Using ECS Core developers have an ideal framework for programming applications in SharePoint Online/ Office 365.
Applications using ECS Core can basically be developed in any programming language that consumes REST Services.
JavaScript library for REST
For programming with ECS REST Services and JavaScript we recommend to use our JavaScript library (tEcs). It’s goal is to make client side programming with JavaScript and ECS easier by encapsulating raw methods of ECS.
App Development
For app development please check out the Knowledge Base articles for typical implementation scenarios:
Provider Hosted App in Sharepoint (+Active Directory)
Office 365 Sharepoint Online App (NAPA)