How does the JavaScript Import work under the hood? -

how import statement work in javascript? read documentation , says places exported code within scope of file. mean code copied on file type import?

for example - in library.js have:

export {export function getusefulcontents(url, callback) {     getjson(url, data => callback(json.parse(data))); } 

in main.js have:

import { getusefulcontents} 'library.js';  getusefulcontents('http://www.example.com',     data => { dosomethinguseful(data); }); 

this allows me call getusefulcontents() in main.js. question is, main.js have contents exported library.js?

is using import same having physically defined getusefulcontents() in main.js?

function getusefulcontents(url, callback) {     getjson(url, data => callback(json.parse(data))); }  getusefulcontents('http://www.example.com',     data => { dosomethinguseful(data); });  

the main reason why ask because want know if using import have effect on main.js file size? or else going on under hood?

thank you!