Postman and API Testing

API testing tools let you send requests to an API and see exactly what it answers, without building a website or app first.

What is an API Request?

When an app talks to a server, it sends an HTTP request with:

The server returns a status code, headers and usually a JSON body.

Postman

Postman is the most popular API testing app. Download the free desktop app, or use it in the browser. Similar free tools are Insomnia, Bruno and the Thunder Client extension for VS Code.

Your First GET Request

1. Click New, then HTTP Request.
2. Keep the method as GET.
3. Enter the URL https://jsonplaceholder.typicode.com/users/1 (a free practice API).
4. Click Send.

Postman shows the response:

Status: 200 OK   Time: 182 ms   Size: 1.2 KB

{
  "id": 1,
  "name": "Leanne Graham",
  "username": "Bret",
  "email": "Sincere@april.biz",
  ...
}

Sending a POST Request

1. Change the method to POST and the URL to https://jsonplaceholder.typicode.com/posts.
2. Open the Body tab, choose raw, and select JSON.
3. Enter the body and click Send:

{
  "title": "Admissions open",
  "body": "New batch starts next Monday.",
  "userId": 1
}

Status: 201 Created

{
  "title": "Admissions open",
  "body": "New batch starts next Monday.",
  "userId": 1,
  "id": 101
}

This practice API pretends to save the post and returns it with a new id.

Status Codes to Know

Headers and Authorization

Many APIs need a key or token. In Postman, open the Authorization tab, choose Bearer Token and paste it, or add a header yourself in the Headers tab:

Authorization: Bearer YOUR_TOKEN_HERE

Collections and Environments

• A collection is a folder of saved requests, for example all requests of your shop API. You can share it with your team.
• An environment stores variables like {{baseUrl}}. Set it to http://localhost:3000 while developing and to your live URL later, and every request switches at once.

Testing from the Terminal with curl

curl is a command-line tool included in Windows 10+, macOS and Linux:

curl https://jsonplaceholder.typicode.com/users/1

curl -X POST https://jsonplaceholder.typicode.com/posts -H "Content-Type: application/json" -d "{\"title\":\"Hello\",\"userId\":1}"

{
  "title": "Hello",
  "userId": 1,
  "id": 101
}

Add -i to also see the status code and headers. curl is handy on servers, where there is no Postman.

Exercise