One of the advantages of Flash/Flex is that you can use vector graphics (SVG), which is nice. I did a bit of searching around and came across this Javascript vector graphic
Just to add an additional answer. the creator of one of the popular vector js (Raphaël) joined Adobe and worked on this (http://snapsvg.io/) for several years.
although the last release is from 2017 its much more maintained and newer than other libraries mentioned here. hope this helps someone looking for svg vector graphics library in js.
if you are looking for a meteorjs build for snap you can get it here (https://atmospherejs.com/mirageglobe/snapsvgcdn) with
$ meteor add mirageglobe:snapsvgcdn