Reverse mapping in numeric enums in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of reverse mapping in numeric enum

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

Typescript

enum Colors {
  Red = 1,
  Green,
  Blue
}

console.log(Colors[2]);

A undefined

B "Green"

C 2

D "Blue"

💡 Hint

Remember numeric enums create a reverse mapping from value to name.

In numeric enums, each numeric value maps back to its name. Since Green has value 2, Colors[2] returns "Green".

❓ Predict Output

intermediate

Reverse mapping with custom numeric values

What will be logged by this code snippet?

Typescript

enum Status {
  Pending = 5,
  Active = 10,
  Completed = 15
}

console.log(Status[10]);

A undefined

B "Pending"

C "Active"

D 10

💡 Hint

Check which enum member has the value 10.

The enum member Active has the value 10, so Status[10] returns "Active".

🧠 Conceptual

advanced

Why reverse mapping does not exist for string enums

Why does reverse mapping not work for string enums in TypeScript?

A Because string enums are not supported in TypeScript

B Because string enum values are not unique and cannot be used as keys

C Because string enums are compiled to objects without reverse mapping properties

D Because TypeScript only generates reverse mapping for numeric enums

💡 Hint

Think about how TypeScript compiles numeric vs string enums.

TypeScript generates reverse mapping only for numeric enums because numeric values can be used as keys. String enums do not have reverse mapping generated.

❓ Predict Output

advanced

Output when accessing reverse mapping with invalid key

What is the output of this code?

Typescript

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

console.log(Directions[5]);

A undefined

B "Right"

C 5

D "Up"

💡 Hint

Check if the enum has a member with value 5.

No enum member has the value 5, so Directions[5] returns undefined.

🧠 Conceptual

expert

How reverse mapping affects enum object size

How does reverse mapping in numeric enums affect the size of the compiled enum object?

A It doubles the number of properties because each numeric value maps back to its name

B It reduces the size by removing duplicate keys

C It has no effect on the size of the enum object

D It triples the size because it adds extra metadata

💡 Hint

Think about how many keys exist in the compiled enum object compared to the number of enum members.

Numeric enums generate both name-to-value and value-to-name mappings, effectively doubling the number of properties in the compiled object.