Runtime type checking strategies in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is runtime type checking in TypeScript?

Runtime type checking means verifying the type of a value while the program is running, not just during compilation. It helps catch errors when data comes from outside sources like user input or APIs.

beginner

Name two common strategies for runtime type checking in TypeScript.

1. Using typeof and instanceof operators.<br>2. Using custom type guard functions that check object shapes.

intermediate

How does a custom type guard function work?

A custom type guard is a function that returns a boolean and uses the is keyword to tell TypeScript the type of a value if the function returns true. It checks properties or structure at runtime.

beginner

What is a limitation of using typeof for runtime type checking?

typeof only works well for primitive types like string, number, and boolean. It cannot check complex objects or arrays accurately.

intermediate

Why might you use a library like zod or io-ts for runtime type checking?

These libraries provide schemas to define expected data shapes and validate data at runtime easily. They help ensure data matches types and give clear error messages when it doesn't.

Which operator is best for checking if a value is a string at runtime in TypeScript?

A instanceof

B typeof

C is

D as

The typeof operator returns 'string' for string values, making it suitable for checking primitive types like strings.

What does a custom type guard function return to inform TypeScript about a type?

A A boolean with a type predicate using 'is'

B A string describing the type

C An object with type info

D A number representing the type

Custom type guards return a boolean and use a type predicate like value is Type to tell TypeScript the type when true.

Which runtime type checking method can verify the shape of an object?

A instanceof operator

B typeof operator

C Custom type guard functions

D as keyword

Custom type guard functions can check object properties to verify the shape and type at runtime.

Why is instanceof not always reliable for runtime type checking?

A It only works with class instances, not plain objects

B It cannot check primitive types

C It changes the type of the variable

D It only works at compile time

instanceof checks if an object is an instance of a class or constructor function, so it doesn't work well with plain objects or interfaces.

What advantage do libraries like zod provide for runtime type checking?

A They disable runtime checks

B They replace TypeScript's compile-time checks

C They convert types to strings

D They automate validation and provide clear error messages

Libraries like zod help define schemas and validate data easily at runtime, giving clear feedback when data doesn't match expected types.

Explain how you would create a custom type guard function in TypeScript to check if a value is a specific object type.

Think about a function that tests properties and tells TypeScript the type when true.

Describe the differences between using 'typeof', 'instanceof', and custom type guards for runtime type checking.

Consider what each method can and cannot check at runtime.