Javascript: Wait for multiple things to finish before continuing

This is not a Forge API related topic, however, I've seen that many people getting started with JavaScript are unaware of Promises and Promise.all() functionality.

How to wait for async calls to finish Javascript

Promises are very useful to make your javascript code readable - instead of passing a callback, and then a callback inside a callback, etc, you can make your code look synchronous by using it along with await:

// note "await" can only be used inside a function flagged as "async"

await new Promise((resolve, reject) => {
  // download first file
  resolve()
})

await new Promise((resolve, reject) => {
  // download second file
  resolve()
})

// etc

In order to be more efficient, we can execute the above functions in parallel with the use of Promise.all()

let first = new Promise((resolve, reject) => {
  // download first file
  resolve()
})

let second = new Promise((resolve, reject) => {
  // download second file
  resolve()
})

await Promise.all([first, second])

alert('Finished all downloads!')

Just like in case of event handlers, the variables you are using from Promises are accessed later than the declaration of the function, so their values might not be what you expect - unless you know how closures work.