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.

Practice

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

easy

A. Prevent enums from being changed at runtime

B. Convert enum names to strings automatically

C. Convert enum numeric values back to their corresponding names

D. Create string enums instead of numeric enums

Solution

1. Step 1: Understand numeric enums

   Numeric enums assign numbers to names and create a two-way map.

2. Step 2: Understand reverse mapping

   Reverse mapping lets you use the number to get the enum name back.

3. Final Answer:

   Convert enum numeric values back to their corresponding names -> Option C

4. Quick Check:

   Reverse mapping = number to name conversion [OK]

Hint: Remember: numeric enums map both ways: name↔number [OK]

Common Mistakes:

• Thinking reverse mapping converts names to strings
• Confusing string enums with numeric enums
• Assuming enums are immutable at runtime

2. Which of the following is the correct syntax to access the name from a numeric enum value 2 in TypeScript?

easy

A. EnumName['name']

B. EnumName[2]

C. EnumName.getName(2)

D. EnumName.2

Solution

1. Step 1: Recall enum reverse mapping syntax

   Numeric enums create a reverse map accessible by bracket notation with the number.

2. Step 2: Identify correct syntax

   Using EnumName[2] returns the name for value 2.

3. Final Answer:

   EnumName[2] -> Option B

4. Quick Check:

   Bracket notation with number = Enum name [OK]

Hint: Use bracket notation with number to get enum name [OK]

Common Mistakes:

• Using dot notation with a number (invalid syntax)
• Trying to call a non-existent getName method
• Using string keys instead of numeric keys

3. Given the enum:

enum Color { Red, Green, Blue }

What is the output of console.log(Color[1])?

medium

A. "Green"

B. "Red"

C. 1

D. undefined

Solution

1. Step 1: Understand enum default values

   By default, Red=0, Green=1, Blue=2 in numeric enums.

2. Step 2: Use reverse mapping

   Color[1] returns the name for value 1, which is "Green".

3. Final Answer:

   "Green" -> Option A

4. Quick Check:

   Color[1] = "Green" [OK]

Hint: Default enum starts at 0, so 1 maps to second name [OK]

Common Mistakes:

• Assuming enum values start at 1
• Expecting numeric output instead of string
• Confusing enum name with value

4. Consider this enum:

enum Status { Active = 1, Inactive, Pending }

What is wrong with this code?

console.log(Status[0]);

medium

A. It returns undefined because 0 is not a valid enum value

B. It causes a syntax error

C. It returns "Active" incorrectly

D. It throws a runtime exception

Solution

1. Step 1: Check enum values

   Status starts at 1, so valid values are 1, 2, 3 for Active, Inactive, Pending.

2. Step 2: Check reverse mapping for 0

   Status[0] does not exist, so it returns undefined.

3. Final Answer:

   It returns undefined because 0 is not a valid enum value -> Option A

4. Quick Check:

   Invalid enum value returns undefined [OK]

Hint: Check enum start value before reverse mapping [OK]

Common Mistakes:

• Expecting 0 to map to first enum name
• Thinking it causes syntax or runtime error
• Assuming reverse mapping always returns a string

5. Given this enum with custom values:

enum Direction { Up = 3, Down = 5, Left = 7, Right = 9 }

What will console.log(Direction[5]) output?

hard

A. 5

B. "Up"

C. undefined

D. "Down"

Solution

1. Step 1: Identify enum values

   Direction enum assigns Up=3, Down=5, Left=7, Right=9 explicitly.

2. Step 2: Use reverse mapping

   Direction[5] returns the name for value 5, which is "Down".

3. Final Answer:

   "Down" -> Option D

4. Quick Check:

   Reverse mapping works with custom numeric values [OK]

Hint: Reverse mapping works even with custom enum numbers [OK]

Common Mistakes:

• Assuming reverse mapping only works with default values
• Expecting numeric output instead of string
• Confusing enum names with values