How do I dynamically reference incremented properties in C#?

sumobj[prop1]

list.Where(o => o.GetType().GetProperty("Source").GetValue(o, null) == "Me").Single()

let daycount = 0,
  day = $('#day');

function time(callback) {
  setInterval(function() {
    daycount++;
    day.html('Day ' + daycount);
    callback(daycount);
  }, 1000);
}

function new_game() {
  function checkCount(daycount) {
    if (daycount == 5) {
      console.log('Is 5');
    } else {
      console.log('Not 5');
    }
  }
  time(checkCount);
}

new_game()

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="day">

class Controller {

  constructor() {
    this.newAttack = 0;
    this.defeatedEnemies = 0;
    this.sullivan = {
      name: "Sullivan",
      attack: 4,
      health: 10
    };
    this.johnson = {
      name: "Johnson",
      counterAttack: 8,
      health: 10
    };
  }

  battleFunction(player, computer) {
    //increments playerAttack
    this.newAttack += this[player].attack;
    this[player].health -= this[computer].counterAttack;

    //deducts attack from computer HP
    if (this.newAttack > this[player].attack) {
      this[computer].health -= this.newAttack;
      console.log(this[computer].health);
    } else {
      this[computer].health -= this[player].attack;
      console.log(this[computer].health);
    }

    if (this[computer].health <= 0) {
      this.defeatedEnemies++
    }
    if (this.defeatedEnemies >= 4) {
      alert("You won!");
      //resetGame();
    }
  };

}
const ctrl = new Controller();
$("#fightBtn").click(() => {
  ctrl.battleFunction($("#playerDiv").children("img").attr("id"), $("#computerDiv").children("img").attr("id"));
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<button id="fightBtn">Fight!</button>

<div id="playerDiv"><img id="sullivan"></div>
<div id="computerDiv"><img id="johnson"></div>