Animate variable inside expression

Question

In After Effects, I created a shape layer, added a path to that, edited the expression of it to contain

var path = thisProperty;
var cx = 100;
var cy = 200;
var r1 = 300;
var ratio =  20;
var quality = 30;
var closedPath = true;
var allPoints = new Array();

for (var a = 0; a < 360; a++) {
    var theta = degreesToRadians(a);
    x0 = Math.cos(theta);
    x = cx + r1 * x0;
    y = cy + 0.75 * r1 * (Math.sin(theta) - Math.sqrt(Math.abs(x0)));
    allPoints.push([x,y]);
}
path.createPath(allPoints, [], [], closedPath);

and added a stroke and a fill to the shape layer and set the width of the stroke to 10 and the color of the fill to light red. I got this shape as a result.

Now, I want to animate the end point of that stroke, you may see that I am looping the variable a from 0 to 360. That 360 value represents the end angle of the shape and I would like to have it animated to make an animation that "draws" the heart.

How do I animate the end value?

Is there a way to add a "dummy effect" so I can use it like

var a_max = effect("dummy")("endpoint")

Answer 1

1. Press the triangle marked "Add" next to the Contents of your shape layer.
2. Add a "trim path"
3. Add a keyframe to the "End" parameter of 0%.
4. Move along your timeline.
5. Add another keyframe of 100%.

Your path will draw on. You probably don't want to fill it until it is drawn.

Answer 2

Tom's answer is a good way to do it, but if you're interested, to do it only in the expression, just add a couple of lines at the start like

myDuration = 5; // the duration of the animation in seconds
let completion = Math.min(time/myduration, 1)

and then in the for loop, substitute "360" with 360 * completion

for (var a = 0; a < (360 * completion); a++) {