Learn ScriptUI and ExtendScript and Create Dockable Scripts for After Effects.
ExtendScript Toolkit CC is used a lot in scripting for the Adobe products like After Effects, Premier, Illustrator and Photoshop. But we can now use Visual Studio Code for Adobe ExtendScript scripting.
A list of all the essential resources you can use while you are scripting for the Adobe suite.
This tutorial will show you how to write a script that applies a expressions to all the properties you have selected. But even better you will also have a good understanding of all the different aspects of scripting.