Module export forms have subtle differences

mod.js

// exporter1.js
let foo = 1;
export { foo as default }; // exports the foo binding
foo = 2;

// exporter2.js
let foo = 1;
export default foo; // creates a new binding named *default* and initializes it to 1.
foo = 2; // assigns to the foo binding which is not exported

// importer1.js
import foo from "exporter1.js";
print(foo); // 2

// importer2.js
import foo from "exporter2.js";
print(foo); // 1