When enums add unnecessary runtime code in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a TypeScript enum?

An enum in TypeScript is a way to define a set of named constants. It creates an object at runtime that maps names to values and sometimes values back to names.

intermediate

Why can enums add unnecessary runtime code?

Because enums generate JavaScript objects at runtime, they add extra code even if you only need compile-time type checking or simple constants.

intermediate

What is a common alternative to enums to avoid runtime code?

Using union types of string or number literals combined with 'as const' objects can provide type safety without generating runtime code.

advanced

How does a const enum differ from a regular enum in TypeScript?

A const enum is inlined at compile time and does not generate runtime code, but it requires 'preserveConstEnums' to be false and can cause issues with some tools.

advanced

What is a downside of using const enums?

Const enums are erased during compilation, so if you use them with external JavaScript or certain build tools, it can cause errors because the enum object does not exist at runtime.

What does a regular TypeScript enum generate in JavaScript?

A A function that returns enum values

B Only type information, no JavaScript code

C An object with keys and values representing the enum members

D A string literal union type

Regular enums create an object at runtime mapping names to values and sometimes values back to names.

Which TypeScript feature avoids generating runtime code for enums?

A const enum

B interface

C class

D namespace

Const enums are inlined at compile time and do not produce runtime JavaScript code.

What is a simple alternative to enums to get type safety without runtime code?

A Using functions

B Union of string literals with 'as const' object

C Using interfaces

D Using classes with static properties

Union types with 'as const' objects provide type safety and no runtime enum object is created.

What problem can const enums cause in some build setups?

A Runtime errors because enum objects do not exist

B Slower runtime performance

C Larger bundle size

D Lack of type safety

Because const enums are erased, if external code expects the enum object, it can cause runtime errors.

When should you avoid using regular enums in TypeScript?

A When you want to use interfaces

B When you need runtime reflection

C When you want to use classes

D When you want to avoid extra runtime code

Regular enums add runtime code, so avoid them if you want minimal JavaScript output.

Explain why regular enums in TypeScript can add unnecessary runtime code and how to avoid it.

Think about what code is generated and alternatives that only exist at compile time.

Describe the trade-offs between using regular enums and const enums in TypeScript.

Consider runtime presence and compatibility.