Edit in JSFiddle

// player.js
(function() {

    window.game = {};

    window.game.player = {

        hp: 10,
        resist: .3,
        poisoned: false,

        damage: function(amount) {
            if (Math.random() < this.resist) {
                amount = Math.round(amount / 2);
            }
            this.hp -= amount;
            document.getElementById("hp").innerHTML = this.hp;
            if (this.hp <= 0) {
                this.die();
            }
        },

        die: function() {
            document.getElementById("controls").innerHTML = "Game Over";
        },

        setPoisoned: function(val) {
            this.poisoned = val;
            document.getElementById("poisoned").innerHTML = val;
        }

    };

}());

// enemy.js
(function() {

    window.game.enemy = {

        damagePlayer: function() {
            window.game.player.damage(3);
        }

    };

}());

// poisonEnemy.js
(function() {

    window.game.poisonEnemy = {

        damagePlayer: function() {
            window.game.enemy.damagePlayer();
            window.game.player.setPoisoned(true);
        }

    };

}());

// set up the ui to test the code in jsfiddle
$("#hp").text(window.game.player.hp);
$("#poisoned").text(window.game.player.poisoned);
$("#attack").on("click", window.game.enemy.damagePlayer);
$("#poisonAttack").on("click", window.game.poisonEnemy.damagePlayer);
HP: <span id="hp"></span>
<br>
Poisoned: <span id="poisoned"></span>
<div id="controls">
    <button id="attack">attack from normal enemy</button>
    <br>
    <button id="poisonAttack">attack from poisonous enemy</button>
</div>