Const enums and optimization in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of const enum in compiled JavaScript

What is the output of the following TypeScript code when compiled to JavaScript and run?

Typescript

const enum Colors {
  Red = 1,
  Green,
  Blue
}

console.log(Colors.Green);

A 1

B undefined

C 2

D "Green"

💡 Hint

Remember that const enums are inlined during compilation.

Const enums are replaced by their values during compilation. Colors.Green is 2 because Red starts at 1 and Green increments by 1.

🧠 Conceptual

intermediate

Why use const enums for optimization?

Which of the following best explains why const enums improve runtime performance in TypeScript?

A They generate extra code to check enum values at runtime.

B They inline enum values, removing object lookups at runtime.

C They create a separate enum object to reduce memory usage.

D They delay enum value evaluation until runtime.

💡 Hint

Think about what happens to enum references in the compiled code.

Const enums are replaced by their literal values during compilation, so no enum object is created and no lookup is needed at runtime.

❓ Predict Output

advanced

Effect of const enum on emitted JavaScript code

Given this TypeScript code, what will the compiled JavaScript output be?

Typescript

const enum Status {
  Success = 0,
  Failure = 1
}

function check(status: Status) {
  if (status === Status.Success) {
    return "OK";
  }
  return "Error";
}

console.log(check(Status.Failure));

A "Error"

B ReferenceError: Status is not defined

C "Failure"

D "OK"

💡 Hint

Const enums are replaced by their values, so no enum object exists at runtime.

The function compares the input to 0 (Success) and returns "OK" if matched, else "Error". Status.Failure is replaced by 1, so the output is "Error".

🔧 Debug

advanced

Why does this code cause a runtime error?

Consider this TypeScript code snippet: const enum Directions { Up, Down, Left, Right } console.log(Directions); Why does this cause a runtime error when compiled and run?

A Because the enum has duplicate values causing a conflict.

B Because the enum values are strings, which cannot be logged directly.

C Because the enum is not exported and cannot be accessed outside its module.

D Because const enums are removed during compilation and no Directions object exists at runtime.

💡 Hint

Think about what happens to const enums in the compiled JavaScript.

Const enums are completely removed during compilation and replaced by their values. The Directions object does not exist at runtime, so logging it causes a ReferenceError.

🚀 Application

expert

Optimizing large enum usage with const enums

You have a large enum used only for numeric constants in performance-critical code. Which approach best optimizes runtime speed and reduces output size?

A Use a const enum so all references are inlined as numeric literals.

B Use a regular enum and access values via the enum object at runtime.

C Use string enums to improve readability and debugging.

D Use a namespace with constant variables instead of enums.

💡 Hint

Inlining values removes object lookups and reduces code size.

Const enums inline their values at compile time, eliminating runtime object lookups and reducing code size, which improves performance in critical code.