Can this function be garbage-collected?

问题:

Consider this piece of cake... ehm, code:

'use strict'

function doWork () {
  return new Promise(function (resolve, reject) {
    // work work work...
    // Done! But... where's the resolve() ???
  })
}

doWork().then(function doMoreWork () {
  // Some more work to do...
})

Once the function in the Promise's constructor finishes...

1. Is the Promise object garbage-collectible?
2. Is doMoreWork() garbage-collectible?

My guess is that doMoreWork() cannot be GC-ed directly because the Promise keeps a reference to it, but once the promise's body finishes and returns the execution context to the upper (?) scope, the stack unwinds (because there is no more statements here to be executed) and the Promise becomes unreachable, thus being garbage-collectible.

Can you confirm that my understanding of this topic is correct?

How could I empirically observe this behaviour? In other words, how can I monitor what objects are being GC-ed and when? I develop purely in Node.js, if that makes any difference.

回答1:

There is nothing keeping reference to the promise so it will be garbage collected. The promise is the only thing keeping reference to the function doMoreWork so it will be garbage collected too.

How could I empirically observe this behaviour? In other words, how can I monitor what objects are being GC-ed and when? I develop purely in Node.js, if that makes any difference.

The GC in V8 never necessarily collects an object. For instance if this is your whole program, it would be a waste of time to run any GC in the first place.

回答2:

1. The Promise object is collectable if it has no references pointing to it. If it is used doWork().then(...) a temporary reference is created. So until .then does not block anymore there is a reference to the object so it cannot be collected
2. You're right, doMoreWork is also not collectible because the Promiseobject has a reference to it

The statement doWork().then(...) can be replaced by

new Promise(function (resolve, reject) {
  // work work work...
}).then(function doMoreWork () {
          // Some more work to do...
        })

So you can imagine that you are using the Promiseobject directly, so the "Upper"-Scope is where the object is used.

Objects are generally collected when there are no more references to it. Even if the code is in a Promise it is just an object and the call to then is chained, so the object is being used

回答3:

To see if an object is garbage-collectible you can create a test and look for memory leak (through task manager). If your code is written properly, everything gets collected.