Numeric enums in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of numeric enum auto-increment

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

Typescript

enum Direction {
  Up,
  Down,
  Left,
  Right
}
console.log(Direction.Up, Direction.Down, Direction.Left, Direction.Right);

A 1 2 3 4

B 0 1 2 3

C Up Down Left Right

D undefined undefined undefined undefined

💡 Hint

Numeric enums start numbering from 0 by default unless specified.

By default, numeric enums start at 0 and increment by 1 for each member. So Up is 0, Down is 1, Left is 2, and Right is 3.

❓ Predict Output

intermediate

Custom starting value in numeric enum

What will this TypeScript code output when run?

Typescript

enum Status {
  Ready = 5,
  Waiting,
  Complete
}
console.log(Status.Ready, Status.Waiting, Status.Complete);

A 0 1 2

B 5 5 5

C 5 6 7

D undefined undefined undefined

💡 Hint

When you assign a number to the first member, the next members increment from that number.

The first member Ready is set to 5, so Waiting becomes 6 and Complete becomes 7 automatically.

🔧 Debug

advanced

Why does this enum cause an error?

This TypeScript enum code causes a compilation error. What is the reason?

Typescript

enum Colors {
  Red = 1,
  Green = Red + "2",
  Blue
}

A Enum members must be initialized with constant values or previous enum members without expressions.

B You cannot use string concatenation in numeric enum values.

C Blue must be assigned a value explicitly after Green uses an expression.

D Enums cannot mix string and numeric values.

💡 Hint

Enum values must be constant expressions for numeric enums.

Numeric enum members must be initialized with constant values or expressions that resolve to constants. Using string concatenation results in a non-constant expression, causing an error.

❓ Predict Output

advanced

Reverse mapping of numeric enums

What is the output of this TypeScript code?

Typescript

enum Weekday {
  Monday = 1,
  Tuesday,
  Wednesday
}
console.log(Weekday[2]);

A undefined

B 2

C "Monday"

D "Tuesday"

💡 Hint

Numeric enums create a reverse mapping from value to name.

Numeric enums in TypeScript create a reverse mapping, so Weekday[2] returns the name of the member with value 2, which is "Tuesday".

🧠 Conceptual

expert

Number of enum members after compilation

Given this TypeScript enum, how many keys will the compiled JavaScript object have?

Typescript

enum Levels {
  Low = 1,
  Medium = 3,
  High
}

A 6

B 3

C 5

D 4

💡 Hint

Remember that numeric enums create both forward and reverse mappings for each member.

Each enum member creates two keys: one for the name to value and one for the value to name. Since High is 4 (3 + 1), there are 3 members, so 3 * 2 = 6 keys.