Postman Introduction

Postman is an application used for creating, sending, testing and debugging API requests.

For example, instead of opening a React Native application to test an API, you can send the request directly from Postman.

Postman
   ↓
REST API
   ↓
Response

Postman makes API development easier because you can test an API independently from the frontend or mobile application.

• Send API requests.
• Test HTTP methods.
• View API responses.
• Check status codes.
• Inspect request headers.
• Test JSON data.
• Test authentication.
• Debug API problems.

Postman works very well with REST APIs because REST APIs communicate using HTTP requests and responses.

Postman
   ↓
HTTP Request
   ↓
REST API
   ↓
HTTP Response
   ↓
Postman

You can inspect the complete communication in Postman.

Postman is not a replacement for React Native. Instead, it can be used to test the backend API before connecting that API to the mobile application.

First:

Postman → REST API

Then:

React Native → REST API

This makes it easier to identify whether a problem is in the API or in the mobile application.

Postman can be used to test many parts of a REST API.

A Postman request contains information such as the HTTP method, URL, headers, parameters and optional request body.

GET
https://example.com/api/students

When you click Send, Postman sends the request to the server.

After sending a request, Postman displays the response returned by the server.

The response can include:

• Status code
• Response time
• Response headers
• Response body
• Cookies

200 OK

{
    "success": true,
    "message": "Students found"
}

Postman provides a method selector that allows you to choose the HTTP method for a request.

GET
POST
PUT
PATCH
DELETE

The selected method determines how the request is sent to the API.

A GET request is commonly used to retrieve information.

GET /api/students

Example response:

{
    "success": true,
    "data": [
        {
            "id": 1,
            "name": "Rahul"
        }
    ]
}

A POST request is commonly used to create new data.

POST /api/students

Example JSON body:

{
    "name": "Rahul",
    "email": "rahul@example.com",
    "course": "React Native"
}

PUT is commonly used to update an existing resource.

PUT /api/students/10

Example body:

{
    "name": "Rahul Kumar",
    "email": "rahul@example.com",
    "course": "React Native"
}

PATCH is commonly used when only part of an existing resource needs to be updated.

PATCH /api/students/10

Example:

{
    "course": "Full Stack Development"
}

DELETE is commonly used to remove a resource.

DELETE /api/students/10

Example response:

{
    "success": true,
    "message": "Student deleted successfully"
}

Every API request requires a URL identifying the endpoint that should receive the request.

https://example.com/api/students

For a local PHP API, the URL may look like:

http://localhost/rest_api/api/students.php

After selecting the method and entering the API URL, the Send button sends the request to the server.

Select Method
      ↓
Enter URL
      ↓
Configure Request
      ↓
Click Send
      ↓
View Response

Headers provide additional information about an HTTP request.

Common API headers include:

Content-Type: application/json
Accept: application/json
Authorization: Bearer TOKEN

Headers are configured in the request's Headers section.

When an API expects JSON, Postman can be used to send a JSON request body.

{
    "name": "Amit",
    "email": "amit@example.com",
    "course": "React Native"
}

For a JSON request, the Content-Type should normally indicate application/json.

Postman displays the HTTP status code returned by the server.

The response body contains the data returned by the API.

{
    "success": true,
    "message": "Student found",
    "data": {
        "id": 1,
        "name": "Rahul"
    }
}

Postman displays the response body so that developers can inspect whether the API is returning the expected data.

Postman is particularly useful for testing a PHP REST API before connecting it to React Native.

Postman
   ↓
PHP REST API
   ↓
MySQL Database
   ↓
PHP Response
   ↓
Postman

This allows you to verify the backend independently.

Suppose our API endpoint is:

http://localhost/rest_api/api/students.php

We can select GET in Postman and click Send.

The API may return:

{
    "success": true,
    "data": [
        {
            "id": 1,
            "name": "Rahul"
        }
    ]
}

Postman can help identify problems in an API by showing the actual request and response.

For example, if an API returns 400 Bad Request, you can inspect:

• URL
• HTTP method
• Headers
• Query parameters
• Request body
• Response body

Postman can also be used to test authenticated APIs.

For example:

Authorization:
Bearer YOUR_JWT_TOKEN

This becomes especially useful when we build JWT authentication for our React Native application.

Create API
    ↓
Test API in Postman
    ↓
Fix API Problems
    ↓
Test Again
    ↓
Connect React Native
    ↓
Test Mobile Application

Testing the backend first can make mobile application development easier.

• Using the wrong API URL.
• Selecting the wrong HTTP method.
• Sending invalid JSON.
• Forgetting the Content-Type header when required.
• Using incorrect authorization information.
• Using the wrong query parameter.
• Testing the wrong endpoint.
• Ignoring the HTTP status code.

For React Native developers, Postman is useful for testing the backend before writing the mobile API integration code.

For example:

First:

Postman → GET /api/students

Then:

React Native → GET /api/students

If the API works correctly in Postman, you can use the same endpoint from React Native.

Postman allows API requests to be organized into collections.

For example, a Student API collection can contain:

• Get Students
• Get Student
• Add Student
• Update Student
• Delete Student
• Login
• Register

Collections make it easier to keep related API requests together.

Open Postman
      ↓
Create Request
      ↓
Select HTTP Method
      ↓
Enter API URL
      ↓
Add Headers
      ↓
Add Body if Required
      ↓
Click Send
      ↓
Check Status Code
      ↓
Check JSON Response
      ↓
Debug if Required

Suppose your PHP REST API is running at:

http://localhost/rest_api/api/students.php

In Postman:

1. Select GET.
2. Enter the API URL.
3. Click Send.
4. Check the status code.
5. Check the response body.

If the API is working correctly, you may receive:

{
    "success": true,
    "message": "Students found",
    "data": []
}

Postman is an important tool for REST API development. It allows us to test the backend independently before connecting it to a React Native application.

Postman
   ↓
HTTP Request
   ↓
PHP REST API
   ↓
MySQL
   ↓
JSON Response
   ↓
Postman

In the upcoming lessons, we will learn how to install and configure Postman and then create different API requests such as GET, POST, PUT, PATCH and DELETE.