How to Enable JavaScript Engine in After Effects

Switching to the modern JavaScript expression engine in Adobe After Effects allows you to use modern ECMAScript standards, resulting in cleaner code, improved expression syntax, and significantly faster processing speeds. This guide provides the exact steps needed to switch your project from the Legacy ExtendScript engine to the JavaScript engine, along with key considerations when making the transition.

Steps to Enable the JavaScript Engine

1. Open your project in Adobe After Effects.
2. Navigate to the top menu and select File > Project Settings... (Alternatively, use the keyboard shortcut Ctrl + Alt + Shift + K on Windows or Cmd + Option + Shift + K on macOS).
3. In the Project Settings dialog box, click on the Expressions tab.
4. Locate the Expressions Engine dropdown menu.
5. Select JavaScript from the list instead of Legacy ExtendScript.
6. Click OK to apply the changes to your project.

Key Differences to Keep in Mind

Once the JavaScript engine is active, your project gains access to modern JavaScript (ES6+) features, including let and const declarations, arrow functions, template literals, and array methods like .map() and .filter().

However, the JavaScript engine is stricter than the older ExtendScript engine. Certain syntax habits—such as omitting semicolons, writing loose equality checks, or using undefined variables—can trigger errors in the JavaScript engine that the Legacy engine previously ignored. If an existing expression breaks after switching engines, inspect the code for non-standard syntax or switch that specific project back to Legacy ExtendScript.