Enum member access in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of accessing enum members by name and value?

Consider the following TypeScript enum and code snippet. What will be printed to the console?

Typescript

enum Direction {
  Up = 1,
  Down,
  Left,
  Right
}

console.log(Direction.Up);
console.log(Direction[2]);

A 1\nDown

B Up\n2

C 1\nDown\nLeft

D 1\nundefined

💡 Hint

Remember that enums in TypeScript can be accessed both by member name and by numeric value.

Direction.Up is assigned 1 explicitly, so console.log(Direction.Up) prints 1. Direction[2] accesses the enum member with value 2, which is 'Down'. So the output is '1' and then 'Down'.

❓ Predict Output

intermediate

What happens when accessing a non-existent enum member by value?

Given this enum and code, what will be the output?

Typescript

enum Status {
  Active = 1,
  Inactive = 3,
  Pending = 5
}

console.log(Status[2]);

A Inactive

B Error: Property does not exist

C 2

D undefined

💡 Hint

Check if the enum has a member with the value 2.

The enum Status has members with values 1, 3, and 5. There is no member with value 2, so Status[2] returns undefined.

🔧 Debug

advanced

Why does this enum member access cause a runtime error?

Examine the code below. Why does accessing Color['Blue'] cause a runtime error?

Typescript

enum Color {
  Red = 'RED',
  Green = 'GREEN'
}

console.log(Color['Blue']);

A It throws a TypeError because string enums do not support reverse mapping.

B Accessing Color['Blue'] throws a ReferenceError because 'Blue' is not declared.

C Color enum does not have a member named 'Blue', so accessing it returns undefined, causing no error.

D It prints 'Blue' because enums allow dynamic keys.

💡 Hint

Check how string enums behave when accessed by keys that don't exist.

Accessing Color['Blue'] returns undefined because 'Blue' is not a member of the enum. This does not cause a runtime error; it just returns undefined.

❓ Predict Output

advanced

What is the output when accessing numeric enum members with computed values?

Look at this enum and code. What will be the output?

Typescript

enum FileAccess {
  None,
  Read = 1 << 1,
  Write = 1 << 2,
  ReadWrite = Read | Write,
  G = '123'.length
}

console.log(FileAccess.ReadWrite);
console.log(FileAccess[3]);
console.log(FileAccess.G);

A 6\nG\n3

B 6\nundefined\n3

C 6\nReadWrite\n3

D 6\nWrite\n3

💡 Hint

Calculate the bitwise OR and the length of the string.

Read is 2 (1 << 1), Write is 4 (1 << 2), so ReadWrite is 6 (2 | 4). FileAccess[3] is undefined because 3 is not assigned to any member. G is 3 ('123'.length). FileAccess.G is 3.

❓ Predict Output

expert

What is the output of this mixed enum member access?

Analyze the following enum and code. What will be printed?

Typescript

enum Mixed {
  A = 1,
  B,
  C = 'C',
  D = 5
}

console.log(Mixed.A);
console.log(Mixed.B);
console.log(Mixed.C);
console.log(Mixed[2]);
console.log(Mixed[5]);
console.log(Mixed['C']);

A 1\n2\nC\nB\nD\nC

B 1\n2\nC\nundefined\nD\nundefined

C 1\n2\nC\nundefined\nD\nC

D 1\n2\nC\nB\nD\nundefined

💡 Hint

Remember that string enum members do not create reverse mappings.

A is 1, B is 2 (auto increment). C is a string 'C'. D is 5. Mixed[2] is 'B' (reverse mapping). Mixed[5] is 'D' (reverse mapping). Mixed['C'] is 'C' (direct property access).