Better code with Typescript aliases

If you've ever worked with Typescript, you surely know what type aliases are for. They helps us create aliases of more complex types, for example arrays of interfaces, union types, etc.

One less-known way they can be leveraged is by simply describing values of primitive types based on their format.

This can accelerate the understanding of a codebase, reduce confusion, and deepen the insights gathered from just reading the types of your code.

For example, we have a DTO with two properties. This is how it would normally be written:

class MyDto {
  id: string;
  lastEdited: number;

Limitations of simple typing

The above is OK, and generally understandable, but can be better. Suppose it's your first day on a new codebase, and you know nothing about it.

You see the above DTO, but you're trying to understand what it is and what actual formats it expects from you:

  • you don't know what's the format of ID
  • you don't know the format of lastEdited. Is it a Unix timestamp, or a Julian day? Or the amount of days since Earth was created? Who knows.

So why not describe what theses values are with type aliases?

Aliasing primitive types based on their format

We can define the types in a global typings file, so they can be reused across the project.

Let's assume that the fields above are, as commonly happen to be, a UUID and a Unix timestamp.

type UUID = string;
type UnixTimestamp = number;

And then, we can assign them to our DTO:

class MyDto {
  id: UUID;
  lastEdited: UnixTimestamp;

In my opinion, the above conveys a lot more information than before, and it's certainly more useful to both existing members and newcomers.

Type Aliases as shortcuts

Type aliases are also useful to shortcut some common type combinations:

Converting inline interfaces to the Record alias:

const myObject: { [key: string]: string } = { property: 'key' };

// can also be written
const myObject: Record<string, string> = { property: 'key' };

The Record type aliases can also be aliased into more granular types:

type StringObject = Record<string, string>;
type UnknownObject = Record<string, unknown>;
type AnyObject = Record<any, any>;

const stringObject: StringObject = { property: 'key' };
const anyObject: AnyObject = {
  1: "3",
  hello: "world"

Aliases are great, sometime forgotten feature in Typescript. Hopefully, this will article will give you some ideas of how you can leverage them in your project!

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