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JS Promises: Why does await have to be inside an async function?

Say I have the following code:
new Promise(res => res(1))
.then(val => console.log(val))
I can achieve the same thing with async/await like this:
let func = async () => {
let val = await new Promise(res => res(1))
console.log (val)
}
func()
I put the async/await code inside a function only because you have to be inside an async function in order to use await.
What I Want To Know: Why is this rule enforced? What would be the problem with just doing
let val = await new Promise(res => res(1))
console.log (val)
Is the reason that await causes the current scope to pause execution, and so forcing you to put the async/await code inside the special scope of an async function prevents JS from pausing the execution of all the rest of your code?

An async function is a different kind of function. It ALWAYS returns a promise. At the point of the first await that it hits, the function execution is suspended, the async function returns that promise and the caller gets the promise and keeps executing whatever code comes next.
In addition, the async function automatically wraps its function body in a try/catch so that any exceptions whether synchronous or unhandled rejected promises from an await are automatically caught by the async function and turned into a rejection of the promise they automatically return.
And, when you return a value from the async function, that return value becomes the resolved value of the promise that it returns.
What I Want To Know: Why is this rule enforced? What would be the problem with just doing...
An async function has a number of behaviors that a regular function doesn't and the JS interpreter wants to know ahead of time which type of function it is so it can execute it properly with the right type of behavior.
I suppose it might have been possible for the interpreter to have discovered when compiling the function body that it contains an await and automatically given the surrounding function an async behavior, but that's not very declarative and simply adding or removing one await could change how the function works entirely. Just my guess here, but the language designers decided it was much better to force an async function to be declared that way, rather than infer its behavior based on the contents of the function body.
The big picture here is to understand that an async function just works differently:
Always returns a promise
Automatically catches exceptions or rejected awaits and turns them into rejections
Suspends function execution upon await
Converts returned value into resolved value of the promise it returns
Chains an explicitly returned promise to the async-returned promise.
And, as such the language is clearer and more declarative if that separate behavior is spelled-out in the declaration with the async keyword rather than inferred from the function body.

Module-scope variable refuses to be overwritten by an async function. Why?

The module-scope variable "output" refuses to be overwritten by the async function "retrieveTextWrapper", and I cannot figure out why. My objective is to output the text on StackOverFlow's homepage. retrieveTextWrapper successfully scrapes this information, but I can't seem to assign this content to the output variable. What am I doing wrong? How can I print the scraped information from the main() function?
Note: I am using electron version 3.0.4 because bypassing CORS is less of a pain on that version.
const {BrowserWindow, app} = require('electron')
output = "this should be overwritten by the retrieveTextWrapper method"
async function main(){
navigate();
win.openDevTools();
await win.webContents.once('dom-ready',retrieveTextWrapper);
console.log(output);
//prints "this should be overwritten by the retrieveTextWrapper method"
}
function navigate() {
win = new BrowserWindow({width:900,height:900});
win.loadURL(`https://stackoverflow.com/`);
}
function retrieveText(){
return `document.querySelector("*").innerText`;
}
async function retrieveTextWrapper(){
output = await win.webContents.executeJavaScript(retrieveText().replace("*", "#content"));
}
app.on('ready',main)

win.webContents.once() does not return a promise (since interfaces generally don't accept both callbacks and return a promise at the same time).
Therefore await doesn't wait for the asynchronous operation to complete. Therefore, you're looking at output before its value has been reassigned. await only does something useful when you await a promise that is connected to the asynchronous operation you're trying to wait for.
To confirm this timing issue, add a unique console.log() statement before and after the await win.webContents.once('dom-ready',retrieveTextWrapper); and inside of retrieveTextWrapper and then you can see the sequencing of these log messages.
Yep, everything changes as it should within retrieveTextWrapper function. And your explanation makes a lot of sense. However, is it possible to wait for the callback to finish (using some other syntax aside from await)? That way, I can use the updated value for other operations in the main function?
You have a couple options.
You could "promisify" win.webContents.once() so you could then use await with it.
You could put the callback inline and put the rest of your code in main inside that callback (a classic way of dealing with asynchronous operations).
Here's an example of promisifying win.webContents.once():
function waitForDomReady() {
return new Promise((resolve, reject) => {
// may want to check if document already has dom-ready and resolve immediately
win.webContents.once('dom-ready', resolve);
});
}
And, you could then use it like this:
async function main(){
navigate();
win.openDevTools();
await waitForDomReady();
await retrieveTextWrapper();
console.log(output);
}
This assumes that the code in retrieveTextWrapper that calls win.webContents.executeJavaScript() does actually return a promise when it's done. If not, you have to promisify that too.

How do I get data from pending resolved promise without async/await?

I have abstraction:
function fetchDataFromAPI() {
const url = `https://api...`
return fetch(url).then(response => response.json())
}
I want to use it in my other piece of code like:
if(something){
const data = fetchDataFromAPI()
return data
}
if I console.log data what I get is resolved pending promise
Promise {<pending>}
__proto__: Promise
[[PromiseStatus]]: "resolved"
[[PromiseValue]]: Object
How do I get that Object in data instead of Promise?

You can not. Here is why:
Promise is a language construct that makes JavaScript engine to continue to execute the code without waiting the return of inner function, also known as the executor function. A promise always run inside the event loop.
var p = new Promise(function(resolve, reject) {
setTimeout(function() {
resolve('foo');
}, 300);
});
console.log(p);
Basically a promise is a glorified syntactic sugar for a callback. We will see how but first lets have a more realistic code:
function someApiCall(){
return new Promise(function(resolve, reject){
setTimeout(()=>{
resolve('Hello');
})
})
}
let data = someApiCall();
console.log(data);
This is a so-called asynchronous code, when JavaScript engine executes it, someApiCall immediately returns a result, in this case pending promise:
> Promise {<pending>}
If you pay attention to the executor, you will see we needed to pass resolve and reject arguments aka callbacks. Yes, they are callbacks required by the language construct. When either of them called, promise will change its state and hence be settled. We don't call it resolved because resolving implies successful execution but a function also can error out.
How do we get the data? Well we need more callbacks, which will be called by the executor function once the promise is settled:
var p = new Promise(function(resolve, reject) {
setTimeout(function() {
resolve('foo');
}, 300);
});
p.then((result) => {
console.log(result); // foo
}).catch((err) => {
console.log(err);
});
Why we need to pass separate callbacks? Because one will be fed to the resolve, and the other to the reject. Then callback will be called by the resolve function, the catch callback by the reject function.
Javascript engine will execute these callbacks later on its leisure, for a regular function it means when the event loop is cleared, for timeout when the time is up.
Now to answer your question, how do we get data out from a promise. Well we can't.
If you look closely, you will see we don't really get the data out but keep feeding callbacks. There is no getting data out, but passing callbacks in.
p.then((result) => {
console.log(result);
}).catch((err) => {
console.log(err);
});
Some say use await:
async function() {
let result = await p;
}
But there is a catch. We have to or wrap it in async function. Always. Why? Because Async/await is another level of abstraction or syntactic sugar, whichever you prefer, on top of promise api. That is why we can not use await directly but always wrap it in async statement.
To sum up, when we use promise or async/await we need to follow certain convention and write terse code with closely knitted callbacks. Either javascript engine or transpilers like babeljs or typescript converts these code to regular javascript to be run.
I can understand your confusion because people keep saying getting data out when talking about promises, but we don't get any data out but pass callback to be executed when the data is ready.
Hope everything is clear now.

No, you cannot without using promises or async/await etc because calling a REST API is an asynchronous operation and is non blocking.
When you make a call to a REST API, the code shouldn't wait until the API returns a value because it may take a lot of time, making program non-responsive, thus by design making a network request is categorized as an asynchronous operation.

To avoid async/await, you'll need to use another .then:
if (something) {
return fetchDataFromAPI()
.then((data) => data /* you can console.log(data) here */)
}

Async / await vs then which is the best for performance?

I have a simple code in JavaScript that execute a request in an API and return the response, simple. But in this case I will have thousands of requests. So, which one of the code options will perform better, and why. Also which one is recommended as good pratices these days?
First options is using the .then to resolve the promises and the seccond one is using async / await.
In my tests the two options had very similar results without significant differences, but I'm not sure in scale.
// Using then
doSomething(payload) {
const url = 'https://link-here/consultas';
return this.axios.get(url, {
params: {
token: payload.token,
chave: payload.chave,
},
}).then(resp => resp.data);
}
// Using Async / await
async doSomething(payload) {
const url = 'https://link-here/consultas';
const resp = await this.axios.get(url, {
params: {
token: payload.token,
chave: payload.chave,
},
});
return resp.data;
}
Any explanation will be of great value.

From a performance point of view, await is just an internal version of .then() (doing basically the same thing). The reason to choose one over the other doesn't really have to do with performance, but has to do with desired coding style or coding convenience. Certainly, the interpreter has a few more opportunities to optimize things internally with await, but its unlikely that should be how you decide which to use. If all else was equal, I would choose await for the reason cited above. But, I'd first choose which made the code simpler to write and understand and maintain and test.
Used properly, await can often save you a bunch of lines of code making your code simpler to read, test and maintain. That's why it was invented.
There's no meaningful difference between the two versions of your code. Both achieve the same result when the axios call is successful or has an error.
Where await could make more of a convenience difference is if you had multiple successive asynchronous calls that needed to be serialized. Then, rather than bracketing them each inside a .then() handler to chain them properly, you could just use await and have simpler looking code.
A common mistake with both await and .then() is to forget proper error handling. If your error handling desire in this function is to just return the rejected promise, then both of your versions do that identically. But, if you have multiple async calls in a row and you want to do anything more complex than just returning the first rejection, then the error handling techniques for await and .then()/.catch() are quite different and which seems simpler will depend upon the situation.

There should be some corrections in this thread. await and .then are going to give very different results, and should be used for different reasons.
await will WAIT for something, and then continue to the next line. It's also the simpler of the two because it behaves mechanically more like synchronous behavior. You do step #1, wait, and then continue.
console.log("Runs first.");
await SomeFunction();
console.log("Runs last.");
.then splits off from the original call and starts operating within its own scope, and will update at a time the original scope cannot predict. If we can put semantics aside for a moment, it's "more asynchronous," because it leaves the old scope and branches off into a new one.
console.log("Runs first.");
SomeFunction().then((value) => {console.log("Runs last (probably). Didn't use await on SomeFunction().")})
console.log("Runs second (probably).");

As more explanation to #user280209 answer let's consider the following function which returns promise and compare its execution with .then() and async await.
function call(timeout) {
return new Promise((resolve, reject) => {
setTimeout(() => {
console.log(`This call took ${timeout} seconds`);
resolve(true);
}, timeout * 1000);
});
}
With .then()
(async () => {
call(5).then((r) => {
console.log(r);
});
await call(2); //This will print result first
await call(1);
})();
When running the above call the logs will be
This call took 2 seconds
This call took 1 seconds
This call took 5 seconds
true
As we can see .then() didn't pause the execution of its below line until it completes.
With async/wait
(async () => {
await call(5); //This will print result first
await call(2);
await call(1);
})();
When run the above function logs will be
This call took 5 seconds
This call took 2 seconds
This call took 1 seconds
So I think if your promise's result won't be used in the following lines, .then() may be better.

For those saying await blocks the code until the async call returns you are missing the point. "await" is syntactic sugar for a promise.then(). It is effectively wrapping the rest of your function in the then block of a promise it is creating for you. There is no real "blocking" or "waiting".
run();
async function run() {
console.log('running');
makePromises();
console.log('exiting right away!');
}
async function makePromises() {
console.log('make promise 1');
const myPromise = promiseMe(1)
.then(msg => {
console.log(`What i want to run after the promise is resolved ${msg}`)
})
console.log('make promise 2')
const msg = await promiseMe(2);
console.log(`What i want to run after the promise is resolved via await ${msg}`)
}
function promiseMe(num: number): Promise<string> {
return new Promise((resolve, reject) => {
console.log(`promise`)
resolve(`hello promise ${num}`);
})
}
The await line in makePromises does not block anything and the output is:
running
make promise 1
promise
make promise 2
promise
exiting right away!
What i want to run after the promise is resolved hello promise 1
What i want to run after the promise is resolved via await hello promise 2

Actually.
Await/Async can perform more efficiently as Promise.then() loses the scope in which it was called after execution, you are attaching a callback to the callback stack.
What it causes is: The system now has to store a reference to where the .then() was called. In case of error it has to correctly point to where the error happens, otherwise, without the scope (as the system resumed execution after called the Promise, waiting to comeback to the .then() later) it isn't able to point to where the error happened.
Async/Await you suspend the exection of the method where it is being called thus preserving reference.

If we just consider performance(time taken) then it actually depends on whether your operations are serial or parallel. If your tasks are serial then there will be no difference between await and .then. But if your tasks are parallel then .then will take less time. Consider the following example
let time = Date.now();
// first task takes 1.5 secs
async function firstTask () {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(1);
},1500)
})
}
// second task takes 2 secs
async function secondTask () {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(2);
},2000)
})
}
// using await
async function call(){
const d1 = await firstTask();
const d2 = await secondTask();
console.log(Date.now()-time, d1+d2)
}
call()
// using .then
async function call2(){
let d1=null,d2=null;
firstTask().then(data => {
d1=data;
if(d2){
console.log(Date.now()-time, d1+d2);
}
})
secondTask().then(data => {
d2=data;
if(d1){
console.log(Date.now()-time, d1+d2);
}
})
}
call2()
Here are the two tasks, first takes 1.5 secs and second takes 2 secs. Call function uses await where as call2 function uses .then . The output is as follows
From call2 2012 3
From call 3506 3
I hope it helps.

As far as I understand .then() and await are not the same thing. An async function won't proceed with the next command until the promise is resolved/rejected since it's basically an implementation of generators. On the contrast, in the case of .then(), the execution of the function will proceed with the next command and the resolve/reject callback will be executed "when there's time" aka when the current event loop (not entirely sure about that part) will be completed.
tldr; on a single promise await and .then() behave similarly but when one promise needs another one to be resolved first then the two of them behave entirely different

Many answer have been provided to this question already. However, to point out key information in the answers above and from my understanding, note below point:
only use await when not handling error return
if no crucial need for error handling use await instead
use .then .catch if returned error message or data is crucial for debugging / or proper error handling instead of try catch for await
Choose any prefer method from code sample below
const getData = (params = {name: 'john', email: 'ex#gmail.com'}) => {
return axios.post(url, params);
}
// anywhere you want to get the return data
// using await
const setData = async () => {
const data = await getData();
}
// to handle error with await
const setData = async () => {
try {
const data = await getData();
}
catch(err) {
console.log(err.message);
}
}
// using .then .catch
const setData = () => {
var data;
getData().then((res) => {
data = res.data; console.log(data)
}).catch((err) => {
console.log(err.message);
});
}

What is meant by 'pause' when async function await expression 'pauses' execution?

I am very new to the concept of asyncronicity in Javascript and I wanted to make sure I am not misinterpreting what I am reading.
Consider this pseudo code in some Angular app:
async ngOnInit() {
this.responseObject = await this.myService.myGetRequest();
//do some more stuff down here
}
My understanding is that the ngOnInit() will 'pause' or stop execution at await, and NOT execute the code below that line until the promise object(data) is returned? Correct?

await used within an async function awaits the fulfillment of a Promise value or converts the variable to a Promise.
Yes, you are correct the code at next line will not be executed until the previous line which uses await has returned a fulfilled Promise or the value is converted to a Promise.
Note, it is not clear what the pattern
await this.responseObject = await this.myService.myGetRequest();
is expected to achieve. The first await should be able to be omitted
this.responseObject = await this.myService.myGetRequest();

The async function declaration will return a Promise that is resolved with the returned valued from your function call.
If you add the await expression, it will stop the async execution and wait until your promise is resolve to continue executing the other instructions in your code, somehow making it behave like a 'synchronous' function.
Do some reading over here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function

I feel the two above answers don't answer the core question. What does javascript mean when it says it's awaiting something? Javascript is singly threaded. So, instead of dealing with async itself, it runs all tasks in a separate task queue provided by the "runtime" (usually either the browser or node). When we await a piece of code, we are placing the promise on the task queue, waiting on the promise to finish, and then taking it off the task queue fo code execution to finish. For example, consider the following code block.
setTimeout(() => console.log("Finished Outer Timeout"), 0);
async function asyncCall() {
console.log("Before Await")
function resolveAfter2Seconds() {
return new Promise(resolve => {
setTimeout(() => {
console.log("Finished Inner Timeout")
resolve('resolved');
}, 2000);
});
}
await resolveAfter2Seconds()
console.log(`After Await`);
}
asyncCall()
If await fully blocked the task queue, we would expect the logging order to be:
Before Await
After Await
Finished Outer Timeout
Finished Inner Timeout
as the timeouts would have to wait on the task queue for the main script to finish. However, the ordering is:
Before Await
Finished Outer Timeout
Finished Inner Timeout
After Await
which indicates that the task queue can still run the two timeout functions even when the main script is awaiting. Note there may be differences depending on your browser, but this is generally how Javascript async/await works.
Work Cited:
https://jakearchibald.com/2015/tasks-microtasks-queues-and-schedules/
https://medium.com/#sadia.islam.badhon/callback-queue-vs-microtask-queue-9dc6a790330e
https://html.spec.whatwg.org/multipage/webappapis.html#queuing-tasks