The majority of JavaScript applications are designed to run on the client side, that is, in the client's browser. Browser-based environments are excellent for development because browsers are ubiquitous, you can work on the code right on your local machine, the interpreter is the browser's JavaScript engine, and all browsers have a developer console. Firefox's FireBug provides very useful error messages and allows for break-points and more, but it's often helpful to run the same code in Chrome and Safari to cross-reference the error output. Even Internet Explorer contains developer tools.

The problem with browsers is that they evaluate JavaScript differently! Though it's not common, it is possible to write code that returns very different results in different browsers. But usually the differences are in the way they treat the document object model and not how prototypes and functions work. Obviously, Math.sqrt(4) method returns 2 to all browsers and shells. But the scrollLeft method depends on the browser's layout policies.

Writing browser-specific code is a waste of time, and that's another reason why libraries should be used.