Running this script:

 let foo = "Foo";
 const bar = "Bar";
 alert(foo + bar);

produces the following error:

 Error 25: Expected: ;.
 Line: 1
 -> let foo = "Foo";

ES6 language features are ostensibly supported, according to this page: https://helpx.adobe.com/after-effects/using/legacy-and-extend-script-engine.html However, only var and const bindings appear to be supported. Replacing "let foo =" with "const foo =" in the example above works as expected.