If you want to build something like draw.io, GraphEditor is the best example to use as a base.
The npm build is here.
mxGraph supports IE 11, Chrome 32+, Firefox 38+, Safari 7.1.x, 8.0.x, 9.1.x and 10.0.x, Opera 20+, Native Android browser 5.1.x+, the default browser in the current and previous major iOS versions (e.g. 10.x and 9.x) and Edge 23+.
mxGraph uses no third-party software, it requires no plugins and can be integrated in virtually any framework. Also provided is server-side functionality in Java and .NET for persistence (open and save) functionality, as well as server-side image generation.
mxGraph is licensed under the Apache 2.0 license. We do not sell any other license, nor do we have an option for paid support.
We created mxGraph in 2005 as a commercial project and it ran through to 2016 that way. Our USP was the support for non-SVG browsers, when that advantage expired we moved onto commercial activity around draw.io. mxGraph is pretty much feature complete, production tested in many large enterprises and stable for many years. We thought it better to open source it rather than letting the code die away.