What is an API?

• API (Application Programming Interface)

  A set of rules and definitions that allow different software applications to communicate with each other.

  Example: A weather app fetches live weather data from a third-party weather service API.

• Why Use APIs?

  • To access functionality or data from another system.
  • To create modular and reusable applications.
  • To allow communication between frontend and backend.

The image below illustrates the fundamental idea behind an API (Application Programming Interface) by showing how a system defines a contract that specifies how a user interacts with it.

Key Points:

• System as a Contract Provider:

  The system defines a contract that includes:

  • What inputs it expects.
  • What outputs it will provide.
  • How the user can interact with the system.

• Browser and JS:

  The lower part of the image shows how the Browser exposes various functionalities through Browser APIs to JavaScript (JS) running in the browser:

  • JavaScript interacts with browser functionalities (like timers, events, etc.) via these APIs.

  • Example:

    setTimeout(fn, t) is a Browser API that schedules a function (fn) to run after t milliseconds.

  Here, the Browser defines the contract (the API) that JS follows to interact with features like timers (functions F1, F2, F3 in the diagram).

<aside> 💡

There are recommended practices for implementing APIs. While not following these practices won't break your code unless there are algorithmic issues, it may make your APIs harder for others to read and understand if they don't follow standard conventions.

</aside>

How Network APIs Work:

1. Client Sends a Request:
   • A user interacts with a UI or a program sends a request over a network.
   • Example: A browser makes a GET request to https://api.example.com/users.
2. Server Receives the Request:
   • The server processes the request — possibly fetching, creating, updating, or deleting data.
   • It then sends a response back to the client.
3. Client Receives the Response:
   • The response typically contains a status code and data (usually in JSON format).
   • The client uses this data to update the UI or perform further operations.

What is a REST API?