How to Create API Documentation with Insomnia Documenter

API documentation are instructions about how to use and integrate with an API. They are basically just a manual containing all the information required to work with the API. The information includes endpoints, request structure, response structure etc. One of the most commonly used way of API documentation is by using Swagger. But there are other ways to do so.

Why API Documentation Matters for Delivery Quality

For product engineering teams, clear API documentation is not just a nice-to-have it’s critical for:

• Faster onboarding: New developers can integrate without endless clarifying questions.
• Reduced rework: Clear contracts mean fewer integration bugs and misunderstandings.
• Better client relationships: External teams can consume your API independently.
• Maintainability: When APIs evolve, documentation prevents breaking changes from going unnoticed.

Our deployment workflow guides often start with proper API contracts because they affect every downstream system.

In this blog, we will create an API documentation using Insomnia Documenter which is in a human-friendly term.

Create a server

You can avoid this step if you already have a running project or you can clone this fastify-boilerplate

First we can create a server with 5 sample API endpoints.

• SignIn
• SignUp
• View User
• Edit User
• Delete User

I am using nodejs and express for this.

If you don’t have nodejs or npm installed in your system, please install it.

1) Create a folder

mkdir sample-api

2) Initialize npm

cd sample-api
npm init -y

3) Add package and create a JS file

npm i express
touch index.js

4) Add the following code to index.js

const express = require('express')

const app = express()
app.use(express.json())

app.listen(process.env.PORT || 3000, () => {
  console.log('Server Running in Port 3000')
})

app.post('/signUp', (req, res) => {
  console.log(req.body)
  res.send({ message: 'signUp Route', data: req.body })
})
app.post('/signIn', (req, res) => {
  console.log(req.body)
  res.send({ message: 'SignIn Route', data: req.body })
})
app.get('/user/:id', (req, res) => {
  console.log(req.params)
  res.send({ message: 'User Display Route', data: req.params })
})
app.put('/user', (req, res) => {
  console.log(req.body)
  res.send({ message: 'User Update Route', data: req.body })
})
app.delete('/user', (req, res) => {
  console.log(req.body)
  res.send({ message: 'User Delete Route', data: req.body })
})

5) Start the server

node index.js

You can see the message Server Running in Port 3000 in the terminal

Test with insomnia

Now into the real stuff.

1. First we need to download and install insomnia.

2. Open insomnia and create a new request collection at top right corner. We will name it blog-insomnia-doc

   You can add environment variables in insomnia, but for an easy implementation lets just avoid that.

3. Create a new folder called Auth using the + icon and create 2 new post requests, inside the Auth folder.

   1. signUp:- http://localhost:3000/signUp
   2. signIn:- http://localhost:3000/signIn

4. In the same way create another folder called User using + sign and add

   1. Display:- http://localhost:3000/user/:id
   2. Edit:- http://localhost:3000/user
   3. Delete:- http://localhost:3000/user

5. You can add JSON body for POST, PUT and DELETE requests as per your interest.

The final result of insomnia would be as below

Add Documentation

• We now need to configure insomnia-documenter.

• You can add description to the requests in the insomnia, which would be displayed in the documentation. We can add description by clicking on the arrow on the right side of the request tab.

• Add description to the requests.

• Export the Insomnia v4 (JSON) using Import/Export…** on the Dashboard/blog-insomnia-doc-

• Open the terminal in the exported folder

• Run the below command

npx insomnia-documenter --config filename.json --output insomnia-final-result

If success

Getting files ready...
Adding Insomnia JSON...

 * * * Done! * * *
Your documentation has been created and it's ready to be deployed!

View Documentation Locally

cd insomnia-final-result
npx serve

This will create a server running in http://localhost:5000 or any other port depending on your system.

• Open a browser and hit the url.

Need help building robust APIs and documentation?

Our product engineering team specializes in building well-documented APIs that scale. Whether you’re modernizing legacy systems or building new web application architecture, we can help ensure your APIs are clear, maintainable, and developer-friendly.

Talk to our product engineering services team to discuss your API strategy.

This article originally appeared on lightrains.com