MOD-10 validator in javascript

gistfile1.js

var validateNum = function(num){
	num = num.replace(/-/g,'');

	var calc, i, check, checksum = 0, r = [2,1]; // alternating routing table
	
	// iterate on all the numbers in 'num'
	for( i=num.length-1; i--; ){
		calc = num.charAt(i) * r[i % r.length];
		//console.log(num.charAt(i), r[i % r.length], [calc]);
		// handle cases where it's a 2 digits number
		calc = ((calc/10)|0) + (calc % 10);
		checksum += calc;
	}
	check = (10-(checksum % 10)) % 10; // make sure to get '0' if checksum is '10'
	checkDigit = num % 10;

	return check == checkDigit;
}

validateNum('740428-0401');

Warning: this is not a proper MOD-10 implementation. It works with most cards, but fails with AMEX cards (likely because they are an odd number of digits). I switched to this implementation instead: https://gist.github.com/DiegoSalazar/4075533