Experimental TypeScript Migration

[SamChou19815]

This is a branch that explores how our codebase can be migrated to TypeScript to boost developer productivity.

Rationale

TypeScript has these advantages over Flow:

• More usage: already over 50% according to 2018 state of JavaScript.
• Better IDE support (remember MS invented both TS and VSCode)
• Better community support
• Better third-party type definitions
• More clear roadmap than Flow

TypeScript has these disadvantages:

• Its type system is less sound than Flow. It's unsound by design.
• It requires more type annotation to avoid 'implicit any'. aka dumb type inference.

According to my investigation when I'm working these branch, the disadvantages can be easily overcome to trade for a much better IDE support. Autocompletion was much faster than that of flow. The code migration was mostly some syntax change, and adding a few type annotations to avoid 'implicit any'.

Merging Comment

Approve the change only if most of us agree it's good. Also consider merging other changes first before merging this.

Infrastructure Change

• setup ESLint with typescript support
• remove old Flow packages
• migrate the codebase to TS
• fix all new linter errors
• setup basic structure for error recovery

Branch Merging Status

• merge in Michael's changes
• merge in Megan's changes

Other Bug Fixes

• Fix Margins/Padding is too narrow #134 (mostly CSS change)
• Fix Monthly and 2 Week view should have same card design as 5D view #136
• Fix Incorrect date range in biweekly view #159
• Fix the position of the focus view toggle in biweekly-view and monthly-view (mostly CSS change)