Using Enums in TypeScript

An enum (enumeration) is a feature of TypeScript that allows you to define a set of constant values under a single name.

To define an enum, the enum keyword is used followed by the name of the enum and a block of braces that contains the values of the enum.

enum NombreEnum {
    VALOR1,
    VALOR2,
    VALOR3
}

Let’s see it with an example,

enum Direccion {
    Norte,
    Sur,
    Este,
    Oeste
}

let direccionActual: Direccion = Direccion.Norte;

console.log(direccionActual); // Output: 0
console.log(Direccion[direccionActual]); // Output: Norte

Numeric Enums

By default, enums in TypeScript are numeric. Each value in the enum is assigned to a number, starting from 0 and increasing by 1 for each subsequent value.

Customizing numeric values

You can customize the numeric values assigned to each member of the enum.

enum Estado {
    Inactivo = 0,
    Activo = 1,
    Pausado = 2
}

let estadoActual: Estado = Estado.Activo;

console.log(estadoActual); // Output: 1

Automatic increment

If a specific numeric value is defined for a member of the enum, subsequent values will automatically increment from that value.

enum Semana {
    Lunes = 1,
    Martes,
    Miercoles,
    Jueves,
    Viernes
}

console.log(Semana.Lunes); // Output: 1
console.log(Semana.Martes); // Output: 2
console.log(Semana.Viernes); // Output: 5

String Enums

In addition to numeric enums, TypeScript also allows defining string enums, where each member of the enum is assigned to a string.

enum Color {
    Rojo = "Rojo",
    Verde = "Verde",
    Azul = "Azul"
}

let colorFavorito: Color = Color.Verde;

console.log(colorFavorito); // Output: Verde

In this example, Color is a string enum that defines three values: Rojo, Verde, and Azul.

Heterogeneous Enums

Although it is not a common practice (nor probably advisable 😅), TypeScript allows defining enums that mix numeric and string values.

enum Mixto {
    No = 0,
    Si = "Sí"
}

console.log(Mixto.No); // Output: 0
console.log(Mixto.Si); // Output: Sí

Practical Examples

Comparisons

Enums are useful for making clear and readable comparisons in code.

enum Respuesta {
    No = 0,
    Si = 1
}

function responder(respuesta: Respuesta): void {
    if (respuesta === Respuesta.Si) {
        console.log("Positive response.");
    } else {
        console.log("Negative response.");
    }
}

responder(Respuesta.Si); // Output: Positive response.

Iterating over enums

You can iterate over the values of an enum using loops and the Object.keys function.

enum Talla {
    Pequeña,
    Mediana,
    Grande
}

for (let talla in Talla) {
    if (isNaN(Number(talla))) {
        console.log(talla); // Output: Pequeña, Mediana, Grande
    }
}