fractal tree in D3 - L-system

Fractal trees created with L-systems in d3.js
Used Daniel Shiffman's p5.js video/code as a foundation

<!doctype html>
<html>
<body>
	<script src="https://d3js.org/d3.v3.min.js" charset="utf-8"></script>
<script>

var w = window.innerWidth,
		h = window.innerHeight;

var theta = -12 * Math.PI / 190,
    length = 29,
    x0 = w/2,
    y0 = h,
    t0 = Math.PI / 2.5;

function tree(string) {
    var tree = [],
        root = { path: "M" + x0 + "," + y0, children: [] },
        step = { x: x0, y: y0, t: t0, branch: root };

string.split('').forEach(function(a) { instructions[a](); });
    return root;
}

var svg = d3.select("body").append("svg")
    .attr("width", w)
    .attr("height", h)
    .datum(tree(line(4, {"F": "F-[-F+F+F-FF]+[+FF-F-F-]"}, "F")))
    .each(grow);

function grow(d) {
    d3.select(this).append("path")
        .attr("stroke", "black")
        .attr("stroke-opacity", 0.45)
        .attr("stroke-width", function(d,i){i * Math.PI/38})
        .attr("fill", "none")
        .attr("d", function(d) { return d.path; });

d3.select(this).selectAll("g")
        .data(d.children)
        .enter().append("g")
      .transition()
        .each("start", grow);
}

function line(n, rules, axiom) {
	if (n == 0){
		return axiom;
	} else{
		return line(--n, rules, axiom.replace(/./g, function(a) { return rules[a] || a; }));
	}
}

</script>
</body>
</html>

https://d3js.org/d3.v3.min.js