Prettier (formatter)

https://prettier.io/playground/

ESLint (linter)

https://eslint.org/demo

JSHint (linter)

http://jshint.com/

Tools for other languages

CSS

HTML

JSON

GraphQL

General purpose (handle more than 1 single syntax)

Download my free books, and check out my upcoming Full-Stack JavaScript Bootcamp! A 4-months online training program. Signups now open, until June 1!