Trusted answers to developer questions
Trusted Answers to Developer Questions

Related Tags

enums
typescript
communitycreator

What are enums in TypeScript?

Programming Bytes

Overview

Enum defines a set of named constants. In TypeScript, the value of the enum can be numeric or string.

Numeric enums

The code below demonstrates how to create numeric enum.

enum Direction {
  Up ,
  Down,
  Left = 8,
  Right,
}

console.log("Direction.Up :", Direction.Up);
console.log("Direction.Down :", Direction.Down);
console.log("Direction.Left :", Direction.Left);
console.log("Direction.Right :", Direction.Right);

console.log("\nThe Direction enum is \n", Direction);

console.log("\nReverse mapping in typescript ")
console.log(`Direction[${Direction.Up}] :`, Direction[Direction.Up]);
console.log(`Direction[${Direction.Down}] :`, Direction[Direction.Down]);
console.log(`Direction[${Direction.Left}] :`, Direction[Direction.Left]);
console.log(`Direction[${Direction.Right}] :`, Direction[Direction.Right]);
Numeric enum in TypeScript

Explanation

  • Lines 1–6: We define an enum, Direction with four members Up, Down, Left, Right. We didn't give an initial value for the first member, Up. TypeScript will initialize with 0 . The following members will have auto-incremented values from the previous member. We have set the 8 as value for the member Left so the value of the next member Right will be 9.
  • Lines 8–11: We print the members of the Direction enum.
  • Line 13: We print the Direction enum. On console, we'll notice that the enum contains additional members. This is due to the reverse mapping process of TypeScript. With this, we can access the value of the member using its name and the name of the member from its value.

String enums

It is similar to numeric enums but instead of the numeric value, it will have a string literal or another string enum member.

enum Greeting {
  HI = "hi",
  HELLO = "hello",
}

console.log("\nThe Greeting enum is \n", Greeting);
String enum in TypeScript

Explanation

In the code above, We create an enum called Greeting with two members, HI and HELLO. These members have the values, "hi" and "hello" respectively.

Note: String enum values need to be individually initialized, otherwise, we'll get compilation error. Also, the reverse mapping is not applied for String enums.

Heterogeneous enums

Heterogeneous enums contain both numeric and string values.

enum  ERROR_CODE{
  A  = 10,
  FILE_NOT_FOUND = "404",
}

console.log(ERROR_CODE)
Heterogeneous enum(contains numeric and string values)

Explanation

In the code above, we create an enum called ERROR_CODE with two members, A and FILE_NOT_FOUND. For the member, A, we assign the numeric value, 10. For the member, FILE_NOT_FOUND, we assign the string value, "404".

Constant enums

We can define a constant enum, which can improve the performance by removing the extra generated code and additional indirection when accessing enum values.

const enum ConstEnum {
  A = 1,
  B = 10,
}

console.log(ConstEnum.A)
console.log(ConstEnum.B)
Using const enums

Explanation

In the code above, We create a constant enum called ConstEnum with two members, A and B, with values 1 and 10 respectively. During the compilation of the above code the, ConstEnum.A and ConstEnum.B will be replaced with the value 1 and 10 respectively.

Computed enums

The values of the enum can be a constant or a computed value.

enum ComputedEnum {
  TEN = getTen(),
  TWENTY = TEN + 10,
}

function getTen(): number{
  return 10;
}
console.log(ComputedEnum)
Computed enum

Explanation

In the code above, we create a constant enum called ComputedEnum with two members, TEN and TWENTY. We create a function getTen that will return 10. We assign the value returned by calling the getTen() method as the value for the member, TEN. For the member TWENTY, the value will be TEN + 10.

Note: A complete guide to enums is present here and here.

RELATED TAGS

enums
typescript
communitycreator
RELATED COURSES

View all Courses

Keep Exploring