external jsx file don't use ecmascript 5

I have a simple .jsx script for AE.

If i use use some ECMAscript 5 statements (like let assignment or arrow functions) the program fail with syntax error:

writeln(app.project.expressionEngine); // ouput javascript-1.0

let a = 1; // Syntax Error: Expected: ;

const something = () => { // Syntax Error: ) does not have a value

     // ...

}

AE project has set javascript as expression engine and expressions inside a property of a composition that use EXCMAScript 5 works correctly.

The problem is only with external jsx file.