Anticipate the resolution of the observable

Question

Anticipate the resolution of the observable

For the past few months, I have been immersed in a project that required me to work within a synchronous environment without the need for HTTP requests or other external asynchronous access. However, the scope of the project has now changed, and I find myself needing to fetch data from HTTP requests. Previously, my code looked something like this:

get foo() { return this.value; }

Now, with the introduction of HTTP requests, I must retrieve this value asynchronously and update the variable once the response is received.

fetchValue() { this.http.get(addr).subscribe(resp => this.value = resp); }
get foo() {  this.fetchData(); return this.value; }

Unfortunately, this approach doesn't work as intended. Since the event loop only triggers after all logic has executed, this.foo will always return undefined. Using a while loop is not an option either, as it would prevent the execution of the HTTP request due to its synchronous nature.

Is there a way to pause the synchronous loop and allow the asynchronous operation to complete before proceeding? Or perhaps, can we wait for a specific function in the event loop to finish?

I'm unable to refactor the existing code as it would require a significant overhaul and considerably more time. Moreover, it would break backward compatibility which is crucial for this project. Therefore, I need foo to return the value only after it has been set; I cannot introduce subscriptions or promise resolutions elsewhere in the codebase to address this issue.

If achieving this is not feasible, could you explain why? I am looking for a clear explanation on the language paradigm that prohibits the implementation of this particular logic.

typescript http async-await rxjs event-loop

Answer 1

Answer №1

There is no way to cease the event loop in JavaScript as it serves as the core mechanism of the language. Disrupting the event loop would essentially halt the entire application's execution :)

To gain a better understanding of the event loop, I highly suggest watching this video - it provides one of the best explanations on this subject.

Now, let's address your code. You can utilize promises to "wait" for asynchronous operations to finish, and with the help of async/await syntax, it can be achieved in an elegant manner.

For starters, when using rxjs, you need to convert your http call into a promise by using the .toPromise() operator.

async fetchValue() {
  this.value = await this.http.get(addr).toPromise();
}

It's important to note that I prefixed the function name with the async keyword, allowing us to employ await within the function. Await anticipates a promise on the right-hand side and pauses the code below it until the promise resolves.

Your second function also requires async/await:

async getFoo() { 
  await this.fetchData();
  return this.value;
}

I'm not entirely sure why you need both functions since the first one accomplishes the same task. Nonetheless, I'm illustrating the concept to you.

Whenever you invoke instance.getFoo(), remember it operates asynchronously. Therefore, utilize async/await or .then within the calling function to handle the result.

Using async/await:

async function someMethod() {
  const foo = await instance.getFoo();
}

Using .then():

function someMethod() {
  instance.getFoo().then(foo => {
    // additional code goes here!
  })
}

Once again, I advise investing time in watching the aforementioned video and studying documentation on promises and async/await.

Answer 2

There is no way to cease the event loop in JavaScript as it serves as the core mechanism of the language. Disrupting the event loop would essentially halt the entire application's execution :)

To gain a better understanding of the event loop, I highly suggest watching this video - it provides one of the best explanations on this subject.

Now, let's address your code. You can utilize promises to "wait" for asynchronous operations to finish, and with the help of async/await syntax, it can be achieved in an elegant manner.

For starters, when using rxjs, you need to convert your http call into a promise by using the .toPromise() operator.

async fetchValue() {
  this.value = await this.http.get(addr).toPromise();
}

It's important to note that I prefixed the function name with the async keyword, allowing us to employ await within the function. Await anticipates a promise on the right-hand side and pauses the code below it until the promise resolves.

Your second function also requires async/await:

async getFoo() { 
  await this.fetchData();
  return this.value;
}

I'm not entirely sure why you need both functions since the first one accomplishes the same task. Nonetheless, I'm illustrating the concept to you.

Whenever you invoke instance.getFoo(), remember it operates asynchronously. Therefore, utilize async/await or .then within the calling function to handle the result.

Using async/await:

async function someMethod() {
  const foo = await instance.getFoo();
}

Using .then():

function someMethod() {
  instance.getFoo().then(foo => {
    // additional code goes here!
  })
}

Once again, I advise investing time in watching the aforementioned video and studying documentation on promises and async/await.

Anticipate the resolution of the observable

Answer №1

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