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What is the difference between using and omitting keyword "await" in these two code blocks?

I'm following along an MDN article on async/await, and I understand the purpose of using the async keyword before functions, but I'm a little more confused about the await keyword. I've read up on on "await" as a result, and I get the general concept, but I'm still unsure when it comes to examples. For instance, here is a trivial piece of code (as shown in the MDN article) using async/await.
async function hello() {
return greeting = await Promise.resolve("Hello");
};
hello().then(value => console.log(value));
As you might expect, this logs "Hello" to the console. But even if we omit "await", it has the exact same output.
async function hello() {
return greeting = Promise.resolve("Hello"); // without await
};
hello().then(value => console.log(value));
Can someone help me understand exactly what the await keyword before Promise.resolve is doing? Why is the output the same even if it's omitted? Thanks.

In up-to-date JavaScript engines, there's no difference, but only because the return await somePromise is at the top level of the function code. If it were within a control block (for instance, try/catch), it would matter.
This is a recent-ish change. Until ES2020 the return await version would have introduced an extra async "tick" into the process. But that was optimized out in a "normative change" (rather than proposal) in ES2020 when the promise being awaited is a native promise. (For a non-native thenable, there's still a small difference.)
For the avoidance of doubt, there's an important difference between:
async function example() {
try {
return await somethingReturningAPromise();
} catch (e) {
// ...do something with the error/rejection...
}
}
and
async function example() {
try {
return somethingReturningAPromise();
} catch (e) {
// ...do something with the error/rejection...
}
}
In the former, a rejection of the promise from somethingReturningAPromise() goes to the catch block, because it's been awaited. In the latter, it doesn't, because you've just returned the promise, which resolves the promise from the async function to the promise somethingReturningAPromise() returns, without subjecting it to the catch.
But in your example, where it's not in any control structure, the await is largely a matter of style at this point.

Keep in mind that async/await is just a fancy API for working with promises.
An async function always and implicitly returns a promise (as soon as the first await or return statement is reached) that will be resolved when the function code itself finally reaches its end. The resolved promise value will be equal to the function's returned value.
async allows you to use await inside of the asynchronous scope. Await only makes sense when preceding a promise (could be another async function call, a promise created in place with new Promise(), etc.). Using await before non promise values will do no harm though.
await indicates, just that, wait. It tells the engine to 'stop' code execution on that scope and to resume it whenever the promise is fulfilled. await will then give you or 'return' the promise resolved value.
Just to give you some practical examples:
Using your first code:
async function hello() {
const greeting = await Promise.resolve("Hello"); // HERE I CAN USE await BECAUSE I'M INSIDE AN async SCOPE.
console.log('I will be printed after above promise is resolved.');
return greeting;
};
hello().then(value => console.log(value));
Using your second code, async could be completely removed:
function hello() {
return Promise.resolve("Hello");
};
hello().then(value => console.log(value));
Please let me know if there is any doubt.

A weird thing with async functions in JavaScript

$ cat x.js
async function print() {
console.log("abc");
}
print();
$ nodejs x.js
abc
How can it be?! print() returns a Promise object that isn't awaited, is it? If it is not awaited, then why is the console.log executed?

yeah, like choz said ,async functions returns a promise, even if you haven't defined a promise in your code.
i think this returned promise turns fulfilled after all promises in await statement get resolved.
i test it in the following code, it also returns promise, which only turns fulfilled after all promises go resolved(after 3000ms in this case):
async function print2() {
await console.log("abc")
await new Promise((res, rej) => {
setTimeout(() => {res(33)},3000)
})
await new Promise((res, rej) => {
setTimeout(() => {res(33)},50)
})
}

You could say that an empty function itself, returns undefined (Which actually doesn't return anything). Take a look at the sample below;
function print() {}
var returnVal = print(); // undefined
You'll notice that returnVal is undefined.
Now, if we have something in the body of test() there, but you still don't pass any return value, then returnVal will still be undefined.
function print() {
console.log('abc');
}
var returnVal = print(); // undefined
So, in order for a function to have a return value, one simply just needs to return it.
function print() {
console.log('abc');
return 1;
}
var returnVal = print(); // 1
When you conver it to async function. Quoting from MDN
The body of an async function can be thought of as being split by zero
or more await expressions. Top-level code, up to and including the
first await expression (if there is one), is run synchronously. In
this way, an async function without an await expression will run
synchronously. If there is an await expression inside the function
body, however, the async function will always complete asynchronously.
Code after each await expression can be thought of as existing in a
.then callback. In this way a promise chain is progressively
constructed with each reentrant step through the function. The return
value forms the final link in the chain.
Now, based on the information above, here's what we know that refer to your question;
Your print() does not return anything, which should be undefined
The async function will complete asynchronously, meaning it will always return a Promise. Either pending, fulfilled or rejected
So, to say it in your question, here's what your function actually does;
async function print() {
console.log("abc"); // Prints 'abc'
}
// Function above is equivalent to;
function printEquivalentInNonAsync() {
console.log("abc"); // Prints 'abc'
}
var returnVal = print(); // `returnVal` is `undefined`
And, to answer your question
If it is not awaited, then why is the console.log executed?
Regardless the async function is awaited, it still executes! - Awaits just to ensure the line execution is halted until the Asynchronous function (Promise) has reached fulfilled or rejected. Note that, the first state of a Promise is pending.

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.

What is the meaning of the `async` keyword?

I have been reading up on async/await in Node.js. I have learnt that the await keyword waits for a promise to be resolved, or throws an exception if it was rejected.
I have also learnt that every function that wants to use await needs to be marked async. However, what does it mean for a function to be marked async?
All the resources and blog posts I was able to find seem to explain await in great detail, but ignore the concept of an async function, or briefly gloss over it. For instance, this author puts it like this:
This makes the function return a Promise implicitly.
What does the async keyword really do? What does it mean for a function to implicitly return a Promise? What are the side effects other than being able to use await?
Alright, so from the answers I have received so far it's clear that it simply wraps the function's return value into a Promise, much like Promise.then would. That just leaves a new question though. Why does a function that uses await need to be async and thus return a Promise?

No matter what you actually return from your function, your async function will still return a Promise. If you return a Number, it actually returns a Promise that resolves to the Number your originally returned. This allows you to write synchronous "looking" code.
Rather than having to write out this:
function foo(){
return Promise.resolve("foo");
}
You can just write this:
async function foo(){
return "foo";
}
and foo() will automagically return a Promise that resolves to "foo".
In response to you comment:
Does it behave like Promise.then in the sense that if you already
return a Promise, it won't wrap it again?
await will peel the Promise until it gets to a value:
async function foo() {
return Promise.resolve(Promise.resolve(true));
}
async function bar() {
return true;
}
(async function () {
let tmp;
tmp = await foo();
console.log(tmp);
tmp = await bar();
console.log(tmp);
console.log("Done");
}());
/*
Prints:
true
true
Done
*/
Why is async needed?
Paraphrasing from #KevinB's comment.
await, just like yield in a generator, pauses the execution of that context until the Promise it's waiting on is no longer pending. This cannot happen in normal functions.
If a function is async but does not contain an await, the promise will be resolved immediately, and any callbacks will be ran on the next tick.

What does async do?
async is syntactic sugar for making your method chain Promise objects.
Take the following method for example:
async function myFunction(a)
{
if (a == 10)
{
await otherFunction();
}
return 10;
}
The JavaScript runtime you use might make more optimized code, but in its simplest it will be something along the lines:
function myFunction(a)
{
if (a === 10)
{
return otherFunction()
.then(() => myFunction_continuation());
}
else
{
return myFunction_continuation();
}
function myFunction_continuation()
{
return Promise.resolve(10);
}
}
For documentation on the Promise type I recommend checking out the Mozilla Developer Network page about the Promise type .
Why do you need to mark it async? Why not just use await?
Because your method needs to be split up into multiple "parts" for it to be able to have code execute after the method being awaited on. The example above has a single continuation, but there could be multiple.
The designers of JavaScript want to make it visible to the developer that the runtime is doing this "magic". But maybe the most important reason is that they don't want to break existing code using await as a variable name. They do this by making await a "contextual keyword". A "contextual keyword" is only a keyword in specific scenarios, in this case: when used inside a method marked as async:
function ABC()
{
var await = 10;
}
The above compiles. But if I add the async keyword to the function declaration it no longer does and throws an Uncaught SyntaxError: Unexpected reserved word.

Asynchronous Task Running
The Basic idea is to use a function marked with async instead of a generator and use await instead of yield when calling a function, such as:
(async function() {
let contents = await readFile('config.json');
doSomethingWith(contents);
console.log('Done');
});
The Async Keyword before function indicates that the function is meant to run in an asynchronous manner. The await keyword signals that the function call to readFile('config.json') should return a promise, and if it doesn't, the response should be wrapped in a promise.
The end result is that you can write asynchronous code as if it were synchronous without overhead of managing an iterator-based state machine.
Understanding ECMACSCRIPT 6 by Nicholas c. Zakas

Is it legitimate to omit the 'await' in some cases?

I am using async/await in several places in my code.
For example, if I have this function:
async function func(x) {
...
return y;
}
Then I always call it as follows:
async function func2(x) {
let y = await func(x);
...
}
I have noticed that in some cases, I can omit the await and the program will still run correctly, so I cannot quite figure out when I must use await and when I can drop it.
I have concluded that it is "legitimate" to drop the await only directly within a return statement.
For example:
async function func2(x) {
...
return func(x); // instead of return await func(x);
}
Is this conclusion correct, or else, what am I missing here?
EDIT:
A small (but important) notion that has not been mentioned in any of the answers below, which I have just encountered and realized:
It is NOT "legitimate" to drop the await within a return statement, if the called function may throw an exception, and that statement is therefore executed inside a try block.
For example, removing the await in the code below is "dangerous":
async function func1() {
try {
return await func2();
}
catch (error) {
return something_else;
}
}
The reason is that the try block completes without an exception, and the Promise object returns "normally". In any function which calls the outer function, however, when this Promise object is "executed", the actual error will occur and an exception will be thrown. This exception will be handled successfully in the outer function only if await is used. Otherwise, that responsibility goes up, where an additional try/catch clause will be required.

If func is an async function then calling it with and without await has different effects.
async function func(x) {
return x;
}
let y = await func(1); // 1
let z = func(1) // Promise (resolves to 1)
It is always legitimate to omit the await keyword, but means you will have to handle the promises in the traditional style instead (defeating the point of async/await in the first place).
func(1).then(z => /* use z here */)
If your return statements use await then you can be sure that if it throws an error it can be caught inside your function, rather than by the code that calls it.

await just lets you to treat promises as values, when used inside an async function.
On the other hand, async works quite the opposite, it tags the function to return a promise, even if it happens to return a real, synchronous value (which sounds quite strange for an async function... but happens often when you have a function that either return a value or a promise based on conditions).
So:
I have concluded that it is "legitimate" to drop the await only directly within a return statement.
In the last return statement of an async function, you just are returning a Promise, either you are return actually a directly a promise, a real value, or a Promise-as-value with the await keyword.
So, is pretty redundant to use await in the return statement: you're using await to cast the promise to a value -in the context of that async execution-, but then the async tag of the function will treat that value as a promise.
So yes, is always safe to drop await in the last return statement.
PS: actually, await expects any thenable, i.e. an object that has a then property: it doesn't need a fully spec compliant Promise to work, afaik.
PS2: of course, you can always drop await keyword when invoking synchronous functions: it isn't needed at all.

An async function always returns a Promise.
So please keep in mind that these writing of an async function are all the same:
// tedious, sometimes necessary
async function foo() {
return new Promise((resolve) => resolve(1)))
}
// shorter
async function foo() {
return Promise.resolve(1)
}
// very concise but calling `foo` still returns a promise
async function foo() {
return 1 // yes this is still a promise
}
You call all of them via foo().then(console.log) to print 1. Or you could call them from another async function via await foo(), yet it is not always necessary to await the promise right away.
As pointed out by other answers, await resolves the promise to the actual return value statement on success (or will throw an exception on fail), whereas without await you get back only a pending promise instance that either might succeed or fail in the future.
Another use case of omitting (i.e.: being careful about its usage) await is that you might most likely want to parallelize tasks when writing async code. await can hinder you here.
Compare these two examples within the scope of an async function:
async function func() {
const foo = await tediousLongProcess("foo") // wait until promise is resolved
const bar = await tediousLongProcess("bar") // wait until promise is resolved
return Promise.resolve([foo, bar]) // Now the Promise of `func` is marked as a success. Keep in mind that `Promise.resolve` is not necessary, `return [foo, bar]` suffices. And also keep in mind that an async function *always* returns a Promise.
}
with:
async function func() {
promises = [tediousLongProcess("foo"), tediousLongProcess("bar")]
return Promise.all(promises) // returns a promise on success you have its values in order
}
The first will take significantly longer than the last one, as each await as the name implies will stop the execution until you resolve the first promise, then the next one.
In the second example, the Promise.all the promises will be pending at the same time and resolve whatever order, the result will then be ordered once all the promises have been resolved.
(The Bluebird promise library also provides a nice Bluebird.map function where you can define the concurrency as Promise.all might cripple your system.)
I only use await when want to work on the actual values. If I want just a promise, there is no need to await its values, and in some cases it may actually harm your code's performance.

I got a good answer above, here is just another explanation which has occurred to me.
Suppose I have this:
async function func(x) {
...
return y;
}
async function func2(x) {
...
return await func(x);
}
async function func3(x) {
let y = await func2(x);
...
}
The reason why I can safely remove the await in the return statement on func2, is that I already have an await when I call func2 in func3.
So essentially, in func3 above I have something like await await func(x).
Of course, there is no harm in that, so it's probably better to keep the await in order to ensure desired operation.