mental_model2_js_implementation.js

function sumOfMultiples(targetNumber, factors) {
  var multiples = [];
  if (factors.length === 0) {
    factors = [3, 5];
  }

  factors.forEach(function(factor) {
    var currentMultiple;
    for (currentMultiple = factor; currentMultiple < targetNumber; currentMultiple += factor) {
      if (multiples.indexOf(currentMultiple) === -1) {
        multiples.push(currentMultiple);
      }
    }
  });

  return multiples.reduce(function(sum, value) {
    return sum + value;
  }, 0);
}

sumOfMultiples(20, [3, 5]);  // returns 78
sumOfMultiples(20, [3]);     // returns 63
sumOfMultiples(20, [5]);     // returns 30
sumOfMultiples(20, []);      // returns 78
sumOfMultiples(1, []);       // returns 0
sumOfMultiples(20, [19]);    // returns 19