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.
SpreadAPI is a free alternative to tools like sheetsu.com, sheetdb.io or sheety.co. With SpreadAPI you get just the same but for free. Moreover, you don't share the data with any third party because the code runs on your account as a Google Apps Script.