How to use a promise (in Typescript)

I’m comfortable with async\await\then etc. but was just experimenting and came across this and wanted to ask:

Let’s say I have this function:

HelloWorld():Promise<string>
  {
    return new Promise(resolve => {
      setTimeout(() => {
        alert("IN");
        resolve('Hello World!');
      }, 2000);
    });
  }

Now, I execute this function from another function like so:

async onClick()
{
    var p:Promise<string> = this.HelloWorld();

    await(p);

    alert(p);
}

So, I’ll see alerts (after 2 seconds) in the following order:

‘IN’

‘[object Promise]’

My question is, by the time the second alert is called the promise has resolved (due to presence the await(p) call). So it’s value is determined (‘Hello World!’).

Can I access it’s value somehow other than using the usual .then type pattern ?

So, something like:

var p:Promise<string> = this.HelloWorld();

await(p);

alert(<VALUE OF PROMISE HERE (which now equals 'Hello World!')>);

Like I say, just experimenting and wondering.

>Solution :

Well, first, await is a keyword, not a function, so i am not sure what you are doing here.

When a promise is resolved, you can access its resolved value as a parameter in the callback of the then method, or, if it is awaited with await, you can assign directly the value to a variable.

So this

myPromise().then((value) => console.log(value));

Is the same as this

const value = await myPromise();
console.log(value);

So in your case you would just have to do this

const value = await p;

Or even

const value = await this.HelloWorld();

Also, no need to explicitly type the value here, as the return type of HelloWorld is already Promise<string>