instanceof type guards in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the instanceof operator do in TypeScript?

It checks if an object is an instance of a specific class or constructor function, helping TypeScript narrow the type within a conditional block.

beginner

How does instanceof help with type safety?

By verifying an object's class at runtime, <code>instanceof</code> lets TypeScript know the exact type, so you can safely access properties or methods specific to that class.

beginner

Example: What type does TypeScript infer inside this block?

if (obj instanceof Date) {
  // What type is obj here?
}

Inside the block, TypeScript knows obj is a Date object, so you can use Date methods safely.

intermediate

Can instanceof be used with interfaces in TypeScript?

No, because interfaces do not exist at runtime. instanceof works only with classes or constructor functions that exist when the code runs.

intermediate

Why might you prefer instanceof over checking a property manually?

Because instanceof is a built-in, reliable way to check an object's type, reducing errors and making your code clearer and easier to maintain.

What does obj instanceof ClassName check?

A If obj is created by ClassName or inherits from it

B If obj has a property named ClassName

C If obj is equal to ClassName

D If obj is a primitive type

instanceof checks if the object was created by the class or inherits from it.

Can instanceof be used to check if an object implements an interface?

A Only if the interface has a constructor

B Yes, always

C No, interfaces are erased at runtime

D Only in strict mode

Interfaces do not exist at runtime, so instanceof cannot check them.

Inside if (obj instanceof Date) { ... }, what type does TypeScript assume for obj?

A Date

B any

C Object

D unknown

TypeScript narrows obj to Date inside the block.

Which of these is a correct use of instanceof?

A if (value instanceof string) { ... }

B if (value instanceof boolean) { ... }

C if (value instanceof number) { ... }

D if (value instanceof Array) { ... }

Array is a class, so instanceof works. Primitive types like string, number, boolean are not classes.

Why is instanceof preferred over checking a property like obj.type === 'Date'?

A Because <code>instanceof</code> is faster

B Because <code>instanceof</code> checks the actual prototype chain

C Because properties can never be trusted

D Because <code>instanceof</code> works with primitives

instanceof checks the real prototype chain, making it more reliable than property checks.

Explain how instanceof helps TypeScript narrow types during runtime checks.

Think about how JavaScript knows an object's class.

Why can't instanceof be used with interfaces in TypeScript? What alternative can you use to check types related to interfaces?

Interfaces disappear after compiling to JavaScript.

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