TypeScript requires an argument for every function parameter by default. Add `?` after a parameter name when callers may omit that argument. Optional parameters must come after required parameters. section 8.2.2
Call a function with or without a discount
In `src/index.ts`: ```ts function calculateTax(amount, discount?) { return (amount * 1.2) - (discount || 0); } console.log(calculateTax(100, 0)); // 120 console.log(calculateTax(100)); // 120 ``` The second call supplies `undefined` for `discount`. The function uses `|| 0` so it can still calculate a result. Making `discount` optional does not permit an extra third argument. section 8.2.2
A default parameter offers another way to allow omission: `discount = 0` assigns the fallback in the parameter declaration instead of the calculation. A rest parameter serves a different purpose: accepting additional arguments. section 8.2.2
