Understanding Enums in TypeScript

In TypeScript, the feature used for defining a variable that can take one of a set of predefined constants is called an Enum. It allows you to define a new type and assign it a limited range of possible values, often used when you want to model a selection between a set of mutually exclusive options or constants.

How to Define an Enum

To define an enum in TypeScript, you'll need enum keyword. With it, you can define a new enum type. The syntax is as follows:

enum DaysOfWeek {
    Monday,
    Tuesday,
    Wednesday,
    Thursday,
    Friday,
    Saturday,
    Sunday
}

Each attribute in the enum holds a numeric value, starting from 0 at the first attribute. However, you can override these values, if desired:

enum DaysOfWeek {
    Monday = 1,
    Tuesday = 2,
    //...
}

Use an Enum

Once the enum is defined, you can then declare a variable of this type. Here's how you do that:

let day: DaysOfWeek;
day = DaysOfWeek.Monday;

In this example, day can only ever take a value that is a member of DaysOfWeek. If you try to assign it another value, TypeScript will throw an error.

Using enums allows you to write more legible and less error-prone code by creating a new type that has a predefined range of constants. It narrows down the possible values and reduce the chances of unexpected behavior.

Further Insights

Though Enums can make our code more readable and less prone to errors, they need to be used thoughtfully. Using Enums everywhere can also make the code harder to read and understand, especially for others. Enums are best used in cases when a variable needs to have one among the limited preset options, as in days of a week, months of a year, or directions (North, South, East, West), and so on.