Upgrade to TypeScript 3.8 and use new features

[AlCalzone]

Blocked by:

• import type in TypeScript babel/babel#10981
• TypeScript 3.8 Syntax Support typescript-eslint/typescript-eslint#1436
• LGTM.com - false positive: New syntax in TypeScript 3.8 github/codeql#2691

TODOs:

• Upgrade Prettier to an official version (v2.0 / v1.20 ?)
• Upgrade babel to an official version (v7.9)
• Check if @babel/core@7.9.x breaks Node 8 support

[lgtm-com]

This pull request introduces 347 alerts and fixes 1 when merging da6f6ce into 8cc48b1 - view on LGTM.com

new alerts:

• 347 for Syntax error

fixed alerts:

• 1 for Useless assignment to local variable

[lgtm-com]

This pull request introduces 333 alerts and fixes 1 when merging 7a90e26 into 8cc48b1 - view on LGTM.com

new alerts:

• 333 for Syntax error

fixed alerts:

• 1 for Useless assignment to local variable

[lgtm-com]

This pull request introduces 329 alerts and fixes 1 when merging 4b2750d into 8cc48b1 - view on LGTM.com

new alerts:

• 329 for Syntax error

fixed alerts:

• 1 for Useless assignment to local variable

[lukescott]

@AlCalzone have you considered using tsc instead of babel?

At work I've found that using babel wasn't helpful when locked into TypeScript features. The only place I still use babel is compiling for the web as the env preset is super helpful for targeting browser versions.

The ts-jest package lets you run tests using tsc instead of going thru babel.

[AlCalzone]

I've been using ts-jest earlier but always ran into problems where tests would use some cache I couldn't get rid of. Not very helpful if the test runner is unreliable or shows false-positives...

Not sure if that has changed but I haven't had any similar problems since I made the switch to babel+jest.

[lgtm-com]

This pull request introduces 335 alerts and fixes 5 when merging 7271670 into b67539a - view on LGTM.com

new alerts:

• 335 for Syntax error

fixed alerts:

• 5 for Missing await

[lgtm-com]

This pull request introduces 337 alerts and fixes 5 when merging 7839c7a into 6ff8d37 - view on LGTM.com

new alerts:

• 337 for Syntax error

fixed alerts:

• 5 for Missing await

[lgtm-com]

This pull request introduces 349 alerts and fixes 5 when merging 3cda87c into df85c58 - view on LGTM.com

new alerts:

• 349 for Syntax error

fixed alerts:

• 5 for Missing await

[lgtm-com]

This pull request introduces 351 alerts and fixes 5 when merging 73626e0 into a73c7da - view on LGTM.com

new alerts:

• 351 for Syntax error

fixed alerts:

• 5 for Missing await