Migrate from JavaScript to TypeScript

[yechs]

Now that I have set up all the infrastructure in #235, people can feel free to convert all javascript pages into typescript.

Some examples can be seen in d99ddbf#diff-ac5d3241834158af861bc3a961a8f04148eef9daed1d8e7f8af4d92f07a6aa0e where I have added all missing types, and extracted some of the common methods into a separate utils file.

For now, you can start by converting some file ending injsx to tsx, and add all missing types.
Most of the types could be inferred (either automatically by code editor or by you). If you face any difficulties, feel free to reach out to us.