Type erasure and its consequences in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is type erasure in TypeScript?

Type erasure means that TypeScript removes all type information when it compiles code to JavaScript. The types exist only during development and are not present at runtime.

beginner

Why does TypeScript use type erasure?

TypeScript uses type erasure to keep the output JavaScript clean and compatible with all JavaScript environments, which do not support types natively.

intermediate

What is a common consequence of type erasure when using generics in TypeScript?

Since types are erased, you cannot check or enforce generic types at runtime. For example, you can't check if a value matches a generic type parameter inside the compiled JavaScript.

intermediate

How does type erasure affect debugging TypeScript code?

Because types are removed, debugging happens on plain JavaScript code. This means you can't see TypeScript types in runtime errors or logs, only JavaScript values.

intermediate

Can you use type information at runtime in TypeScript? Why or why not?

No, you cannot use type information at runtime because TypeScript removes all types during compilation. To use type-like checks at runtime, you must write explicit JavaScript code or use libraries.

What happens to TypeScript types after compilation?

A They are converted to JavaScript classes

B They are converted to runtime type checks

C They are kept as comments in the JavaScript

D They are removed and do not exist in the output JavaScript

TypeScript removes all types during compilation, so the output JavaScript has no type information.

Why can't you check generic types at runtime in TypeScript?

A Because TypeScript does not allow generics

B Because generics are only a compile-time feature and erased in JavaScript

C Because JavaScript does not support functions

D Because generics are converted to strings

Generics exist only during compilation and are erased, so runtime checks are not possible.

Which of these is a consequence of type erasure?

A TypeScript code cannot be compiled

B Your code runs slower because of type checks

C You cannot rely on types during runtime

D Types are available as JavaScript objects

Since types are erased, you cannot use them at runtime.

How can you perform type-like checks at runtime in TypeScript?

A By writing explicit JavaScript checks or using libraries

B By relying on TypeScript's runtime type system

C By enabling a special compiler flag

D By using TypeScript enums

TypeScript types are erased, so runtime checks must be done manually or with helper libraries.

What does type erasure help TypeScript achieve?

A Compatibility with JavaScript environments

B Faster runtime type checking

C Automatic type inference at runtime

D Keeping types in the final code

Removing types ensures the output runs anywhere JavaScript runs.

Explain what type erasure is and why TypeScript uses it.

Think about what happens to types when TypeScript turns into JavaScript.

Describe one consequence of type erasure when working with generics in TypeScript.

Consider what you cannot do with generic types after compilation.