TypeScript Enum

Summary: in this tutorial, you’ll learn about the TypeScript enum type and how to use it effectively.

What is an enum

An enum is a group of named constant values. Enum stands for enumerated type.

To define an enum, you follow these steps:

• First, use the enum keyword followed by the name of the enum.

• Then, define constant values for the enum.

The following shows the syntax for defining an enum:

enum name {constant1, constant2, ...};

In this syntax, the constant1, constant2, etc., are also known as the members of the enum.

TypeScript enum type example

The following example creates an enum that represents the months of the year:

enum Month {
    Jan,
    Feb,
    Mar,
    Apr,
    May,
    Jun,
    Jul,
    Aug,
    Sep,
    Oct,
    Nov,
    Dec
};

In this example, the enum name is Month and constant values are Jan, Feb, Mar, and so on.

The following declares a function that uses the Month enum as the type of the month parameter:

function isItSummer(month: Month) {
  let isSummer: boolean;
  switch (month) {
    case Month.Jun:
    case Month.Jul:
    case Month.Aug:
      isSummer = true;
      break;
    default:
      isSummer = false;
      break;
  }
  return isSummer;
}

And you can call it like so:

console.log(isItSummer(Month.Jun)); // true

This example uses constant values including Jan, Feb, Mar, … in the enum rather than magic values like 1, 2, 3,… This makes the code more obvious.

How TypeScript enum works

It’s a good practice to use the constant values defined by enums in the code.

However, the following example passes a number instead of an enum to the isItSummer() function. And it works.

console.log(isItSummer(6)); // true

This example uses a number (6) instead of a constant defined by the Month enum. And it works.

Let’s check the generated Javascript code of the Month enum:

var Month;
(function (Month) {
    Month[Month["Jan"] = 0] = "Jan";
    Month[Month["Feb"] = 1] = "Feb";
    Month[Month["Mar"] = 2] = "Mar";
    Month[Month["Apr"] = 3] = "Apr";
    Month[Month["May"] = 4] = "May";
    Month[Month["Jun"] = 5] = "Jun";
    Month[Month["Jul"] = 6] = "Jul";
    Month[Month["Aug"] = 7] = "Aug";
    Month[Month["Sep"] = 8] = "Sep";
    Month[Month["Oct"] = 9] = "Oct";
    Month[Month["Nov"] = 10] = "Nov";
    Month[Month["Dec"] = 11] = "Dec";
})(Month || (Month = {}));

And you can output the Month variable to the console:

{
  '0': 'Jan', 
  '1': 'Feb', 
  '2': 'Mar', 
  '3': 'Apr', 
  '4': 'May', 
  '5': 'Jun', 
  '6': 'Jul', 
  '7': 'Aug', 
  '8': 'Sep', 
  '9': 'Oct', 
  '10': 'Nov',
  '11': 'Dec',
  Jan: 0,     
  Feb: 1,     
  Mar: 2,     
  Apr: 3,     
  May: 4,
  Jun: 5,
  Jul: 6,
  Aug: 7,
  Sep: 8,
  Oct: 9,
  Nov: 10,
  Dec: 11
}

The output indicates that a TypeScript enum is an object in JavaScript. This object has named properties declared in the enum. For example, Jan is 0 and Feb is 1.

The generated object also has number keys with string values representing the named constants.

That’s why you can pass a number into the function that accepts an enum. In other words, an enum member is both a number and a defined constant.

Specifying enum members’ numbers

TypeScript defines the numeric value of an enum’s member based on the order of that member that appears in the enum definition. For example, Jan takes 0, Feb gets 1, etc.

It’s possible to explicitly specify numbers for the members of an enum like this:

enum Month {
    Jan = 1,
    Feb,
    Mar,
    Apr,
    May,
    Jun,
    Jul,
    Aug,
    Sep,
    Oct,
    Nov,
    Dec
};

In this example, the Jan constant value takes 1 instead of 0. The Feb takes 2, and the Mar takes 3, etc.

When to use an enum

You should use an enum when you:

• Have a small set of closely related fixed values.

• And these values are known at compile time.

For example, you can use an enum for the approval status:

enum ApprovalStatus {
    draft,
    submitted,
    approved,
    rejected
};

Then, you can use the ApprovalStatus enum like this:

const request =  {
    id: 1,
    status: ApprovalStatus.approved,
    description: 'Please approve this request'
};

if(request.status === ApprovalStatus.approved) {
    // send an email
    console.log('Send email to the Applicant...');
}

Summary

• A TypeScript enum is a group of constant values.

• Under the hood, an enum is a JavaScript object with named properties declared in the enum definition.

• Do use an enum when you have a small set of fixed values that are closely related and known at compile time.