function sum (arr) {
  let sum = 0;
  for (num of arr)
    sum += Number(num);
  return sum;
}

const assert = require('chai').assert;

describe.only('sum', function () {
  it('should return the sum of the values of all elements inside the array',
    () => {
      let numbers = [1, 2, 3];
      let result = sum(numbers);
      assert.strictEqual(result, 6);
    })
});