Heynote is a dedicated scratchpad for developers. It functions as a large persistent text buffer where you can write down anything you like. Works great for that Slack message you don't want to accidentally send, a JSON response from an API you're working with, notes from a meeting, your daily to-do list, etc.
The Heynote buffer is divided into blocks, and each block can have its own Language set (e.g. JavaScript, JSON, Markdown, etc.). This gives you syntax highlighting and lets you auto-format that JSON response.
Download the appropriate (Mac, Windows or Linux) version from the latest Github release (or from [heynote.com](https://heynote.com)). The Windows build is not signed, so you might see some scary warning (I can not justify paying a yearly fee for a certificate just to get rid of that).
Heynote is built upon [CodeMirror](https://codemirror.net/), [Vue](https://vuejs.org/), [Electron](https://www.electronjs.org/), [Math.js](https://mathjs.org/), [Prettier](https://prettier.io/) and other great open-source projects.