Fix all typescript errors when compiled in strict mode #1

[karrtikr]

For #611 part 1

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• Has sufficient logging.
• Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated
• Test plan is updated as appropriate
• package-lock.json has been regenerated by running npm install (if dependencies have changed)

[codecov]

Codecov Report

Merging #4410 into master will increase coverage by 21%.
The diff coverage is 93%.

@@           Coverage Diff            @@
##           master   #4410     +/-   ##
========================================
+ Coverage      57%     78%    +21%     
========================================
  Files         364     439     +75     
  Lines       15495   20524   +5029     
  Branches     2408    3291    +883     
========================================
+ Hits         8752   15813   +7061     
+ Misses       6163    4707   -1456     
+ Partials      580       4    -576

Flag	Coverage Δ
#Linux	66% <58%> (?)
#Windows	66% <58%> (?)
#macOS	66% <58%> (?)