export.js

export default () => {
  console.log("default export")
}
export function printArg(arg) {
  console.log(arg)
}
export function throwError() {
  throw Error("error!");
}

export2.js

export default () => {
  console.log("default export2")
}
export function printArg(arg) {
  console.log(arg, '2')
}
export function throwError() {
  throw Error("error2!");
}

import.js

let importModule;
if (someBool) {
  importModule = import('./export.js');
} else {
  importModule = import('./export2.js'); 
}
importModule.then(module => {
    module.default();
    module.printArg('Hello');
    module.throwError();
  }).catch(e => {
    console.error(e)
  });