Code quality

Use a code quality standard
no loose equals (==) to avoid implicit coercion bugs
capitalize constants with underscores (js convention)
Do not use indexes as a key in a list to avoid rerendering of the while list if an item gets added in the middle
Do not use var
Do not use insecure packages (check with npm audit, yarn audit)
Do not use subscribe inside subscribe to avoid memory leaks(Rxjs)
Do not use map inside map. Try to solve with reduce
Write tests
Write utils for frequently used functions
Use component based design

Frontend quality

Use alt for every image tag
Use different image sizes for different screen widths
Use a-tags instead of routerlinks so you can hover over it and see the link + open it with right mouse click
Use responsive design
Follow WCAG guidelines for accessibility
Add validation to all input fields