Const enums and optimization in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a <code>const enum</code> in TypeScript?

A <code>const enum</code> is a special kind of enum that is completely inlined at compile time, meaning no JavaScript code is generated for the enum itself. Instead, enum values are replaced directly with their numeric or string values.

intermediate

How does using <code>const enum</code> improve performance?

Using <code>const enum</code> reduces the generated JavaScript code size and runtime overhead because enum references are replaced with literal values, avoiding extra object lookups.

advanced

What happens if you try to use a <code>const enum</code> with the <code>--isolatedModules</code> flag in TypeScript?

The compiler will give an error because <code>const enum</code> values are inlined and not emitted as code, which conflicts with isolated modules that require each file to be independently compilable.

beginner

Example: What is the output JavaScript for this TypeScript code?<br><pre>const enum Colors { Red, Green, Blue }
let c = Colors.Green;</pre>

The compiled JavaScript will be:<br><pre>let c = 1;</pre><br>Because <code>Colors.Green</code> is replaced with its numeric value <code>1</code> directly.

intermediate

Why should you avoid using <code>const enum</code> in libraries published as npm packages?

Because <code>const enum</code> values are inlined at compile time, consumers of the library must also compile the TypeScript source. If they use plain JavaScript or different compilation settings, the enum values may not be available, causing errors.

What does a const enum generate in the compiled JavaScript?

A An error during compilation

B A full enum object with keys and values

C No enum object; values are inlined as literals

D A function to access enum values

Const enums are fully inlined, so no enum object is generated in JavaScript.

Which TypeScript compiler flag can cause errors when using const enum?

A --isolatedModules

B --noImplicitAny

C --strictNullChecks

D --esModuleInterop

The --isolatedModules flag requires each file to be independently compilable, which conflicts with const enums.

Why might const enum improve runtime speed?

A Because it avoids object property lookups

B Because it caches enum values in memory

C Because it uses async functions

D Because it compiles to WebAssembly

Inlining enum values avoids the cost of looking up properties on an enum object at runtime.

What is a risk of using const enum in distributed JavaScript code?

A It causes runtime memory leaks

B Enum values may be missing if code is not compiled with TypeScript

C It slows down the code execution

D It increases bundle size

Since const enums are inlined at compile time, if the consumer uses plain JavaScript, enum values won't exist.

Which of these is a correct way to declare a const enum?

A const Direction enum { Up, Down }

B enum const Direction { Up, Down }

C enum Direction const { Up, Down }

D const enum Direction { Up, Down }

The correct syntax is const enum Name { ... }.

Explain what a const enum is and how it helps optimize TypeScript code.

Think about how the enum values appear in the final JavaScript.

Describe a situation where using const enums might cause problems in a project.

Consider how code is shared and compiled across different environments.