JavaScript's primary and original environment is the web browser. It was created to add interactivity to web pages, and it is executed by a JavaScript engine embedded within browsers like Chrome (V8), Firefox (SpiderMonkey), and Safari (JavaScriptCore). While server-side environments like Node.js exist, JavaScript's most fundamental and defining characteristic is its role as the scripting language for the client-side web, running directly inside the user's browser to manipulate the DOM and communicate with servers.

B. JavaScript code does not automatically implement across all browsers; developers must often handle cross-browser compatibility issues.

C. JavaScript is an interpreted or just-in-time (JIT) compiled language, meaning it does not require a separate, explicit compilation step before execution.

D. Client-side JavaScript is not hidden. The source code is sent to the browser and can be viewed by the user via developer tools.

1. MDN Web Docs. (2023). What is JavaScript?. Mozilla. Retrieved from https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Firststeps/WhatisJavaScript. (See the "So what is it, really?" section).

2. Ecma International. (2023). ECMAScript® 2023 Language Specification. Standard ECMA-262, 14th edition. Retrieved from https://tc39.es/ecma262/. (The entire specification defines the language that runs in browsers).

3. MIT OpenCourseWare. (2014). Lecture 1: Introduction, HTTP, HTML, CSS. 6.170 Software Studio. Retrieved from https://ocw.mit.edu/courses/6-170-software-studio-spring-2014/resources/mit6170s14lec1/. (The lecture introduces JavaScript as a client-side language for browsers).