find all require() calls by walking the AST

 This modules allows one to determine modules depends by
 walking AST tree and detecting require() call.
 .
 In computer science, an abstract syntax tree (AST),
 or just syntax tree, is a tree representation of the abstract
 syntactic structure of source code
 .
 This a dependency of Browserify is a JavaScript tool that allows
 developers to write Node.js-style modules that compile for use in
 the browser.
 .
 Node.js is an event-based server-side JavaScript engine.