Improve JavaScript cheatsheet

[Brymmobaggins]

I work through the javascript.md I spotted some typos. I'd also like to add some cheatsheet.

[Suryansh1720001]

Hello @Brymmobaggins
Go for it. And go through the readme and contributing file first before sending the pull request.
( Also please keep this in mind - don't send any Plagiarized content. )
Thank you