Programmatically use RGBa values in fillStyle in <canvas>?

You just need to concatenate the r_a variable to build the string correctly:

var r_a = 0.3; 
ctx.fillStyle = "rgba(32, 45, 21, " + r_a + ")"; 

ctx.fillStyle = "rgba(32, 45, 21, "+r_a+")"; 

It is string concatenation.

I'm very late to the party but I had a similar problem and solved it in a slightly different way that may be useful.

Since I needed to reuse the fill colours at different alpha levels, I used the JavaScript replace method:

colurfill = "rgba(255, 255, 0, [[opacity]])";
ctx.fillStyle = colurfill.replace("[[opacity]]", "0.5");
:
:
ctx.fillStyle = colurfill.replace("[[opacity]]", "0.75");

Obviously it doesn't have to be the alpha level that varies, it could be one of the other channels.

var r_a = 0.3
ctx.fillStyle = `rgba(32, 45, 21, ${r_a})`;

These are called template literals https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals

The back-ticks `` are often found left of the number 1 on the keyboard on US keyboards.