Apply discount

Learn percentage discount logic.

Percentage discount

A percentage discount is applied by multiplying the amount by (1 - percent / 100). For example, a 20% discount on 100 gives 80: 100 * (1 - 20/100) = 80.

function applyDiscount(amount: number, percent: number): number {
  return amount * (1 - percent / 100);
}

Invalid rate check

The discount rate must be between 0 and 100. Otherwise it is safer to return the amount unchanged. This prevents accidentally applying negative or very large discounts.

function applyDiscount(amount: number, percent: number): number {
  if (percent < 0 || percent > 100) {
    return amount;
  }
  return amount * (1 - percent / 100);
}

Exercise Write the applyDiscount function

Write a function applyDiscount(amount: number, percent: number): number. If percent is not between 0 and 100, return amount unchanged. Otherwise return amount * (1 - percent / 100). Print three example calls: (100, 20), (50, 10), (200, 150).

Show example solution

function applyDiscount(amount: number, percent: number): number {
  if (percent < 0 || percent > 100) {
    return amount;
  }
  return amount * (1 - percent / 100);
}

console.log(applyDiscount(100, 20));
console.log(applyDiscount(50, 10));
console.log(applyDiscount(200, 150));

Test yourself

What is the price of 100 units after a 20% discount?
- 100
- 20
- 80
- 120
Show answer

80 — Percentage discount: amount * (1 - percent / 100). 100 * (1 - 20/100) = 100 * 0.8 = 80.

Complete the discount formula

return amount * (1 - ____ / 100);

Show answer

return amount * (1 - percent / 100);

Discount parts

percent / 100

Convert rate to decimal
1 - percent / 100

Remaining rate
amount * (1 - percent / 100)

Discounted price
percent < 0 || percent > 100

Invalid rate check

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