Javascript function that converts roman numeral style ordered list strings to number

romantonumber.js

function romanToNumber( str ) {
	
	var string = str.toUpperCase(),
		letters = string.split( '' ),
		length = letters.length,
		i,
		mapping = { I: 1, V: 5, X: 10, L: 50, C: 100, D: 500, M: 1000 },
		values = [],
		valueA,
		valueB,
		number = 0;
	
	// Validate string
	if( ! string.match( /^M{0,3}(CM)?(CD|D?C{0,3})(XC)?(XL|L?X{0,3})(IX)?(IV|V?I{0,3})$/ ) ) {
		
		throw new Error( 'Not a valid roman numeral string' );
		
	}
	
	// Convert all roman numerals to numbers
	for( i = 0; i < length; i++ ) {
		
		values[ i ] = mapping[ letters[ i ] ];
		
	}
	
	// If only one value return it
	if( length == 1 ) {
		
		return values[ 0 ];
		
	}
	
	// Calculate number from values
	for( i = 0; i <= length; i++ ) {
		
		valueA = values[ i ];
		
		if( typeof valueB !== 'undefined' ) { // Not first
			
			if( valueA > valueB ) { // Left number smaller than right so negate
				
				number -= valueB;
				
			} else {
				
				number += valueB; // Left number is greater than right or right is undefind so add
				
			}
			
		}
		
		valueB = valueA;
		
	}
	
	return number;
	
}