Reverse mapping in numeric enums in Typescript

Introduction

Reverse mapping helps you find the name of an enum member when you have its numeric value. It makes working with enums easier and clearer.

When you get a numeric value and want to know which enum name it matches.

When debugging and you want to print the enum name instead of just a number.

When you receive numeric enum values from an API and want to convert them back to readable names.

Syntax

Typescript

enum EnumName {
  Member1 = 0,
  Member2 = 1,
  Member3 = 2
}

// Reverse mapping example:
let name = EnumName[1]; // 'Member2'

Numeric enums automatically create reverse mappings from value to name.

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

Examples

This prints the name 'Green' because 1 maps to Green in the enum.

Typescript

enum Colors {
  Red = 0,
  Green = 1,
  Blue = 2
}

console.log(Colors[1]);

Since Waiting is 6 (5 + 1), Status[6] returns 'Waiting'.

Typescript

enum Status {
  Ready = 5,
  Waiting,
  Done
}

console.log(Status[6]);

Reverse mapping finds 'Left' from the numeric value 3.

Typescript

enum Direction {
  Up = 1,
  Down = 2,
  Left = 3,
  Right = 4
}

let dirName = Direction[3];
console.log(dirName);

Sample Program

This program shows how to get the enum name from a number using reverse mapping.

Typescript

enum Weekday {
  Monday = 1,
  Tuesday,
  Wednesday,
  Thursday,
  Friday
}

// Get name from number
const dayName = Weekday[3];
console.log(`Day 3 is ${dayName}`);

Important Notes

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

If you assign custom numbers, reverse mapping uses those numbers to find names.

Be careful: if two members have the same value, reverse mapping returns the last one defined.

Summary

Numeric enums create a two-way map: name to number and number to name.

Reverse mapping helps convert numbers back to enum names easily.

This feature is useful for debugging and working with numeric enum values.