Reverse mapping in numeric enums in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is reverse mapping in numeric enums in TypeScript?

Reverse mapping means you can get the name of an enum member by using its numeric value. TypeScript creates this automatically for numeric enums.

intermediate

How does TypeScript create reverse mapping for numeric enums?

TypeScript creates an object where both the name-to-value and value-to-name mappings exist. For example, Enum[Enum.Value] returns the name as a string.

beginner

Can reverse mapping be used with string enums in TypeScript?

No, reverse mapping only works with numeric enums. String enums do not have reverse mapping because string values are not unique keys.

beginner

Example: What will be the output of this code?

enum Color { Red, Green, Blue }
console.log(Color[1]);

The output will be "Green" because the numeric value 1 corresponds to the enum member Green.

intermediate

Why is reverse mapping useful in TypeScript numeric enums?

It helps to get the name of an enum member from its numeric value, which is useful for debugging, logging, or displaying friendly names.

What does reverse mapping in numeric enums allow you to do?

A Prevent enums from being changed

B Get the enum member name from its numeric value

C Automatically assign string values to enums

D Convert string enums to numeric enums

Reverse mapping lets you use the numeric value to find the enum member's name.

Does reverse mapping work with string enums in TypeScript?

A No, never

B Only if values are unique

C Yes, always

D Only in TypeScript 4.0+

Reverse mapping only works with numeric enums, not string enums.

Given enum Status { Active = 1, Inactive, Pending }, what is Status[2]?

A "Pending"

B "Active"

C "Inactive"

D undefined

Since Active=1, Inactive=2, Pending=3, Status[2] returns "Inactive".

What type of object does TypeScript create for numeric enums?

A Both name-to-value and value-to-name mappings

B Only value-to-name mapping

C Only name-to-value mapping

D No mapping, just constants

TypeScript creates an object with both mappings for numeric enums.

Why might reverse mapping be helpful when debugging?

A It shows the numeric value instead of the name

B It prevents errors in enum usage

C It converts enums to strings automatically

D It lets you see the enum member name from a numeric value

Reverse mapping helps by showing the enum member name from its numeric value.

Explain how reverse mapping works in numeric enums in TypeScript and why it is useful.

Think about how you can find the name if you only have the number.

Describe the difference between numeric enums and string enums regarding reverse mapping.

Consider how keys and values are stored in the enum object.