Programmatically use RGBa values in fillStyle in <canvas>?

Using <canvas>, I want to set the RGBa value of the rectangle using a variable.

for example:

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

works fine, but using it with a variable:

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

doesn't work.

Apparently fillStyle only accepts a string. So how do I set a value of the rgba value using some variable instead of explicitly defining the values?