C's #include

gistfile1.js

//looks for comments at the beginning of a line looking like this:
//#build fileName
//and replaces them with fileName contents
//throws an error when fileName points to a file which doesn't exist

//first parameter is the file on which to operate,
//second parameter is a function which is called at the of operation with the
// new content
//no changes are done to the original file

var fs = require( 'fs' ),
	path = require( 'path' );

var build = exports.build = function ( filePath, cb ) {
	var lastIndex = 0, index,
		source,
		instruction = '//#build ';

	fs.readFile( filePath, 'utf8', function ( err, data ) {
		if ( err ) {
			throw err;
		}
		source = data;

		preprocess(function cont () {
			//this is the continuation function, which is called when either
			// the processor finished doing its job, or if the processor
			// skipped a match for any reason.
			// it sets the index for the next match to begin with, and
			// calls the preprocess function again. recursion ftw
			lastIndex = index + instruction.length;
			preprocess( cont );
		});
	});

	function preprocess ( next ) {
		index = source.indexOf( instruction, lastIndex );

		if ( index < 0 ) {
			finish();
			return;
		}

		//check to see if you're at the beginning of a line or at the beginning
		// of the file
		var offset, targetName = '';

		if ( index === 0 || source[index-1] === '\n' ) {
			offset = index + instruction.length;

			//capture the filename:
			//#build blah.js
			//       [-----]  <-- this part, everything until a newline or EOF
			while (
				source[offset] &&
				source[offset] !== '\n' && source[offset] !== '\r'
			) {
				targetName += source[ offset++ ];
			}

			//check to see if the file requested exists
			path.exists( targetName, function ( exists ) {
				if ( !exists ) {
					throw new Error(
						'Cannot #build unexisting file ' + targetName +
						' (in ' + filePath + ')'
					);
				}

				embedFile( targetName );
			});
		}
		else {
			next();
		}

		function embedFile ( targetName ) {
			fs.readFile( targetName, 'utf8', function ( err, data ) {
				if ( err ) {
					throw err;
				}

				//replace the comment with the file content
				//TODO: make this better
				//this seems INCREDIBLY inefficient, and should be replaced with
				// some better solution
				source =
					source.slice( 0, index ) +
					data +
					source.slice(
						index + instruction.length + targetName.length
					);
				next();
			});
		}
	}

	function finish () {
		cb( source );
	}
};

//if we were executed directly,
if ( module === require.main ) {
	var inpName = process.argv[ 2 ] || 'test.js',
		outName = process.argv[ 3 ] || 'result.js';

	build( inpName, function ( data ) {
		fs.writeFile( outName, data, 'utf8', function ( err ) {
			if ( err ) {
				throw err;
			}

			console.log( 'complete' );
		});
	});
}