find sum of multiples 3 and 5, JS

I’m given a number and I need to find the sum of the multiples of 3 and 5 below the number. For example: 20 => 78 = 3 + 5 + 6 + 9 + 10 + 12 + 15 + 18

My code works, but not for numbers greater than 1,000,000 (I tested it for 100,000 – it gives the result with 2sec delay). So, it should be optimized. Could someone help me? Why is my code slow? Thanks.

My logic is as follows:

add multiples to an array
filter duplicate values
sum all values

my code:

  function sumOfMultiples(number) {

    let numberBelow = number - 1;  
    let numberOfThrees = Math.floor(numberBelow / 3);
    let numberOfFives = Math.floor(numberBelow / 5);  
    let multiples = [];
    let multipleOfThree = 0;
    let multipleOfFive = 0;

    for (var i = 0; i < numberOfThrees; i++) {
      multiples.push(multipleOfThree += 3);
    }

    for (var j = 0; j < numberOfFives; j++) {
      multiples.push(multipleOfFive += 5);
    }

    return multiples
              .filter((item, index) => multiples.indexOf(item) === index)
              .reduce((a, b) => a + b);    
 }

Answer

You can just run a loop from 1 to number, and use the modulo operator % to check if i divides 3 or 5:

function sumOfMultiples(number) {
  var result = 0;
  
  for (var i = 0; i < number; i++) {
    if (i % 5 == 0 || i % 3 == 0) {
      result += i;
    }
  }

  return result;
}

console.log(sumOfMultiples(1000));
console.log(sumOfMultiples(100000));
console.log(sumOfMultiples(10000000));

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Answer