If you’ve ever touched a more traditional Object-Oriented Language (C, C#, Java), you’ve probably seen an enum data type. While JavaScript does not have support for enums, Flow’s typechecking can make up for it.

Where are Enums Useful?

Let’s say you’re writing a program that deals with poker chips. The color of a chip is a string (i.e. ‘red’, 'blue’, et cetera). So your Chip type would look something like:

type Chip = {
  color: string,
};

…and a function that would create chips would be:

function createChip(color: string): Chip {
  return {
    color,
  };
}

But you want to make sure that it only uses valid colors:

// This should be valid.
createChip('red');
// This should not.
createChip('cyan');
// This isn't even a color.
createChip('Nicomachean Ethics');

Creating an Enum

The simplest way to define an enum is as a union of string literals:

// 'red', 'blue', and 'green' are valid colors.
type Color = 'red' | 'blue' | 'green';

From here, you can use the type Color in place of string:

type Chip = {
  // `color` can only be a valid color.
  color: Color,
};

// `createChip` only takes values of type `Color`.
function createChip(color: Color): Chip {
  return {
    color,
  };
}

Creating an Enum From an Existing Value

Poker chips have value depending on the color. So let’s say that you have an object to look up values somewhere in your project:

const chipValues = {
  red: 5,
  blue: 10,
  green: 25,
};

The Color type is repeating the keys of chipValues It’s not so bad when there are just three, but it’s still duplication and it’ll get annoying if more are added. Flow provides a way to derive an enum from the keys of an object with the $Keys type:

type Color = $Keys<typeof chipValues>;