In JavaScript we can separate a program into separate files. How do we make a function we define in a file available to other files?

You typically write a function, like this:

function sum(a, b) {
  return a + b
}

and you can make it available for any other file using this syntax:

export default sum

We call it a default export.

The files that need the function exported will import it using this syntax:

import sum from 'myfile'