Google Apps Script is a scripting language for light-weight application development in the Google Apps platform. It is based on JavaScript 1.6 with some portions of 1.7 and 1.8 and provides subset of ECMAScript 5 API, however instead of running on the client, it gets executed in the Google Cloud. According to Google, Google Apps Script "provides easy ways to automate tasks across Google products and third party services." Apps Script is also the tool that powers the add-ons for Google Docs, Sheets and Slides.
Build web apps and automate tasks with Google Apps Script
Google Apps Script is a JavaScript cloud scripting language that provides easy ways to automate tasks across Google products and third party services and build web applications.
https://www.google.com/script/start/