Questions about using the After Effects scripting API using Adobe Extendscript. Do not use for questions that are about expressions, they are NOT the same thing.

Adobe® Extendscript is a platform-independent scripting language, based on Javascript (ES5) that you can use to control many features and automate many tasks in Adobe® applications. Anything that can be achieved with the application's UI can be done with scripting, which makes it useful for repetitive or time-consuming tasks. Scripting is not able to access the pixels of a layer, or change the pixels in a composition, to do that you need to use the SDK for C++ developers. Documentation is available here: