let population = [{ fitness: 97 }, { fitness: 1 }, { fitness: 2 }];

  population.sort((a, b) => b.fitness - a.fitness);

  let sum = 0;

  for (let i = 0; i < population.length; i++) {

    population[i].rank = population.length - i;

    sum += population[i].rank;

  }

  for (let i = 0; i < population.length; i++) {

    population[i].probability = population[i].rank / sum;

  }

  console.log(population);

}

​