instanceof type guards in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the object is an instance of the class Dog.

Typescript

if (pet [1] Dog) {
  console.log("It's a dog!");
}

Drag options to blanks, or click blank then click option'

Aequals

Btypeof

Cin

Dinstanceof

Attempts:

3 left

💡 Hint

Common Mistakes

Using typeof instead of instanceof

Using in operator which checks properties, not types

The instanceof operator checks if an object is an instance of a class or constructor function.

2fill in blank

medium

Complete the function to return true if the pet is a Cat using instanceof.

Typescript

function isCat(pet: any): pet is Cat {
  return pet [1] Cat;
}

Drag options to blanks, or click blank then click option'

Ain

Binstanceof

Ctypeof

Dequals

Attempts:

3 left

💡 Hint

Common Mistakes

Using typeof which returns 'object' for objects

Using in which checks for properties, not types

The instanceof operator is used here to check if pet is an instance of Cat.

3fill in blank

hard

Fix the error in the code to correctly check if vehicle is an instance of Truck.

Typescript

if (vehicle [1] Truck) {
  vehicle.loadCargo();
}

Drag options to blanks, or click blank then click option'

Ainstanceof

Btypeof

C===

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using == or === instead of instanceof

Using typeof which returns 'object' for objects

Only instanceof correctly checks if vehicle is an instance of Truck. Using == or === compares references, not types.

4fill in blank

hard

Fill both blanks to create a type guard that checks if pet is a Bird and calls fly().

Typescript

if (pet [1] Bird) {
  pet.[2]();
}

Drag options to blanks, or click blank then click option'

Ainstanceof

Brun

Cfly

Dwalk

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong operator like typeof

Calling a method not defined on Bird like walk or run

The instanceof operator checks the type, and fly() is the method called on Bird instances.

5fill in blank

hard

Fill all three blanks to create a type guard that checks if shape is a Circle and returns its radius.

Typescript

function getRadius(shape: Shape): number | null {
  if (shape [1] Circle) {
    return shape.[2];
  } else {
    return [3];
  }
}

Drag options to blanks, or click blank then click option'

Ainstanceof

Bradius

Cnull

Ddiameter

Attempts:

3 left

💡 Hint

Common Mistakes

Returning diameter instead of radius

Using typeof instead of instanceof

Returning 0 instead of null when not a Circle

instanceof checks if shape is a Circle, then returns its radius. Otherwise, it returns null.

Practice

1. What does the instanceof operator check in TypeScript?

easy

A. If a variable is a primitive type

B. If an object is an instance of a specific class

C. If a function returns a boolean

D. If a type is an interface

Solution

1. Step 1: Understand the purpose of instanceof

   The instanceof operator checks whether an object was created from a particular class.

2. Step 2: Differentiate from other checks

   It does not check primitive types, function returns, or interfaces, only class instances.

3. Final Answer:

   If an object is an instance of a specific class -> Option B

4. Quick Check:

   instanceof checks class instance = A [OK]

Hint: Remember: instanceof works only with classes, not interfaces [OK]

Common Mistakes:

• Thinking instanceof works with interfaces
• Confusing instanceof with typeof
• Assuming instanceof checks primitive types

2. Which of the following is the correct syntax to check if obj is an instance of class Car?

easy

A. if (obj instanceof Car) { }

B. if (obj instance Car) { }

C. if (obj is Car) { }

D. if (obj type Car) { }

Solution

1. Step 1: Recall correct instanceof syntax

   The correct syntax uses the keyword instanceof between the object and the class name.

2. Step 2: Identify invalid options

   Options B, C, and D use incorrect keywords or missing syntax.

3. Final Answer:

   if (obj instanceof Car) { } -> Option A

4. Quick Check:

   Correct syntax uses instanceof keyword = A [OK]

Hint: Use 'instanceof' keyword exactly between object and class [OK]

Common Mistakes:

• Using 'is' or 'type' instead of 'instanceof'
• Omitting parentheses around condition
• Writing 'instance' instead of 'instanceof'

3. What will be the output of this code?

class Dog { bark() { return 'woof'; } }
class Cat { meow() { return 'meow'; } }
function speak(animal: Dog | Cat) {
  if (animal instanceof Dog) {
    return animal.bark();
  } else {
    return animal.meow();
  }
}
console.log(speak(new Cat()));

medium

A. 'woof'

B. TypeError at runtime

C. 'meow'

D. undefined

Solution

1. Step 1: Identify the object passed

   The function speak is called with new Cat(), so animal is a Cat instance.

2. Step 2: Check the instanceof condition

   The condition animal instanceof Dog is false, so the else block runs, returning animal.meow().

3. Final Answer:

   'meow' -> Option C

4. Quick Check:

   Cat instance triggers else block = D [OK]

Hint: Trace instanceof condition to pick correct method call [OK]

Common Mistakes:

• Assuming instanceof works with interfaces
• Confusing which method runs for which class
• Expecting 'woof' when passing Cat instance

4. Identify the error in this code snippet:

interface Bird { fly(): void; }
class Eagle implements Bird { fly() { console.log('Flying'); } }
function check(bird: Bird) {
  if (bird instanceof Eagle) {
    bird.fly();
  }
}

medium

A. No error, code is correct

B. Missing return type in fly() method

C. Cannot use instanceof with interfaces

D. Eagle class must be abstract

Solution

1. Step 1: Understand instanceof usage

   The instanceof operator is used with the class Eagle, which is valid even though bird is typed as the interface Bird.

2. Step 2: Verify no errors

   There are no syntax errors, type mismatches, or runtime issues. TypeScript compiles this code and narrows bird to Eagle inside the if block.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   instanceof with class on interface-typed param works = D [OK]

Hint: Remember: instanceof works with classes even on interface-typed variables [OK]

Common Mistakes:

• Thinking instanceof can't be used on interface-typed variables
• Confusing instanceof with direct interface checks
• Thinking implementing class must be abstract

5. Given these classes:

class Animal { name: string; constructor(name: string) { this.name = name; } }
class Fish extends Animal { swim() { return 'swimming'; } }
class Bird extends Animal { fly() { return 'flying'; } }

Write a function move that takes animal: Animal and returns 'swimming' if it's a Fish, 'flying' if it's a Bird, or 'unknown' otherwise. Which implementation is correct?

hard

A. function move(animal: Animal) { if (animal instanceof Animal) return 'unknown'; else if (animal instanceof Fish) return animal.swim(); else return animal.fly(); }

B. function move(animal: Animal) { if (animal instanceof Fish) return 'flying'; else if (animal instanceof Bird) return 'swimming'; else return 'unknown'; }

C. function move(animal: Animal) { if (animal instanceof Fish) return animal.fly(); else if (animal instanceof Bird) return animal.swim(); else return 'unknown'; }

D. function move(animal: Animal) { if (animal instanceof Fish) return animal.swim(); else if (animal instanceof Bird) return animal.fly(); else return 'unknown'; }

Solution

1. Step 1: Check instanceof order and method calls

   The function must check if animal is a Fish or Bird using instanceof and call the correct method.

2. Step 2: Validate each option

   function move(animal: Animal) { if (animal instanceof Fish) return animal.swim(); else if (animal instanceof Bird) return animal.fly(); else return 'unknown'; } correctly checks Fish first, calls swim(), then Bird with fly(), else returns 'unknown'. function move(animal: Animal) { if (animal instanceof Animal) return 'unknown'; else if (animal instanceof Fish) return animal.swim(); else return animal.fly(); } wrongly checks Animal first, which always matches. function move(animal: Animal) { if (animal instanceof Fish) return animal.fly(); else if (animal instanceof Bird) return animal.swim(); else return 'unknown'; } swaps methods incorrectly. function move(animal: Animal) { if (animal instanceof Fish) return 'flying'; else if (animal instanceof Bird) return 'swimming'; else return 'unknown'; } returns incorrect strings directly without calling methods.

3. Final Answer:

   function move(animal: Animal) { if (animal instanceof Fish) return animal.swim(); else if (animal instanceof Bird) return animal.fly(); else return 'unknown'; } -> Option D

4. Quick Check:

   Correct instanceof checks and method calls = B [OK]

Hint: Check instanceof order and call matching methods [OK]

Common Mistakes:

• Checking base class first, blocking subclass checks
• Calling wrong methods for classes
• Returning strings instead of calling methods