SOURCE

function koch(ctx, x1, y1, x2, y2, n, m) {    //ctx为绘图对象，x1，y1,x2,y2为两头的点，n为当前维度，m为维度    
    if (m == 0) {
        ctx.moveTo(x1, y1);
        ctx.lineTo(x2, y2);
        ctx.stroke();
        return false;
    }
    var x3 = (x2 - x1) / 3 + x1;
    var y3 = (y2 - y1) / 3 + y1;
    var x4 = (x2 - x1) / 3 * 2 + x1;
    var y4 = (y2 - y1) / 3 * 2 + y1;
    var x5 = x3 + ((x2 - x1) - (y2 - y1) * Math.sqrt(3)) / 6;
    var y5 = y3 + ((x2 - x1) * Math.sqrt(3) + (y2 - y1)) / 6;

    n = n + 1;

    if (n == m) {
        ctx.moveTo(x1, y1);
        ctx.lineTo(x3, y3);
        ctx.lineTo(x5, y5);
        ctx.lineTo(x4, y4);
        ctx.lineTo(x2, y2);
        ctx.stroke();
        return false;
    }

    koch(ctx, x1, y1, x3, y3, n, m)
    koch(ctx, x3, y3, x5, y5, n, m)
    koch(ctx, x5, y5, x4, y4, n, m)
    koch(ctx, x4, y4, x2, y2, n, m)
}

function draw() {
    var canvas = document.getElementById("canvas");
    if (canvas.getContext) {
        canvas.height = canvas.height;
        var ctx = canvas.getContext("2d");
        ctx.strokeStyle = "#00";
        ctx.beginPath();
        var x1 = 400.00;
        var y1 = 150.00;

        var x2 = 100.00;
        var y2 = 150.00;


        var x11 = x2 + (x1 - x2) / 2;
        var y11 = y1 + Math.sin(Math.PI / 3) * (x1 - x2);

        var depth =6;  //取得一个文本框的值，可以调整维度，这里没有进行输入判断。

        koch(ctx, x1, y1, x2, y2, 0, depth);
        koch(ctx, x11, y11, x1, y1, 0, depth);
        koch(ctx, x2, y2, x11, y11, 0, depth);

    } else {
        alert("不支持Canvas");
    }
}

draw();

<canvas id="canvas" width=600 height=600 style="background:red"></canvas>