Hero.js

function NPC(name, hitpoints) {
    this.name = name;
    this.hitpoints = hitpoints;
}
function Hero(name) {
    NPC.call(this, name, 100);
    this.damage = 10;
    this.critical = false;
}

Hero.prototype.attack = function (defender) {

    if (defender instanceof Hero && defender.critical === true && this instanceof Hero) {
        defender.hitpoints -= 15;

        if (defender.hitpoints > 0) {
            defender.critical = true;
            this.critical = false;
        } else {
            this.critical = false;
        }

    } else {
        defender.hitpoints -= 10;
    }
    console.log(this.name + " attacked " + defender.name + ". Defender hitpoints = " + defender.hitpoints);
};

Hero.prototype.status = function () {
    return this.name + " " + this.hitpoints;
};

//Creating list of characters
let N = Math.floor(Math.random() * 41) + 10;
let characters = [];
for (let i = 0; i < N; i++) {
    let random = Math.floor(Math.random() * 2) + 1;
    random === 1 ? characters.push(new Hero("Hero_" + i)) : characters.push(new NPC("NPC_" + i, 100));
}
//end of Creating list of characters

//Choosing random Hero player for critical damage
while (true) {
    let random = Math.floor(Math.random() * characters.length);
    if (characters[random] instanceof Hero) {
        characters[random].critical = true;
        break;
    }
}
console.log(characters);
//end of Choosing random Hero player for critical damage

//Battle of titans
let go = true;
function battle(characters) {
    while (go) {
        for (let i = 0; i < characters.length; i++) {

            while (true) {
                var random = Math.floor(Math.random() * characters.length);
                if (random !== i) {
                    break;
                }
            }

            if (characters[i] instanceof Hero) {
                characters[i].attack(characters[random]);
                if (characters[random].hitpoints <= 0) {
                    characters.splice(random, 1);
                }
                console.log(characters[i].status());
            }

            if (characters.length === 1) {
                console.log("Winner: " + characters[0].name + ", hitpoints: " + characters[0].hitpoints + "!");
                go = false;
                break;
            }
        }
    }
}
//end of Battle of titans
battle(characters);