An example of how to add default arguments to a named function in es6.

es6-default-arguments.js

function calculateBill(total, tax = 0.10, tip = 0.15) {
    return total + (total * tax) + (total * tip);
}
console.log(calculateBill(100));

// Calculate bill with custom tip value and default tax value. Pass undefined
// expliclitely to use a default value that's not the first or last function argument
const bill = caluculateBill(100, undefined, 0.20);
console.log(bill);