Heterogeneous enums in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a heterogeneous enum value

What is the output of this TypeScript code snippet?

Typescript

enum Status {
  Ready = 1,
  Waiting = "WAIT",
  Done = 3
}
console.log(Status.Waiting);

A undefined

B 3

C "WAIT"

D 1

💡 Hint

Look at the value assigned to the Waiting member.

The enum member Waiting is explicitly assigned the string "WAIT", so logging Status.Waiting outputs "WAIT".

❓ Predict Output

intermediate

Value of enum member with mixed types

Given this heterogeneous enum, what is the value of Status.Done?

Typescript

enum Status {
  Ready = 1,
  Waiting = "WAIT",
  Done = 3
}
const doneValue = Status.Done;
console.log(doneValue);

A 3

B "WAIT"

C 1

D undefined

💡 Hint

Check the explicit value assigned to Done.

Done is explicitly assigned the numeric value 3, so doneValue is 3.

❓ Predict Output

advanced

Reverse mapping behavior of heterogeneous enums

What happens if you try to get the enum key from a string value in a heterogeneous enum?

Typescript

enum Status {
  Ready = 1,
  Waiting = "WAIT",
  Done = 3
}
console.log(Status["WAIT"]);

A Throws a runtime error

B "Waiting"

C 1

D undefined

💡 Hint

Reverse mapping works only for numeric enum members.

Reverse mapping is only generated for numeric enum members. Since "WAIT" is a string, Status["WAIT"] is undefined.

❓ Predict Output

advanced

Accessing enum keys from numeric values in heterogeneous enums

What is the output of this code?

Typescript

enum Status {
  Ready = 1,
  Waiting = "WAIT",
  Done = 3
}
console.log(Status[3]);

A "Done"

B "Waiting"

C 3

D undefined

💡 Hint

Reverse mapping works for numeric values.

Since Done is assigned 3, Status[3] returns the key "Done".

🧠 Conceptual

expert

Why are heterogeneous enums discouraged in TypeScript?

Which of the following is the main reason why using heterogeneous enums (mixing string and numeric values) is discouraged in TypeScript?

A They cause syntax errors during compilation.

B They break reverse mapping for string members, causing inconsistent behavior.

C They prevent enums from being used in switch statements.

D They increase runtime performance overhead significantly.

💡 Hint

Think about how reverse mapping works in enums.

Heterogeneous enums break reverse mapping for string members, which can cause confusion and bugs.