This is how I would solve euler 1 without resulting to passing functions to functions (beginner code)

Euler-1.js

// If we list all the natural numbers below 10 that are
// multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of
// these multiples is 23.
//
// Find the sum of all the multiples of 3 or 5 below 1000.

if (typeof(Array.prototype.flatMap) != "function") {
  // warning, we are mutating the actual base prototype for Array here
  Array.prototype.flatMap = function(fn) {
    return Array.prototype.concat.apply([], this.map(fn));
  };
}

var sumMultiplesOf3and5 = (function() {

  var naturalNumbers = function(from, to) {
    var nums = [];
    for(var i=from; i < to; i++) {
      nums.push(i);
    };
    return nums;
  };

  var multiplesOf = function(multiples, nums) {
    return multiples.flatMap(function(multiple) {
      return nums.filter(function(num) {
        return num % multiple == 0;
      });
    });
  };

  var sum = function(nums) {
    return nums.reduce(function(acc, num) { return num + acc });
  };

  var sumMultiplesOf3and5 = function(from, to) {
    return sum(multiplesOf([3,5], naturalNumbers(from, to)));
  }

  return {
    call: sumMultiplesOf3and5,
  };

}());

console.log(sumMultiplesOf3and5.call(1, 10));