run.js

'use strict';

const sortModule = require('./sort');

/* Example array */
const array = [
  {
    name: 'Kostas',
    age: 28
  },
  {
    name: 'Helen',
    age: 25
  },
  {
    name: 'George',
    age: 32
  }
];

const a = sortModule.sortArray(array, 'age');
console.log(a);

sort.js

'use strict';

module.exports.sortArray = (array, sortingField) => {

  const clone = array.slice(0); // clone so the original stays intact
  return clone.sort((a, b) => sorter(a, b, sortingField));
};

const sorter = (a, b, sortingField) => {

    const n1 = a[sortingField];
    const n2 = b[sortingField];

    if (!Number.isInteger(n1) || !Number.isInteger(n2)) {
        throw new Error('Not an integer passed');
    }

    return n1 > n2 ? 1 : -1;
};

test.js

'use strict';

const assert = require('assert');
const sortModule = require('./sort');

/* Example array */
const array = [
  {
    name: 'Kostas',
    age: 28
  },
  {
    name: 'Helen',
    age: 25
  },
  {
    name: 'George',
    age: 32
  }
];

const a = sortModule.sortArray(array, 'age');
assert.equal(a[0].age, 25);
assert.equal(a[1].age, 28);
assert.equal(a[2].age, 32);