React (Frontend Framework)
Created by Facebook, React has a lead over the other frontend frameworks in being statistically the most-popular web framework on the frontend. Also, I have found that most job postings for frontend JavaScript positions will usually require at-least some familiarity with React.
Outside of its utility in the job market, React also takes a lot of the headache out of traditional webpage loading on the frontend. Traditionally, rendering HTML elements dynamically using JavaScript requires a lot of repeated code to be written. DOM elements have to be created manually and added using a sequence of steps. By extrapolating this process down into components, React makes the process more-scalable and reduces the chances of bugs caused by code duplication.
Angular (Frontend Framework)
The second leading frontend framework on the list is Angular. Made by Google, Angular takes a different approach on the component-based design introduced by React. Built in TypeScript rather than JavaScript, pages created with React are compiled into JavaScript by the TypeScript compiler. This supposedly reduces the chance of hard-to-catch bugs, since the TypeScript compiler will pick up data types for you as you go.
In addition to its usage of TypeScript, Angular is known for having a full suite of developer tools that connect the frontend and the backend. Angular is also popular in the single-page web application development space.
Express (Backend Framework)
In contrast to the other two JavaScript frameworks mentioned that work primary on the frontend, Express is a server-side framework for the popular Node.JS JavaScript runtime.
In contrast to most backend frameworks that require knowledge of object-oriented programming languages such as C# or Java, Node.JS allows you to apply the knowledge that you learned in JavaScript to code server-side web applications. This allows you to work with form data on the back end (such as respond to HTTP POST requests) as well as connect to and interact with a database.
As the most popular backend framework for Node.JS, Express simplifies the API introduced in vanilla Node.JS. For example, express allows you to serve static (non-changing) web content without hard-coding each file on your web server. Express also makes it more-convenient to read form data from HTTP requests.
Summary
Since JavaScript is such a versatile language, your choice of framework will largely depend on your use-case. For example, React may be more-suitable for a team of frontend developers who simply want to reduce code redundancy without importing a huge framework. Angular may be a better fit for full-stack developers who want increased code validation or want to create singe-page web applications. Finally, for those frontend developers who know JavaScript and want to delve into backend web development, Express is a great backend framework that takes a lot of the hassle out of the vanilla Node.JS API.