dto

A generic typescript utility type that transforms a given type into something that can go over the wire. For example, Sets are turned into arrays and optional values (denoted by ? or | undefined) are converted to null.

I made this playground as a way to safely iterate on the DTO type without regression

But why?

Here's a blog post where I explain: https://dev.to/tamj0rd2/dto-a-typescript-utility-type-4o3m

How this repo works

The DTO is located in src/dto.ts

The tests are located in the fixtures folder.

Each test declares a type that uses a DTO. For example, type numberDTO = DTO<number>. The lines following that are test cases, some of which I expect to cause errors and some of which I expect to compile successfully.

For example, taking a look at /fixtures/primitive/optional-string:

import { Dto } from '~dto'

// this is a DTO of a string
export type StringDto = Dto<string>

// I expect this not to give compiler errors because a string can be sent over the wire without any transformations
export const dtoGood: StringDto = 'Hello world!'

// I expect this to fail because numbers are not strings
export const dtoBadNumber: StringDto = 28947

// I expect this to fail because we haven't made our string optional
export const dtoBadUndefined: StringDto = undefined

// I expect this to fail because bools are not strings
export const dtoBadBool: StringDto = false

// I expect this to fail because we haven't made our string optional
export const dtoBadNull: StringDto = null

// I expect this to fail because objects are not strings
export const dtoBadObject: StringDto = { hello: 'world' }

If you open up the fixtures files in your editor, you should easily be able to see which lines pass and which lines fail

Run npm run test to see and capture the typescript compiler output as a jest snapshot