divide-bills.js

(function () {
    let Andrew = { name: 'Andrew', debts: []}, 
        Justin = { name: 'Justin', debts: []}, 
        Jon = { name: 'Jon', debts: []}, 
        Zach = { name: 'Zach', debts: []};

    let people = [ 
        Andrew, 
        Justin, 
        Jon, 
        Zach 
    ];

    let evenSplit = () => 1 / people.length;

    let bills = [
        {
            name: 'Internet',
            amount: 95,
            billPayer: Justin,
            getShare: evenSplit
        },
        {
            name: 'Gas',
            amount: 45,
            billPayer: Jon,
            getShare: evenSplit
        },
        {
            name: 'Rent',
            amount: 1500,
            billPayer: Andrew,
            getShare: (person) => {
                if (person.name === Jon.name || person.name === Zach.name)
                    return 1 / 6;
                else 
                    return 2 / 6;
            }
        },
        {
            name: 'Electric',
            amount: 90,
            billPayer: Jon,
            getShare: evenSplit
        }
    ];

    // asign each person their debts
    people.forEach((payer) => {
        bills.forEach((bill) => {
            if (payer.name !== bill.billPayer.name) {
                let amountOwed = bill.getShare(payer) * bill.amount;

                // if the payer already has a debt to the person, bundle it
                const index = payer.debts.findIndex(debt => debt.debtor.name === bill.billPayer.name);

                if (index > -1) {
                    payer.debts[index].amount += amountOwed;
                } else {
                    payer.debts.push({ debtor: bill.billPayer, amount: amountOwed});
                }
            }
        });
    });

    // consolidate debts between people
    people.forEach((payer) => { 
        payer.debts.forEach((debt) => { 

            // if the person we own money to owes us money or vice versa, consolidate 
            const index = debt.debtor.debts.findIndex(debt => debt.debtor.name === payer.name);

            if (index > -1) {
                const amountOwed = debt.debtor.debts[index].amount;

                // balence it
                if (amountOwed === debt.amount) {
                    debt.debtor.debts[index].amount = 0;
                    debt.amount = 0;
                } else if (amountOwed > debt.amount) {
                    debt.debtor.debts[index].amount -= debt.amount;
                    debt.amount = 0;
                } else {
                    debt.amount -= debt.debtor.debts[index].amount;
                    debt.debtor.debts[index].amount = 0;
                }
            }
        });
    });

    // clear out empty debts
    people.forEach((payer) => { 
        payer.debts = payer.debts.filter(item => item.amount > 0);
    });

    // report
    people.forEach((payer) => { 
        payer.debts.forEach((debt) => {   
            console.log(payer.name + ' owes ' + debt.debtor.name + ' $' + debt.amount);
        });
    });
})();

This is the simple version that doesn't do advanced consolidation.
It only consolidates depts between 2 parties, not the whole pot.....yet

you forgot to send me 1% of the sum of all debts owed.