Use strict-equal instead of lossy-equal

[KashEight]

Overview

• Use strict-equal (===, !==) instead of lossy-equal (==, !=)
• Add eslint rule 'eqeqeq'
  • ref: eqeqeq - ESLint - Pluggable JavaScript Linter
• Add TODO comments to lines needed to refactor
  • e.g.) comparison between objects, null and undefined

Testing

• Only eslint testing
  • Some errors (I cannot replace with strict-equal) are suppressed using eslint-disable-next-line

Comment

Basically, I checked the type of variables and whether there is at least a primitive such as "str", 42 in comparison or not.
I add null-check or undefined-check in some comparisons not to break.

Funny things: after adding rule 'eqeqeq':

$ yarn run lint:eslint
✖ 240 problems (200 errors, 40 warnings)
  39 errors and 0 warnings potentially fixable with the `--fix` option.