Based on Simon Willison's original 2005 eTech talk, Simon expands this into a full article covering the expressive functionality of JavaScript. He covers the basics of JavaScript: literals, variables, functions, scope, control flow, objects, classes, inner functions and closures. Everything a developer needs to know before diving into more advanced JavaScript.
Jim Ley covers the intricacies of type conversion, implicit and explicit. He covers type conversion into boolean, string, number, undefined, null; and parsing into floats and integers. This is backed up by conversion tables for quick reference. There's also a useful section on regular expressions for form field validation.