When should I use Async? ¶ You should use Async when you desire explicit concurrency in your program. That means you want to run multiple tasks at the same time, and you want to be able to wait for the results of those tasks.

Instance methods Instances of Models are documents. Documents have many of their own built-in instance methods. We may also define our own custom document instance methods. // define a schema const animalSchema = new Schema({ name: String, type: String }, { // Assign a function to the "methods" object of our animalSchema through schema options. // By following this approach, there is no need to create a separate TS type to define the type of the instance functions. methods: { findSimilarTypes(cb) { return mongoose.model('Animal').find({ type: this.type }, cb); } } }); // Or, assign a function to the "methods" object of our animalSchema animalSchema.methods.findSimilarTypes = function(cb) { return mongoose.model('Animal').find({ type: this.type }, cb); }; Now all of our animal instances have a findSimilarTypes method available to them. const Animal = mongoose.model('Animal', animalSchema); const dog = new Animal({ type: 'dog' }); dog.findSimilarTypes((err, dogs) => { console.log(dogs); // woof }); Overwriting a default mongoose document method may lead to unpredictable results. See this for more details. The example above uses the Schema.methods object directly to save an instance method. You can also use the Schema.method() helper as described here. Do not declare methods using ES6 arrow functions (=>). Arrow functions explicitly prevent binding this, so your method will not have access to the document and the above examples will not work.

async for (not used here) iterates over an asynchronous stream.

async with allows awaiting async responses and file operations.

So, you're doing some async stuff, repeatedly, many times.

I was getting an error indicating I was using an invalid access_token. It turns out that I wasn't waiting for getLoginStatus to complete prior to making an API call

because the value isn't there yet. A promise is just a marker that it will be available at some point in the future. You cannot convert asynchronous code to synchronous, though. If you order a pizza, you get a receipt that tells you that you will have a pizza at some point in the future. You cannot treat that receipt as the pizza itself, though. When you get your number called you can "resolve" that receipt to a pizza. But what you're describing is trying to eat the receipt.

async is a concurrency technique. If you need concurrency, async is required for node to work properly (not "better"). If you don't have concurrency, you don't need async. The point is you need to actually understand what async does for you and why. It's not inherently "better" for no reason and you don't need to memorize it as a "best practice". If the OP is writing a command line utility to alter a JSON file then exit, async complicates the code for no reason as the concurrency is not required.

async vs. sync depends exactly on what you are doing in what context. If this is in a network service, you need async. For a command line utility, sync is the appropriate paradigm in most simple cases, but just knee-jerk saying "async is better" is not correct. My snippet is based on the OP snippet for context.

However, if one uses aiohttp, one chooses asynchronous programming, a paradigm that makes the opposite trade-off: more verbosity for better performance

They are:doers: self-motivated or intrinsically driven to achieve and learndrivers: proactive and decisive; don't wait for orderspromoters: passionate and excited to share what they create for their own benefit and the benefit of others

As said in the chapter, there’s an "implicit try..catch" around the function code. So all synchronous errors are handled. But here the error is generated not while the executor is running, but later. So the promise can’t handle it.

My gut told me calling an async function from the setTimeout callback was a bad thing. Since the setTimeout machinery ignores the return value of the function, there is no way it was awaiting on it. This means that there will be an unhandled promise. An unhandled promise could mean problems if the function called in the callback takes a long time to complete or throws an error.

test2 being marked async does wrap your return value in a new promise:

but has a critical difference: the expression console.log("before 2"); does not and cannot depend on the resolved value result. The throw propagates through all chained promises, and when it stops, there is no remaining undefined behavior! No piece of code is left in an unclear state, and therefore there is no reason to crash.

The power of await is that it lets you write asynchronous code using synchronous language constructs.

I ended up writing a custom store that "buffers" sets for both a small time interval and ensuring only one async action is in flight (and triggering an extra round of async processing if a set was seen after the last async action was launched).

If you return a Promise from load, SvelteKit will delay rendering until the promise resolves.

Remember that async functions always return promises. This promise rejects if any uncaught error occurs in the function. If your async function body returns a promise that rejects, the returned promise will reject too.

I refactored quite a bit of tarball-fetcher now to use actual Promises and asyncawait instead of passing resolve / reject callbacks around. This makes the code quite a bit easier to follow in my opinion, but let me know if anything should be changed there.

Note that when using sass (Dart Sass), synchronous compilation is twice as fast as asynchronous compilation by default, due to the overhead of asynchronous callbacks.

Unlike "native" events, which are fired by the DOM and invoke event handlers asynchronously via the event loop, dispatchEvent() invokes event handlers synchronously.

Derived data can move between being synchronous and asynchronous without modifying the components that use it.

here I wrapped the function call in an IIFE - that's what that (async () => {....})() is if you've never seen it. This is simply because we need to wrap the await call in a function that uses the async keyword, and we also want to "immediately invoke" the function (IIFE = "Immediately Invoked Function Execution") in order to call it.

Here we store the three Promise objects in variables, which has the effect of setting off their associated processes all running simultaneously. Next, we await their results — because the promises all started processing at essentially the same time, the promises will all fulfill at the same time

By only adding the necessary handling when the function is declared async, the JavaScript engine can optimize your program for you.

$: (async() => filtered = await getItems())();

Your solution is a very basic. The case above is more complex because using your solution you can't manipulate with fetched data outside of template and even outside {#await / } tag. So, if you need a read-only solution it's good but otherwise, it won't help you.

there is a particularly unconventional mechanism by which these coroutines actually get run. Their result is an attribute of the exception object that gets thrown when their .send() method is called.

If Python encounters an await f() expression in the scope of g(), this is how await tells the event loop, “Suspend execution of g() until whatever I’m waiting on—the result of f()—is returned. In the meantime, go let something else run.”

A coroutine is a specialized version of a Python generator function

Asynchronous routines are able to “pause” while waiting on their ultimate result and let other routines run in the meantime. Asynchronous code, through the mechanism above, facilitates concurrent execution. To put it differently, asynchronous code gives the look and feel of concurrency.

What’s important to know about threading is that it’s better for IO-bound tasks. While a CPU-bound task is characterized by the computer’s cores continually working hard from start to finish, an IO-bound job is dominated by a lot of waiting on input/output to complete.