Rich Domain

25th Jan 2019

Rich Domain

In this post I want to go beyond React and Redux and explore ways in which we can improve the readability of our code by creating a rich domain of types that describe the data better.

This series:

Using Type Aliases to Build a Rich Description of the Domain

The point of the rich domain is to enhance the underlying types in such a way that it adds further semantic meaning. Secondly to provide more certainty to what data is being passed around

In this post we'll be looking at a few kinds of data which are ambiguous when described as a primitive type. We'll then explore some examples of how we can add more semantic meaning along with some guidelines to help prevent some of the problems you might find.


Let's start by looking at dates. Dates are a good example of a piece of data which can be described in many forms and stored in multiple ways.

A date could be passed around in memory as a JavaScript Date object, you often see dates saved using a library like Moment which enhance the date object. If it has been stored as a result of an api call it could be in a serialised format like a string or a number. If it was a string it is more than likely going to be in an ISO format.

Seeing a variable typed as a Date or a Moment is often sufficient. However, let's consider the serialized types of string and number which don't convey any "rich" meaning.

Dates as Strings

Describing a dateOfBirth as a string is ok, but you gain a lot more meaning from describing it as a ISODateString.

Here are three examples of a string type, they will all compile if you interchange the values but the types communicate more about what kind of string is expected.

type ISODateString = string
type ISODateTimeString = string
const birthName: string
const dateOfBirth: ISODateString = '2018-01-01'
const created: ISODateTimeString = '2018-01-01T10:00Z'

Dates as Numbers

Describing a dateOfBirth as a number is also ok for the compiler, but you gain a lot more meaning for the developer by describing it as MillisecondsDateNumber or SecondsDateNumber. These are two real-world ways in which numbers are used to describe a date.

type MillisecondsDateNumber = number
type SecondsDateNumber = number
const age: number = 39
// 01 Jan 1980
const dateOfBirth: SecondsDateNumber = 315532800
// 01 Jan 2019
const created: MillisecondsDateNumber = 1546300800000

Looking at IDs

IDs are another example of values that come in all shapes and sizes. Giving them more semantics really helps developers understand the data they are working with.

IDs as Strings

Describing an id field as a string would be accurate for a whole range of identifiers but there is a very big difference between a user id of davetayls, a JIRA id of PU-1234, a UUID of ab39ae77-c1cd-4490-a114-eb9776c2f321 or a MondoDb ObjectId of 507f1f77bcf86cd799439011

type UserIdString = string
type JiraIdString = string
type UUIDString = string
type ObjectIdString = string
const id: UserIdString = 'davetayls'
const id: JiraIdString = 'PU-1234'
const id: UUIDString = 'ab39ae77-c1cd-4490-a114-eb9776c2f321`'
const id: ObjectIdString = '507f1f77bcf86cd799439011'

Guidelines for Naming Rich Types

There are some gotchas with these but following these good practices for naming rich types will help prevent them.

Masked primitive type

It is easy to mask the underlying primitive type such that it actually loses semantic meaning and makes the type harder to understand.

To make sure this doesn't happen, always include the primitive name with the rich name. For example, you should append "String" or "Number" at the end of the name.

The type UserId is quite ambiguous but UserIdString is not.


I have found a rich domain of types to be a real boost to developer understanding of the data they are working with and I would whole-heartedly recommend you make use of them within your codebase.