Fix all typescript errors when compiled in strict mode #3

[karrtikr]

For #611 part 3

[codecov]

Codecov Report

Merging #4419 into master will increase coverage by 20%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #4419     +/-   ##
========================================
+ Coverage      58%     78%    +20%     
========================================
  Files         369     445     +76     
  Lines       15946   21071   +5125     
  Branches     2507    3423    +916     
========================================
+ Hits         9134   16241   +7107     
+ Misses       6190    4824   -1366     
+ Partials      622       6    -616

Flag	Coverage Δ
#Linux	65% <80%> (?)
#Windows	66% <80%> (?)
#macOS	65% <80%> (?)

[d3r3kk]

I think this should be fine, it's not changing any of the logic from the original versions. However, the use of null instead of undefined seems suspect. Please have a quick chat with @DonJayamanne on the places where I make mention of this before merging.

[d3r3kk]

CI timeout/matplotlib problem (should be fixed now).