On Windows, After Effects uses the V8 open-source JavaScript engine. On macOS, After Effects uses the JavaScriptCore framework. Both of these engines provide a more modern version of JavaScript than available in ExtendScript, which only supports the 3rd Edition of the ECMA-262 Standard.

To choose whether a project uses the JavaScript engine or Legacy ExtendScript, in the Project Settings dialog, click the Expressions tab, and change the Expressions Engine option. Projects saved in previous versions of After Effects default to Legacy ExtendScript.

• Higher mathematical precision in floating-point operations. 
• Array and string methods from modern JavaScript can be used.
• Other modern JavaScript objects like typed arrays, usage of keywords 'let' and 'const', and many other additions made in recent versions of the ECMA-262 Standard are available.

• The syntax for if/else statements is strict in the JavaScript engine and need to be written for standardized JavaScript.
• When accessing the indexes of characters on a Text Layer as an array, use text.sourceText.value[i] instead of text.sourceText[i].
• Older snake_case expression methods  such as this_comp and to_world, which had been previously deprecated in ExtendScript) are not supported.
• Simplified 'this(arg)' syntax is not supported.