API stands for an application programming interface. It is a piece of code that makes it easier for your site's API to send or receive data from other sites. Basically, it's the bridge between your software and external tools in order to integrate them together. Similar to how Twitter, Facebook, Instagram, and other social media sites offer APIs on their platforms so other developers can build tools that tie into their products.

We can build API using any backend framework, But here are the best frameworks that are preferred and most used to build an API-based application.

1.Express.js

Express.js is an open-source, flexible, and modular web framework for Node.js server applications. Express offers a minimal set of features without imposing any particular structural hurdle on projects where the primary goal is rapid development, or to save time.

This framework has been written with scalability and information reuse in mind so that you can rapidly build powerful, highly scalable network applications without losing the sense.

2.Lumen

The Lumen Framework is a good backend framework used for API. It offers a lot of benefits such as: enabling rapid prototyping and deployment of scalable web applications, great performance, and decreased server resources.

Lumen is a minified, minimal version of the Laravel PHP framework and simultaneously speeds up development and reduces deployment maintenance.

3.Flask

Flask is one of the most popular Python backend frameworks for API. Flask is a microframework and as such, it doesn't require any dependencies, extensions, or changes to the standard library.

Nowadays flask is used for machine learning projects. One of the reasons why it is popular for ML engineers is because of the simplicity of use and the fact that it has a fairly high level of adaptability, and due to easy access to pandas, the python data analysis library, and other machine learning libraries.

4.Fast API

FastAPI is a high-performance web framework for building APIs with Python 3.6+, based on standard Python type hints. FastAPI is a backend framework for building APIs with all the things you need out of the box. It has a high performance and easily scales to millions of requests per minute, so it's ideal for handling huge amounts of traffic.

The advantages of using Fast API are fast coding, efficient development, fewer bugs, intuitive and easy to use.

5.Spring Boot

Spring Boot is a Java application framework for creating new applications, usually to serve front-end and cloud needs. It's a standard framework that is actually based on the more complex Java frameworks of Spring MVC, Spring WebSocket, and Spring Boot.

Spring Boot simplifies deploying applications and includes a number of features such as Embed Tomcat or Jetty for free without needing to deploy a WAR file. With "starters", Spring Boot allows you to create applications quicker.

Spring Boot provides production-ready features such as metrics, health checks, and externalized configuration without the need for XML configuration.