Javascript function that converts alphabetical styled ordered list letters to number

lettertonumber.js

function letterToNumber( str ) {
	
	var string = str.toUpperCase(),
		length = string.length,
		value,
		number = 0;
	
	// Validate string
	if( ! string.match( /^[A-Z]+$/ ) ) {
		
		throw new Error( 'String contains non-alphabetic characters' );
		
	}
	
	for( i = 0; i < length; i++ ) {
		
		value = string.charCodeAt( i ) - 64;
		
		number += value * Math.pow( 26, length - i - 1 );
		
	}
	
	return number;
	
}