Real live example of OpenAPI in a front-end project

By Maarten Van Hoof

2 min read


In the previous parts of this series, we introduced you to the OpenAPI specification, how to describe an API provider in an OpenAPI document, the approaches we can take incorporating it on workflows helping us build and integrate with RESTful APIs, and the tooling that is available to optimise our OpenAPI workflows. In this final part of the series, we will show you how we can use OpenAPI in a front-end monorepo project.

I've created a demo repository showing the power of OpenAPI and automation in a front-end project.

vanhoofmaarten/openapi-monorepo-demo (

Project structure

The project is set up as a monorepo. Each domain lives as a separate entity in the packages folder, but because they are very much interdependent, they are within the same repository.

β”œβ”€β”€ packages/
β”‚   β”œβ”€β”€ frontend/
β”‚   β”‚   β”œβ”€β”€ index.html
β”‚   β”‚   β”œβ”€β”€ main.ts
β”‚   β”‚   └── ...
β”‚   β”œβ”€β”€ mock/
β”‚   β”‚   └── ...
β”‚   β”œβ”€β”€ openapi/
β”‚   β”‚   β”œβ”€β”€ petstore.yml
β”‚   β”‚   └── ...
β”‚   └── sdk/
β”‚       β”œβ”€β”€ (generated API client)
β”‚       └── ...
β”œβ”€β”€ package.json
└── ...

OpenAPI document

The OpenAPI document is fetched from a different repository using Git submodules. It contains its own linting rules and own automation. We include it in our project to refer to the OpenAPI document itself.


The sdk folder only contains the configuration and scripts for generating a TypeScript-based API client by openapi-typescript-codegen. Git ignores generated files.

"scripts": {
  "dev": "npm run generate",
  "generate": "openapi --input ../openapi/petstore.yml --output ./src && tsc"


The mocks folder contains the configuration and scripts to run the Prism mocking server.

"scripts": {
  "dev": "prism mock -d ../openapi/petstore.yml"


The front-end folder contains a Vite application dependent on the SDK folder. In the main.ts file, we import the PetsService, set the SDK base URL, and fetch data with the listPets method. The retrieved data will be outputted in the HTML element with the id preview.

// Import the generated API client
import { OpenAPI, PetsService } from '../sdk/dist'

// Set the base URL of the API client
OpenAPI.BASE = 'http://localhost:4010'

// Fetch data
const data = await PetsService.listPets(3243)

// Output the data in the HTML
const preview = document.getElementById('preview')

if (preview) {
  preview.innerHTML = JSON.stringify(data, null, 2)


With the help of Lerna, a tool to orchestrate monorepo tasks, when running npm run dev, our entire development will start with just one command. First, it will generate the SDK. Then it will start the Vite dev environment and the mocking server in parallel.

➜ npm run dev

> dev
> lerna run --parallel --scope @mrtnvh/*-sdk dev && lerna run --parallel --scope @mrtnvh/*-frontend --scope @mrtnvh/*-mock dev

lerna notice cli v5.1.6
lerna notice filter including "@mrtnvh/*-sdk"
lerna info filter [ '@mrtnvh/*-sdk' ]
lerna info Executing command in 1 package: "npm run dev"
@mrtnvh/frontend-loves-openapi-demo-sdk: > @mrtnvh/frontend-loves-openapi-demo-sdk@0.0.0 dev
@mrtnvh/frontend-loves-openapi-demo-sdk: > npm run generate
@mrtnvh/frontend-loves-openapi-demo-sdk: > @mrtnvh/frontend-loves-openapi-demo-sdk@0.0.0 generate
@mrtnvh/frontend-loves-openapi-demo-sdk: > openapi --input ../openapi/petstore.yml --output ./src && tsc
lerna success run Ran npm script 'dev' in 1 package in 2.0s:
lerna success - @mrtnvh/frontend-loves-openapi-demo-sdk

lerna notice cli v5.1.6
lerna notice filter including ["@mrtnvh/*-frontend","@mrtnvh/*-mock"]
lerna info filter [ '@mrtnvh/*-frontend', '@mrtnvh/*-mock' ]
lerna info Executing command in 2 packages: "npm run dev"
@mrtnvh/frontend-loves-openapi-demo-frontend: > @mrtnvh/frontend-loves-openapi-demo-frontend@0.0.0 dev
@mrtnvh/frontend-loves-openapi-demo-frontend: > vite

@mrtnvh/frontend-loves-openapi-demo-mock: > @mrtnvh/frontend-loves-openapi-demo-mock@0.0.0 dev
@mrtnvh/frontend-loves-openapi-demo-mock: > prism mock -d ../openapi/petstore.yml

@mrtnvh/frontend-loves-openapi-demo-frontend:   vite v2.9.13 dev server running at:
@mrtnvh/frontend-loves-openapi-demo-frontend:   > Local: http://localhost:3000/
@mrtnvh/frontend-loves-openapi-demo-frontend:   > Network: use `--host` to expose
@mrtnvh/frontend-loves-openapi-demo-frontend:   ready in 81ms.

@mrtnvh/frontend-loves-openapi-demo-mock: [2:51:01 PM] β€Ί [CLI] …  awaiting  Starting Prism…
@mrtnvh/frontend-loves-openapi-demo-mock: [2:51:01 PM] β€Ί [CLI] β„Ή  info      GET
@mrtnvh/frontend-loves-openapi-demo-mock: [2:51:01 PM] β€Ί [CLI] β„Ή  info      POST
@mrtnvh/frontend-loves-openapi-demo-mock: [2:51:01 PM] β€Ί [CLI] β„Ή  info      GET
@mrtnvh/frontend-loves-openapi-demo-mock: [2:51:01 PM] β€Ί [CLI] β–Ά  start     Prism is listening on

When we visit the front-end development server on the provided address, typically at http://localhost:3000, we see our list of mocked data, verifying that our solution works.


Using this project structure as default in all our projects has helped us reduce project setup time significantly. We can now focus on the end-user, building features that will improve the final product, rather then having to spend time on bootstrapping it.

If you'd made it here so far, thank you for reading! I hope you enjoyed this and that OpenAPI help you improve your API development workflow.