JavaScript snippet for generating the checkdigit of an USPS tracking number

genCheckDigitUSPSTracking.js

// using USPS Barcode Construct N04

// padding function, if not available
Object.prototype.pad = function(size) {
	var targetHelper = String(this);
	if (targetHelper.length<size){
		targetHelper = Array((size - targetHelper.length)+1).join("0") + targetHelper;
	}
	return targetHelper;
}

// modulo10 function
function USPSMod10(trackingNumberRaw){
	// take all numbers on even positions
	var evenNumbers = 0;
	var oddNumbers = 0;
	for (var i=0; i<=20; i++){
		if (i % 2 == 0){
			evenNumbers += parseInt(trackingNumberRaw[i]);
		}
		if (i % 2 == 1){
			oddNumbers += parseInt(trackingNumberRaw[i]);
		}
	}
	evenNumbers *= 3;
	var addedNumbers = evenNumbers + oddNumbers;
	// subtract from 10 to get the number that needs to be added
	var checkDigit = (10 - (addedNumbers % 10)) % 10;
	return String(checkDigit);
}

var trackingNumber="";
// this fields have to be populated accordingly
var servicetypecode = "";
var mailerid = "";
var packageid =""; 

// channelAppID
trackingNumber += "94";
// ServiceTypeCode
trackingNumber += servicetypecode.pad(3);
// Source Identifier
trackingNumber += "20";
// MailerId
trackingNumber += mailerid.pad(9);
// Package Id
trackingNumber += packageid.pad(5);

// CheckDigit
trackingNumber += USPSMod10(trackingNumber);

// this generates a clickable link
var trackingLinkFormula = 'https://tools.usps.com/go/TrackConfirmAction_input?qtc_tLabels1=' + trackingNumber;