comparator.js

function compareNumber(a, b) {
  return a - b;
}

function compareString(a, b) {
  return a.localeCompare(b);
}

const comparatorMap = {
  age: compareNumber,
  name: compareString
};

function comparatorFactory(...keys) {
  return (a, b) => {
    for (const key of keys) {
      const comparison = comparatorMap[key](a[key], b[key]);
      
      if (comparison !== 0) {
        return comparison;
      }
    }
    
    return 0;
  }
}

let comparator = comparatorFactory('age', 'name');

let people = [
  {
    "age": 31,
    "name": "Eduardo"
  },
  {
    "age": 31,
    "name": "Abril"
  },
  {
    "age": 31,
    "name": "Zaratustra"
  },
  {
    "age": 13,
    "name": "Alan"
  },
  {
    "age": 25,
    "name": "Eduardo"
  }
];

lowest-unassigned-integer.js

function lowestUnassignedInt(array) {
  array.sort();
  
  let previous = 0;
  
  for (const i of array) {
    if (i !== previous + 1) {
      break;
    }
    
    previous++;
  }
  
  return previous + 1;
}