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If the user’s After Effects is set to use the legacy ExtendScript engine, use this to prompt the user to change it to the new JavaScript engine. Returns false/true, so you can also use it for validation purposes.

function SetJavascriptEngine() {
var currentEnginge = app.project.expressionEngine;
if (currentEnginge !== "javascript-1.0" && currentEnginge !== undefined) {
var confirmChange = confirm("Confirm changing to the new JavaScript engine?");
if (confirmChange == true) {
try{
app.project.expressionEngine = "javascript-1.0";
alert("Success!");
return true;
} catch (e) {
alert("Something Went Wrong...")
return false;
}
} else {
return false;
}
} else return true;
}
SetJavascriptEngine();