JavaScript has evolved into a crucial tool for modern web development, facilitating both frontend and backend tasks, and a significant part of its functionality involves interacting with APIs. This guide explores how to use REST APIs with JavaScript, detailing key concepts such as CRUD operations—Create, Read, Update, and Delete—and how they correspond to HTTP request methods like POST, GET, PUT, and DELETE. It emphasizes the importance of understanding endpoints in API interactions and demonstrates how to use the axios library for handling API requests. The tutorial walks through creating a simple JavaScript application that integrates with an API to build a 'Comment Sentiment Analyzer' app, which analyzes user comments for sentiment using the Text-Processing API available on RapidAPI. By registering for an API key and testing endpoints, developers can integrate API functionality into their applications, exemplified by a step-by-step creation of a sentiment analysis tool that provides user feedback on comment tone.