sum(range(start, end))

range.js

function range(num1, num2) {

  const array = [];

  if (typeof num1 !== 'number' || typeof num2 !== 'number') {
    throw new Error('Please provide 2 numbers.');
  }

  for (let i = num1; i < num2; i++) {
      array.push(i);
  }

  return array;
}

sum.js

function sum(array) {

  if (typeof array !== 'object') {
      throw new Error('Please provide an array.');
  }

  let total = 0;

  for (let i = 0; i < array.length; i++) {
      total += array[i];
  }

  return total;
}