How to calculate a percentage

Short answer

To find what percentage one number is of another, divide them and multiply by 100:

const done = 45;
const total = 60;
 
console.log((done / total) * 100);

Output:

To take 15% of a sum, multiply the sum by 15 and divide by 100:

const price = 80;
const discount = (price * 15) / 100;
 
console.log(discount, price - discount);

Output:

12 68

The change is the difference divided by the old value:

const oldPrice = 40;
const newPrice = 50;
const change = ((newPrice - oldPrice) / oldPrice) * 100;
 
console.log(change + '%');

Output:

25%

Intl.NumberFormat with style: 'percent' takes a share from 0 to 1 and adds the sign itself:

const format = new Intl.NumberFormat('en-US', { style: 'percent', maximumFractionDigits: 1 });
 
console.log(format.format(2 / 3));

Output:

66.7%

Dividing by zero does not throw an error, it gives NaN or Infinity. Check the total before dividing:

const percent = (part, total) => (total === 0 ? 0 : (part / total) * 100);
 
console.log((0 / 0) * 100);
console.log(percent(0, 0));

Output:

NaN
0