This tutorial explains the mathematics behind collision detection using bounding circle method. Source code and online demo is available with the tutorial.
Just a suggestion: get rid of the sqrt operation. It is slow and unnecessary. The same test can be expressed as,
var sum_radii = sprite1Radius + sprite2Radius; var sum_radii_sqr = sum_radii * sum_radii; var dist_sqr = (distanceX * distanceX) + (distanceY * distanceY); return dist_sqr <= sum_radii_sqr