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.

Practice

1.

What is a heterogeneous enum in TypeScript?

easy

A. An enum that contains both string and numeric values.

B. An enum that only contains numeric values.

C. An enum that only contains string values.

D. An enum that cannot be used with functions.

Solution

1. Step 1: Understand enum value types

   Enums in TypeScript can hold numeric or string values.

2. Step 2: Define heterogeneous enum

   A heterogeneous enum mixes both string and numeric values in the same enum.

3. Final Answer:

   An enum that contains both string and numeric values. -> Option A

4. Quick Check:

   Heterogeneous enum = mixed string and number values [OK]

Hint: Remember: mixed types in one enum means heterogeneous [OK]

Common Mistakes:

• Thinking enums can only have one type of value
• Confusing heterogeneous with union types
• Assuming enums can't mix strings and numbers

2.

Which of the following is the correct syntax to declare a heterogeneous enum?

enum Status {
  Active = 1,
  Inactive = "NO"
}

easy

A. Incorrect because enum members must be all strings.

B. Incorrect because enums cannot have string values.

C. Incorrect because enum members must be all numbers.

D. Correct syntax with mixed number and string values.

Solution

1. Step 1: Check enum member types

   The enum has one numeric value (1) and one string value ("NO").

2. Step 2: Validate TypeScript enum rules

   TypeScript allows heterogeneous enums mixing numbers and strings.

3. Final Answer:

   Correct syntax with mixed number and string values. -> Option D

4. Quick Check:

   Mixed values in enum = valid heterogeneous enum [OK]

Hint: Look for mixed number and string values in enum members [OK]

Common Mistakes:

• Assuming enums can't have string values
• Thinking all enum members must be same type
• Confusing enum syntax with object literals

3.

What will be the output of the following TypeScript code?

enum Mixed {
  Yes = 1,
  No = "NO"
}
console.log(Mixed.Yes);
console.log(Mixed.No);
console.log(Mixed[1]);

medium

A. Yes, NO, 1

B. 1, NO, Yes

C. 1, NO, undefined

D. NO, 1, Yes

Solution

1. Step 1: Check enum values

   Mixed.Yes is 1, Mixed.No is "NO".

2. Step 2: Understand reverse mapping

   Numeric enums have reverse mapping, so Mixed[1] returns "Yes".

3. Final Answer:

   1, NO, Yes -> Option B

4. Quick Check:

   Numeric enum reverse lookup works, string does not [OK]

Hint: Only numeric enum members have reverse mapping [OK]

Common Mistakes:

• Expecting string enum members to have reverse mapping
• Confusing enum member names with values
• Assuming Mixed[1] returns 1

4.

Identify the error in this heterogeneous enum declaration:

enum Colors {
  Red = "RED",
  Green,
  Blue = 3
}

medium

A. Enums cannot mix string and number values.

B. Blue cannot be assigned a number after a string member.

C. Green must have an explicit string or number value.

D. Red cannot be assigned a string value.

Solution

1. Step 1: Check enum member values

   Red is assigned a string "RED". Green has no value assigned.

2. Step 2: Understand enum auto-increment rules

   After a string member, numeric auto-increment is not allowed. Green must have explicit value.

3. Final Answer:

   Green must have an explicit string or number value. -> Option C

4. Quick Check:

   Members after string need explicit values [OK]

Hint: After string enum member, next must have explicit value [OK]

Common Mistakes:

• Assuming auto-increment works after string member
• Thinking enums can't mix types at all
• Ignoring missing value for Green

5.

You want to create a heterogeneous enum to represent user roles with numeric IDs and string labels. Which enum declaration correctly achieves this?

enum UserRole {
  Admin = 1,
  Guest = "GUEST",
  Member = 2
}

hard

A. This enum is valid and mixes numbers and strings correctly.

B. This enum is invalid because Guest must be a number.

C. This enum is invalid because Admin and Member must be strings.

D. This enum is invalid because Member cannot be a number after a string.

Solution

1. Step 1: Analyze enum member assignments

   Admin=1 (number), Guest="GUEST" (string), Member=2 (number). All explicitly assigned.

2. Step 2: Validate heterogeneous enum rules

   TypeScript allows explicit mixing of numeric and string values in enums.

3. Final Answer:

   This enum is valid and mixes numbers and strings correctly. -> Option A

4. Quick Check:

   Explicit assignments allow mixing numbers after strings [OK]

Hint: Explicit numeric assignments work after string members [OK]

Common Mistakes:

• Thinking numbers cannot follow strings even explicitly
• Confusing explicit assignments with auto-increment rules
• Assuming strict ordering prevents mixing