Enum vs union literal type trade-offs in Typescript - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of enum vs union literal type usage

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

Typescript

enum Direction {
  Up = "UP",
  Down = "DOWN"
}

function move(dir: Direction | "LEFT" | "RIGHT") {
  switch (dir) {
    case Direction.Up:
      return "Moving up";
    case Direction.Down:
      return "Moving down";
    case "LEFT":
      return "Moving left";
    case "RIGHT":
      return "Moving right";
  }
}

console.log(move(Direction.Up));
console.log(move("LEFT"));

A

"Moving up"
"Moving left"

B

"Moving Up"
"Moving Left"

C TypeError at runtime

D

undefined
undefined

💡 Hint

Check how enum values and union literal strings are compared in the switch.

The enum Direction has string values "UP" and "DOWN". The function move accepts either Direction enum or union literals "LEFT" or "RIGHT". The switch matches the exact string values, so Direction.Up is "UP" which matches case Direction.Up and returns "Moving up". The string "LEFT" matches case "LEFT" and returns "Moving left".

🧠 Conceptual

intermediate

Trade-off: Enum vs Union Literal Types for Type Safety

Which statement best describes a key trade-off between using enums and union literal types in TypeScript?

A Enums cannot be used in switch statements, but union literals can.

B Enums provide runtime objects and can be iterated, but union literals offer better type narrowing and simpler code.

C Union literals create runtime objects, while enums exist only at compile time.

D Union literals allow adding new values at runtime, enums do not.

💡 Hint

Think about what exists at runtime and how TypeScript narrows types.

Enums compile to real JavaScript objects, so you can iterate over them or access their values at runtime. Union literal types exist only at compile time and provide better type narrowing and simpler syntax but no runtime representation.

🔧 Debug

advanced

Why does this enum code cause a runtime error?

Consider this TypeScript code snippet: ```typescript enum Status { Active, Inactive, Pending } function checkStatus(s: Status) { if (s === Status.Pending) { console.log("Pending status"); } } checkStatus(3); ``` What is the cause of the runtime error?

A The function checkStatus is missing a return statement.

B Enums cannot be compared with === operator.

C The enum values are numeric and 3 is not a valid Status value, causing undefined behavior.

D The enum Status is not exported, causing a reference error.

💡 Hint

Check the numeric values assigned to enum members and what happens when you pass 3.

The enum Status assigns Active=0 (default), Inactive=1, Pending=2. Passing 3 is allowed because numeric enums accept any number, but 3 does not correspond to a defined enum member value (Status.Pending is 2), so the comparison fails and it can cause logic errors or undefined behavior if invalid values are not handled.

📝 Syntax

advanced

Which option causes a syntax error in union literal type declaration?

Which of the following union literal type declarations is invalid TypeScript syntax?

A type Directions = Up | Down | Left | Right;

B type Colors = "red" | "green" | "blue";

C type Sizes = 'small' | 'medium' | 'large';

D type Status = "active" | "inactive" | "pending";

💡 Hint

Check if the literals are quoted strings or identifiers.

Union literal types must be string or number literals (quoted). Option A uses unquoted identifiers (Up, Down, Left, Right) which are not valid string literals and cause a syntax error.

🚀 Application

expert

Choosing between enum and union literal for a feature flag system

You are designing a feature flag system in TypeScript where flags can be "enabled", "disabled", or "experimental". You want to ensure: - The flags have a runtime representation for logging. - The code is easy to extend with new flags. - Type safety is strict to prevent invalid flag values. Which approach best balances these requirements?

A Use a string enum with explicit string values for each flag.

B Use a union literal type of strings without any enum.

C Use numeric enums and map them to strings at runtime.

D Use a plain object with string constants and a union literal type derived from its values.

💡 Hint

Consider runtime representation, type safety, and ease of extension.

Using a plain object with string constants provides a runtime object for logging and iteration. Deriving a union literal type from the object's values ensures strict type safety and easy extension by adding new keys. String enums provide runtime objects but are less flexible to extend and can be verbose. Numeric enums add complexity with mapping. Union literals alone lack runtime representation.