'Hello, my name is Alice'.

❓ Predict Output

advanced

Output when method is extracted from instance

What is the output of this code?

Javascript

class Dog {
  constructor(name) {
    this.name = name;
  }
  bark() {
    return `${this.name} says woof!`;
  }
}
const dog = new Dog('Buddy');
const barkFn = dog.bark;
console.log(barkFn());

A "undefined says woof!"

B "Buddy says woof!"

C TypeError

D ReferenceError

💡 Hint

When a method is called without its object, 'this' is undefined in strict mode.

Extracting the method and calling it loses the original 'this' binding. So 'this.name' is undefined, resulting in 'undefined says woof!'.

🧠 Conceptual

expert

Why use instance methods instead of static methods?

Which is the best reason to use instance methods instead of static methods in a class?

A Instance methods can access and modify individual object data, while static methods cannot.

B Instance methods run faster than static methods.

C Static methods cannot be called from outside the class.

D Static methods automatically bind 'this' to the instance.

💡 Hint

Think about what data each method type can access.

Instance methods work with data specific to each object instance. Static methods belong to the class itself and cannot access instance properties.