SpreadAPI is a free Google Apps Script that allows you to add REST API to any spreadsheet in Google Sheets in a few minutes. The API can be accessed over HTTPS from back-end as well as front-end apps.
It’s currently possible to publish a Google Sheet as JSON to get the data in a third-party application, but it’s harder to allow for posting new data back to that sheet. SpreadAPI is a script that creates a full-function REST API for a Google Sheet. Each implementation is set up individually and provides multiple authentication methods for specific users or even anonymous users.