This is amazing and fast. I'm complaining about it actually being easily broken only because I really wished this worked well, so I could build on it. It crashed for me on literally the very first thing I tried. I opened up the demo page, typed in 4 numbers in a column (in positions a5,a6,a7,a8), then typed "=sum(a5:a8)" in position a9, hit enter, and the demo crashed (with canvas all messed up). There's a traceback in the console log that starts "Uncaught TypeError: Cannot read property 'render' of undefined at cell.js:146". I'm using Chrome 71. I refreshed and tried the same test and it crashed again. There is a test suite (https://github.com/myliang/x-spreadsheet/tree/master/test) but the files have mostly not been touched in four months. In any case, awesome work - please make it even better!