Edit in JSFiddle

// player.js

(function() {
    
    window.game = {};

    window.game.player = {

        x: 5,
        y: 10,

        distanceTo: function(x, y) {
            return Math.sqrt(Math.pow(x - this.x, 2) +
                             Math.pow(y - this.y, 2));
        }

    };

}());

// enemy.js

(function() {

    window.game.enemy = {
        
        x: 5,
        y: 20,

        move: function(x, y) {
            this.x = x;
            this.y = y;
        }

    };

}());

// crate.js

(function() {

    window.game.crate = {

        hp: 30,
        
        damage: function(amount) {
            this.hp -= amount;
        }

    };

}());