A discount calculator needs three operations: find the sale price, find the savings, and reverse a sale price back to the original. In JavaScript, each one is a single line of arithmetic. The harder part is validating input and handling stacked discounts correctly, because two 10% discounts are not the same as one 20% discount.
The core formula
For an original price P and a discount percentage d, the sale price is:
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salePrice = originalPrice * (1 - discountPercent / 100)
savings = originalPrice - salePriceA $80 item at 25% off sells for 80 * (1 - 25 / 100), which is $60. You save $20. The discount is applied to the original price, so the savings are simply the difference between the two prices.
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Sometimes you know only the sale price and the discount, such as a tag that reads “$75, 25% off.” To recover the original, divide by the fraction that remains after the discount:
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originalPrice = salePrice / (1 - discountFraction)
// discountFraction = discountPercent / 100For $75 at 25% off, the original is 75 / 0.75 = 100. This formula only works when the discount is below 100%. At exactly 100%, the denominator is zero, and a free item cannot be reverse-calculated to any original price.
Stacked discounts do not add together
When two percentage discounts are applied in sequence, each one reduces the running price, so the second discount applies to an already-reduced amount. The formula is:
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final = price * (1 - discount1) * (1 - discount2)Take a $100 item with 20% off, then an additional 10% off. The result is 100 × 0.80 × 0.90 = 72. The price has dropped by 28%, not 30%. The effective discount is 28% of the original price.
| Scenario (on $100) | Additive guess | Sequential (actual) | Effective discount |
|---|---|---|---|
| 20% then 10% | 30% off, $70 | $72 | 28% |
| 50% then 50% | 100% off, $0 | $25 | 75% |
| 25% then 25% | 50% off, $50 | $56.25 | 43.75% |
The sequential column is what a checkout that applies one discount after another will charge, assuming each discount is calculated on the price left after the previous one. Some retailers apply all percentages to the original price instead, so check which method a promotion uses before modeling it.
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Validating input before calculating
Form fields return text, and text needs to be converted and checked before any arithmetic happens. Two JavaScript behaviors cause most bugs here:
Number("")returns0, notNaN. An empty price field will quietly become a free item unless you reject empty strings first.Number.isFinite()returnsfalseforNaN, positive and negativeInfinity, and non-number types. It does not convert strings, so you must parse the value before calling it. MDN documents the behavior of Number.isFinite().
A complete calculator function
The following function separates parsing, validation, and calculation. It returns raw numbers and leaves display formatting to a separate step.
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function parseAmount(text) {
const trimmed = String(text).trim();
if (trimmed === "") throw new Error("Enter a value.");
const n = Number(trimmed);
if (!Number.isFinite(n)) throw new Error("Enter a valid number.");
return n;
}
function applyDiscount(originalPrice, discountPercent) {
const price = parseAmount(originalPrice);
const pct = parseAmount(discountPercent);
if (price < 0) throw new RangeError("Price cannot be negative.");
if (pct < 0 || pct > 100) throw new RangeError("Discount must be between 0 and 100.");
const salePrice = price * (1 - pct / 100);
return { salePrice, savings: price - salePrice };
}
function stackDiscounts(price, percents) {
const base = parseAmount(price);
return percents.reduce((running, p) => {
const pct = parseAmount(p);
if (pct < 0 || pct > 100) throw new RangeError("Each discount must be between 0 and 100.");
return running * (1 - pct / 100);
}, base);
}
function reverseDiscount(salePrice, discountPercent) {
const sale = parseAmount(salePrice);
const pct = parseAmount(discountPercent);
if (pct < 0 || pct >= 100) throw new RangeError("Discount must be at least 0 and below 100.");
return sale / (1 - pct / 100);
}Formatting the result as currency
Keep the numbers as numbers and format them only for display. Intl.NumberFormat handles currency symbols, separators, and digit counts, and it is more reliable than concatenating a “$” onto a value.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallconst usd = new Intl.NumberFormat("en-US", { style: "currency", currency: "USD" });
usd.format(60); // "$60.00"
const eur = new Intl.NumberFormat("de-DE", { style: "currency", currency: "EUR" });
eur.format(60); // German formatting with the euro signAlways pass an explicit locale and a currency code such as USD, EUR, or JPY. The default number of fraction digits follows each currency’s minor unit under ISO 4217: USD and EUR use two decimal places, while JPY uses none. MDN’s Intl.NumberFormat reference describes these options.
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Two decimal places are a display choice, not a rule for how a store must round its totals. Tax, per-line rounding, and rounding of the final charge are set by merchant policy and local rules, and this calculator does not model them.
Edge cases to test
- Empty field: should show a message, not a $0 result.
- Text such as “abc” or “1,000”:
Number("1,000")isNaN. Strip thousands separators deliberately, and decide whether a comma means a decimal point in your locale. - Discount of 0 or 100: 0 leaves the price unchanged; 100 makes the sale price zero, which the reverse function correctly refuses.
- Floating-point drift: binary floating point can produce tiny errors in intermediate results. Round only when displaying or storing final money values.
What the $2,400 figure does and does not show
The title’s claim that the calculator saved its author $2,400 last year is a personal estimate. It has not been independently measured or audited, and no outside source confirms it. The math above can tell you what any single purchase costs after a discount. It cannot tell you how many discounts you actually used or whether you would have bought the item at full price, so a savings total depends on your own purchase records.
If you want to track real savings, log each purchase’s original price and final charge, then sum the differences. That gives you a figure you can verify against receipts.
The Tool Desk
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Quick Recap
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Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
