The reference solution allows that lines will be drawn moving up or left. The description does not specify this will not happen.

The tests allow solutions that only allow lines drawn moving down or right.

Set chai.config.truncateThreshold to 0, to make error messages like expected [] to deeply equal [ Array(10) ] actually report the expected value. Right now, they're pretty much useless.

Representing an array of characters as a string instead might reduce the visual clutter in the description ( and everywhere ).

No random tests.

Tests produce useless error messages.

Size is an integer which will produce an n x n canvas

What is n?