Can you explain how this promise functions within the context of the mutation observer, even without an argument?

Question

Can you explain how this promise functions within the context of the mutation observer, even without an argument?

Recently, I came across a mutation observer in some TypeScript code that has left me puzzled. This particular implementation of a promise within the mutation observer seems unconventional to me:

  const observer = new MutationObserver((mutations: MutationRecord[]) => {
    Promise.resolve().then(someNextMethod)
    this.somethingThatTakesAWhile()
  })

  observer.observe(HTMLtarget)

Surprisingly, when the observation is triggered, the someNextMethod executes after this.somethingThatTakesAWhile(). I'm struggling to grasp how the promise in this scenario does not have any arguments yet still knows how to resolve. Can someone shed light on the underlying mechanics of this code? The sequence of execution here has got me intrigued. Thank you for your help!

javascript typescript promise mutation-observers thenable

Answer 1

Answer №1

Exploring the concept behind something like this:

 Promise.resolve().then(someNextMethod)

is essentially to trigger someNextMethod() after the current chain of events has completed. This scenario can be compared to a browser executing the following:

setTimeout(someNextMethod, 0);

However, using Promise.resolve() would give priority to this operation over setTimeout() if other tasks are in queue.

In the provided example, the purpose of these two lines:

Promise.resolve().then(someNextMethod)
this.somethingThatTakesAWhile()

is to execute someNextMethod() once this.somethingThatTakesAWhile() finishes and the callback from MutationObserver is processed, allowing for notification of other observers before someNextMethod() is invoked.

The reason why someNextMethod() is executed later is because all .then() handlers wait until the current thread of execution completes (and control is returned to the event loop), even if the attached promise has already been resolved. This behavior aligns with the specifications outlined in the Promise system.

The specific rationale behind implementing this approach varies depending on the context, and as the code provided is purely hypothetical, the true motive remains unspecified.

Answer 2

Exploring the concept behind something like this:

 Promise.resolve().then(someNextMethod)

is essentially to trigger someNextMethod() after the current chain of events has completed. This scenario can be compared to a browser executing the following:

setTimeout(someNextMethod, 0);

However, using Promise.resolve() would give priority to this operation over setTimeout() if other tasks are in queue.

In the provided example, the purpose of these two lines:

Promise.resolve().then(someNextMethod)
this.somethingThatTakesAWhile()

is to execute someNextMethod() once this.somethingThatTakesAWhile() finishes and the callback from MutationObserver is processed, allowing for notification of other observers before someNextMethod() is invoked.

The reason why someNextMethod() is executed later is because all .then() handlers wait until the current thread of execution completes (and control is returned to the event loop), even if the attached promise has already been resolved. This behavior aligns with the specifications outlined in the Promise system.

The specific rationale behind implementing this approach varies depending on the context, and as the code provided is purely hypothetical, the true motive remains unspecified.

Answer 3

Answer №2

Here's a comparison:

 Promise.resolve().then(someNextMethod)

This is the same as:

 Promise.resolve().then(() => someNextMethod())

If we reverse engineer this, it becomes:

const myNewMethod = () => someNextMethod()
Promise.resolve().then(myNewMethod)

Whether defining a function inline or referring to a function, it's just a difference in syntax. Any arguments passed through then will be passed to the referenced function, although in this case there are none since the promise has an empty return value.

In essence, the method doesn't require any parameters. This particular approach is actually an old JavaScript hack/trick to ensure the code runs at the end of the call stack.

Answer 4

Here's a comparison:

 Promise.resolve().then(someNextMethod)

This is the same as:

 Promise.resolve().then(() => someNextMethod())

If we reverse engineer this, it becomes:

const myNewMethod = () => someNextMethod()
Promise.resolve().then(myNewMethod)

Whether defining a function inline or referring to a function, it's just a difference in syntax. Any arguments passed through then will be passed to the referenced function, although in this case there are none since the promise has an empty return value.

In essence, the method doesn't require any parameters. This particular approach is actually an old JavaScript hack/trick to ensure the code runs at the end of the call stack.

Can you explain how this promise functions within the context of the mutation observer, even without an argument?

Answer №1

Answer №2

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