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?<br>

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

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.