JavaScript is a powerful, high-level, dynamic programming language that is among critical pillars in web development. As a staple of the World Wide Web, JavaScript enables dynamic interactivity on websites when applied on an HTML document. It can update and change both HTML and CSS, calculate, manipulate and validate data.

Developed by Mozilla Foundation's Brendan Eich in 1995, JavaScript started as a client-side scripting language for adding interactivity on websites. It has now evolved to be an integral part of full-stack development being used on both front-end and back-end through Node.js. Consequently, JavaScript powers millions of applications on the global web today.

Expertise in JavaScript involves understanding and using features such as AJAX for asynchronous programming, DOM manipulation, Event Handling, Promises, and Application Programming Interfaces (APIs). You'll need to learn and understand different JavaScript frameworks (like AngularJS, ReactJS, VueJS) and libraries that make development quicker and more efficient.

A competent JavaScript developer should also have a detailed understanding of JavaScript specific syntax and rules such as ES6 features, control flow and error handling. Proficiency in using version control/Git could be advantageous. Also, a real-world JavaScript developer needs to understand concepts like asynchronous programming, closures, types, scoping, inheritance, 'this' keyword, function passing, JSON parsing, and JavaScript object literals among others.

Understanding the basics of web technologies like HTML and CSS is required as a foundation for learning JavaScript. Prior knowledge or experience in other programming languages (such as Python or Java) can also be beneficial though not strictly mandatory. Key competencies that would aid the learning or mastering of JavaScript include problem-solving skills, strong mathematical abilities, resilience, persistence and importantly, an appetite for continuous learning given the rapidly evolving nature of JavaScript and the wider world of web development.

JavaScript is not just a useful skill but a powerful tool for creating dynamic and interactive web applications. It has become an essential skill for any modern developer, therefore, mastery of JavaScript opens up a wide array of opportunities not only in web development but also in other areas such as mobile app development, server-side development, game development and even in building internet of things (IoT) devices.