This project is badly in need of an update. It's using an ancient 4 yo. version of tesseract.js. The current version:
https://github.com/naptha/tesseract.js
is based on tesseract v4.1.1, a newer your Ubuntu 18.04's.
The 4.0 version added new neural network system based on LSTMs, with major accuracy gains.
https://fossies.org/diffs/tesseract/4.0.0_vs_4.1.0/ChangeLog...