Simple heat transfer simulation

function shuffle(array) {
    var m = array.length, t, i;
    while (m) {
        i = Math.floor(Math.random() * m--);
        t = array[m];
        array[m] = array[i];
        array[i] = t;
    }
    return array;
}
var Grid = (function () {
    function Grid(width, height) {
        this.width = width;
        this.height = height;
        this.content = new Array(width);
        var i;
        for (i = 0; i < this.width; i++)
            this.content[i] = new Array(height);
        this.reset();
        this.traversal = 0;
    }
    Grid.prototype.reset = function () {
        var i;
        var j;
        for (i = 0; i < this.width; i++)
            for (j = 0; j < this.height; j++)
                this.content[i][j] = 0;
    };
    Grid.prototype.draw = function (ctx) {
        var i;
        var j;
        for (i = 0; i < this.width; i++)
            for (j = 0; j < this.height; j++) {
                ctx.fillStyle = "rgb(" + this.content[i][j] + ",0,0)";
                ctx.fillRect(i * GRID_STEP, j * GRID_STEP, GRID_STEP, GRID_STEP);
            }
    };
    Grid.prototype.adjacent = function (i, j) {
        var res = [];
        if (i > 0)
            res.push([i - 1, j]);
        if (i < this.width - 1)
            res.push([i + 1, j]);
        if (j > 0)
            res.push([i, j - 1]);
        if (j < this.height - 1)
            res.push([i, j + 1]);
        return res;
    };
    Grid.prototype.get = function (p) {
        return this.content[p[0]][p[1]];
    };
    Grid.prototype.set = function (p, n) {
        this.content[p[0]][p[1]] = n;
    };
    Grid.prototype.processCell = function (i, j) {
        var adj = this.adjacent(i, j);
        var min = 255;
        var p;
        for (p = 0; p < adj.length; p++) {
            if (this.get(adj[p]) < min)
                min = this.get(adj[p]);
        }
        var adjMin = adj.filter(function (p) { return this.get(p) == min; }, this);
        if (adjMin != [] && this.content[i][j] > 0 && min < 255) {
            var delta = Math.floor((this.content[i][j] - min) / 2);
            var r = shuffle(adjMin)[0];
            this.set(r, this.get(r) + delta);
            this.content[i][j] -= delta;
        }
    };
    Grid.prototype.dissipate = function () {
        var i;
        var j;
        if (this.traversal == 0) {
            for (i = this.width - 1; i >= 0; i--)
                for (j = this.height - 1; j >= 0; j--)
                    this.processCell(i, j);
        }
        else if (this.traversal == 1) {
            for (i = 0; i < this.width; i++)
                for (j = 0; j < this.height; j++)
                    this.processCell(i, j);
        }
        else if (this.traversal == 2) {
            for (i = 0; i < this.width; i++)
                for (j = this.height - 1; j >= 0; j--)
                    this.processCell(i, j);
        }
        else {
            for (i = this.width - 1; i >= 0; i--)
                for (j = 0; j < this.height; j++)
                    this.processCell(i, j);
        }
        this.traversal = (this.traversal + 1) % 4;
    };
    return Grid;
}());
var GRID_STEP = 10;
var w = Math.floor(window.innerWidth / (GRID_STEP * 2));
var h = Math.floor(window.innerHeight / (GRID_STEP + 1));
function canvasInit() {
    var canvas = document.getElementById("canvas");
    canvas.width = w * GRID_STEP;
    canvas.height = h * GRID_STEP;
    return canvas;
}
var canvas = canvasInit();
var ctx = canvas.getContext("2d");
var grid = new Grid(w, h);
var mouseDown = false;
function onMouseMove(e) {
    if (mouseDown) {
        var x = e.pageX - canvas.offsetLeft;
        var y = e.pageY - canvas.offsetTop;
        var i = Math.floor(x / GRID_STEP);
        var j = Math.floor(y / GRID_STEP);
        grid.content[i][j] = 255;
        grid.draw(ctx);
    }
}
function onMouseDown(e) {
    mouseDown = true;
}
function onMouseUp(e) {
    mouseDown = false;
}
function onKeyDown(e) {
    if (e.keyCode == 32) {
        grid.dissipate();
        grid.draw(ctx);
    }
}
grid.draw(ctx);
canvas.addEventListener("mousedown", onMouseDown);
canvas.addEventListener("mouseup", onMouseUp);
canvas.addEventListener("mousemove", onMouseMove);
window.onkeydown = onKeyDown;


<html>
    <head>
        <title>code</title>
    </head>
    <body>
        <p align=center>
            <canvas id=canvas></canvas>
        </p>
        <style type="text/css">
            canvas { display:block; }
        </style>
        <script type="text/javascript" src="main.js"></script>
    </body>
</html>