In recent years, javascript has been extended to support modules.

Documentation

MDN modules https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules

Example

<html lang="en">
  <head>
    <script type="module" src="main.js"></script>   <!-- add main.js to file -->
  </head>
</html>

import * as Foo from './modules/foo.js'
import { Bar, Baz } from './modules/foo.js'
import { Bar as barbar } from './modules/foo.js'

export { foo, bar, baz };
export * from 'foo.js'
export { name } from 'foo.js'

// ${project}/modules/math.js

export default function sum(a, b) {
    return a + b;
}

import * as Math from './modules/math.js'

Math.default(1, 1) // -> 2  ('default' is the 'sum' function)