How to Access the Value of a Promise

How to access the value of a promise?

promiseA's then function returns a new promise (promiseB) that is immediately resolved after promiseA is resolved, its value is the value of the what is returned from the success function within promiseA.

In this case promiseA is resolved with a value - result and then immediately resolves promiseB with the value of result + 1.

Accessing the value of promiseB is done in the same way we accessed the result of promiseA.

promiseB.then(function(result) {
    // here you can use the result of promiseB
});

Edit December 2019: async/await is now standard in JS, which allows an alternative syntax to the approach described above. You can now write:

let result = await functionThatReturnsPromiseA();
result = result + 1;

Now there is no promiseB, because we've unwrapped the result from promiseA using await, and you can work with it directly.

However, await can only be used inside an async function. So to zoom out slightly, the above would have to be contained like so:

async function doSomething() {
    let result = await functionThatReturnsPromiseA();
    return result + 1;
}

how to get the value from a Promise resolve?

Your convertFileToString looks a little wrong: you should be invoking resolve() in the onload handler, not the other way round:

convertFileToString(file){
  return new Promise((resolve, reject)=>{
      let fileReader = new FileReader();
      fileReader.readAsText(file);
      fileReader.onload = (event) => {
          resolve(event.target.result);
      }
    })
}

How to access values of a Promise?

How to extract data out of a Promise

NO you can't get the data synchronously out of a promise like you suggest in your example. The data must be used within a callback function. Alternatively in functional programming style the promise data could be map()ed over.

If your are OK using async/await (you should it's awesome) then you can write code that looks synchronous yet retain the asynchronicity of a promise (see @loganfsmyth comments).

const { foo, bar }  = await iAmAPromise.then(result => result.data);

Overall since you are already using ES6 I assume you are also using a transpiler. In which case you should definitely give async/await a try.
Just be sure to weight in the decision that as today they are not yet a ratified specification.

Get value of resolved Promise in sync

No, it is not possible to do this. This is by design.

The Promise A+ specification is meant to be used as a simple, consistent way to deal with asynchronous operations. One of the constraints is that passing a new callback on to then() on an already resolved promise will always execute on a later tick in the event loop, so things are consistent.

Adding a secondary way to inspect promise results would not have been impossible. It would probably have been quite easy to do so, but there's at least 2 problems with adding this to the specification:

1. If you're looking to build a specification, you want it to be as simple as possible. The specification itself actually only defines the then() function.
2. By adding this feature, you're opening the door to hordes of developers getting even more confused about something that's already hard to grok.

Promises and asynchronous operations are hard for people to understand. I see questions here daily about promises and not 'getting' it. If non-async way to access promise results would be added to the default promise, I'd imagine that this would be an even larger amount. It's good to try and enforce 'the right way' of doing things.

However, this decision is not simply made for you. You're not stuck there. You can very easily build your own version of a promise that has this feature, and still works with existing promises. As long as your object has a then() function that behaves according to Promises/A+ you can do with the rest of the object whatever you want.

Is there a way to save [[Promise Result]] as a variable?

There are 3 ways for solving this:

1. Since you return a promise, use .then to get the returned value.

function currentloginid() {
  return fetch('http://localhost/gaq/api/api.php?action=userid', {
      method: 'GET',
    })
    .then(function(response) {
      return response.json();
    })
    .then(function(data) {
      var userid = JSON.parse(data);
      console.log(userid);
      return userid;
    })
}

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