Heterogeneous enums in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a heterogeneous enum in TypeScript?

A heterogeneous enum is an enum that contains both string and numeric values in its members.

intermediate

Why might you use a heterogeneous enum?

You might use a heterogeneous enum to represent values that need both numbers and strings, for example, when some values are codes and others are descriptive labels.

beginner

How do you declare a heterogeneous enum in TypeScript?

You declare it by assigning some members string values and others numeric values explicitly inside the enum block.

beginner

Example: What is the output of this enum member access?

enum Example {
  No = 0,
  Yes = "YES"
}
console.log(Example.Yes);

The output will be the string "YES" because the member Yes is assigned the string value "YES".

intermediate

Can heterogeneous enums cause any issues in TypeScript?

Yes, they can make code harder to understand and maintain because the enum members are not consistent in type, which can confuse type checking and usage.

Which of the following is a valid heterogeneous enum in TypeScript?

A enum Mixed { A = 1, B = 2 }

B enum Mixed { A, B, C }

C enum Mixed { A = "A", B = "B" }

D enum Mixed { A = 1, B = "B" }

Only option A mixes numeric and string values, making it a heterogeneous enum.

What type of values can heterogeneous enums contain?

A Both numbers and strings

B Only strings

C Only numbers

D Only booleans

Heterogeneous enums contain both numeric and string values.

What will be the output of this code?

enum Status {
  Active = 1,
  Inactive = "NO"
}
console.log(Status.Inactive);

A 1

B "NO"

C undefined

D 0

Status.Inactive is assigned the string "NO", so that is printed.

Is this enum valid?

enum Test {
  One = "1",
  Two = 2
}

A No, enums cannot mix types

B No, string values are not allowed

C Yes, it's a heterogeneous enum

D Yes, but only if all values are strings

TypeScript allows enums to mix string and numeric values, making this a valid heterogeneous enum.

What is a potential downside of using heterogeneous enums?

A They can confuse type checking and maintenance

B They are slower to compile

C They cannot be used in switch statements

D They always cause runtime errors

Mixing types in enums can make code harder to understand and maintain.

Explain what a heterogeneous enum is and give a simple example.

Think about enums that mix numbers and strings.

Describe one advantage and one disadvantage of using heterogeneous enums.

Consider why mixing types might help or hurt your code.

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