Code snippets to be used in two separate Hubspot workflows to create a referral program. The first will generate a random number (referral ID) the second will be used to attribute referrals back to the referring contact.

workflow-1

//1. Import required libaries
const Promise = require("bluebird"); // Javascript Promise Library
const randomNumber = require("random-number-csprng"); // Javascript Random Number Generator

exports.main = (event, callback) => {

    //2. Using a Promise, we first try to generate a random number
    Promise.try(function() {
        return randomNumber(10000, 50000);  // Generate Random Number between a defined range
    }).then(function(number) { // When the number is successfully generated we do this
        // 3. Pass data back to worklfow using outputFields - make sure to also define data output types
        callback({
            outputFields: {
                referrer_id: number
            }
        });
    }).catch({ // If there are any issues we can throw an error
        code: "RandomGenerationError"
    }, function(err) {
        console.log("Something went wrong!");
    });
}

workflow-2

//1. Import required libraries
const hubspot = require('@hubspot/api-client'); // HubSpot Node Client will allow us to make calls to HubSpot API

exports.main = (event, callback) => {

    //2. Create our client
    const hubspotClient = new hubspot.Client({
        accessToken: process.env.ReferralProgram
    });

    //3. Store the referrer ID linked to the enrolled contact in a variable
    const referrer = event.inputFields['referrer'];

  	//4. Create and configure our search filters
    const filter = {
        propertyName: 'referrer_id',
        operator: 'EQ',
        value: referrer
    }
    const filterGroup = {
        filters: [filter]
    }

    const properties = ['referrer_id', 'number_of_referrals', 'firstname', 'lastname', 'email']
    const publicObjectSearchRequest = {
        filterGroups: [filterGroup],
        properties,
    }


    //5. Search the CRM for the contact using the query defined above
    hubspotClient.crm.contacts.searchApi.doSearch(publicObjectSearchRequest).then(results => {

        //6. Store data in variables
        let contactId = results.results[0].id; // Id of the referring contact
        let referred_by = results.results[0].properties.firstname + " " + results.results[0].properties.lastname; // full name of the referring contact
        let totalReferrals = 0;

        //7. Check to see if this is their first referral. If it is we set "totalReferrals" property to "0"
        if (results.results[0].properties.number_of_referrals === null || results.results[0].properties.number_of_referrals === undefined || results.results[0].properties.number_of_referrals === "") {
            totalReferrals = 0;
        } else { // Otherwise we retrieve the current number of referrals
            totalReferrals = parseInt(results.results[0].properties.number_of_referrals)
        }

        let totalReferralsUpdated = totalReferrals + 1; // Increment referrals

        //8. Create a new date (todays date)
        var d = new Date();
        d.setUTCHours(0, 0, 0, 0);

        //9. Update Referred Contact's "referred by" property  	
        hubspotClient.crm.contacts.basicApi.update(event.object.objectId, {
            "properties": {
                "referred_by": referred_by
            }
        })

        //10. Update Referring Contacts "number of referrals" and "recent referral date" properties      	
        hubspotClient.crm.contacts.basicApi.update(contactId, {
            "properties": {
                "number_of_referrals": totalReferralsUpdated,
                "recent_referral_date": d
            }
        })
    });
}

Please find the images for reference @jackcoldrick90