Learn Skills - Understand everything

# Practice: Understand the Discount Function Three exercises grounded in your code. Attempt each before opening the solutions. Bring your answers back for verification against the source. | Exercise | Your task | |---|---| | Recreate | Rebuild the behavior without copying | | Debug | Find the mistake in a plausible rewrite | | Transfer | Apply the same reasoning to refunds | ### 1. Recreate the behavior Read the original function, then hide it. Write your own `discountedTotal(prices, discount)` that: - Accepts an array of prices. - Defaults to no discount. - Rejects discounts below `0` or above `1`. - Returns the total after applying the discount. Before running your implementation, predict its output for: ```js discountedTotal([40, 60], 0.25) discountedTotal([40, 60]) discountedTotal([], 0.25) ``` Explain how one input value travels through your function to become part of the result. ### 2. Debug the rewrite **A teammate changes the return statement** ```js return prices.reduce((sum, price) => sum + price, 0) * discount; ``` **They say:** “We’re applying the discount, so multiplying by it makes sense.” **Your task** 1. Choose an input that exposes the mistake. 2. Predict the original and rewritten results. 3. Correct the expression. 4. Explain what happens at discounts of `0` and `1`. ### 3. Apply the pattern elsewhere A shop needs to calculate revenue retained after proportional refunds. **Implement** ```js retainedRevenue(payments, refundFraction) ``` **Requirements** - `payments` contains payment amounts. - `refundFraction` defaults to `0`. - Fractions outside `[0, 1]` throw a `RangeError`. - The result is the amount the shop keeps. Use `[40, 60]` with a refund fraction of `0.25` to explain your implementation. Which parts of your reasoning stayed the same when moving from discounts to refunds? --- ```html <details> ``` <summary>Open solutions — after attempting the exercises</summary> ### 1. Recreate **One valid implementation** ```js export function discountedTotal(prices, discount = 0) { if (discount < 0 || discount > 1) { throw new RangeError('discount'); } const subtotal = prices.reduce((sum, price) => sum + price, 0); return subtotal * (1 - discount); } ``` The three calls return **75**, **100**, and **0**. **Source:** the supplied function, lines 1–3. ### 2. Debug For `[40, 60]` and `0.25`: - Original: `100 × 0.75 = 75`. - Rewrite: `100 × 0.25 = 25`. The rewrite calculates the discounted portion instead of the remaining amount. **The correct multiplier is** ```js 1 - discount ``` A discount of `0` keeps the full total; a discount of `1` leaves zero. **Source:** the supplied function, line 3. ### 3. Transfer ```js export function retainedRevenue(payments, refundFraction = 0) { if (refundFraction < 0 || refundFraction > 1) { throw new RangeError('refundFraction'); } const total = payments.reduce((sum, payment) => sum + payment, 0); return total * (1 - refundFraction); } ``` For `[40, 60]` and `0.25`, retained revenue is **75**. **The shared pattern is:** add the amounts, then multiply by the fraction remaining. The meaning changes from discounted prices to retained revenue. **Source:** the supplied function’s validation and calculation, lines 2–3; the refund requirements above. **Send your first attempt when ready. I’ll check it against the code. If something doesn’t match, you’ll get a targeted question and a source reference before seeing the answer.**