Javascript while循环，其中条件是一个承诺

Question

Is it possible to make a native JavaScript while loop where the condition is a promise?

Edit: What I'm trying to do is implement a check before uploading a file to firebase storage, to see if a file with the same name already exists in the firebase storage. If there is already a file with the same name then add a random suffix and check again.

var storageRef = firebase.storage().ref().child(fileName);

while(storageRef.getDownloadURL()) {
    // create random number in between 0 and 100
    var random = Math.floor((Math.random() * 100) + 1);    
    storageRef = firebase.storage().ref().child(fileName + random);
}

storageRef.put(file);

Answer 1

Is it possible to make a native JavaScript while loop where the condition is a promise?

No, it is not.

There are a couple problems with trying to do that:

1. With ES6 standard promises, there is no way to directly test the value of a promise. The only access to the eventually resolved value is with .then() . You can't do while (p !== resolved) .

2. Even if you could loop on the promises value, because Javascript is event driven and single threaded, if you did do a loop, then the asynchronous operation of the promise could never run and the promise could never get resolved anyway.

Instead, you simply use:

p.then(function(result) {
    // process result here
}).catch(function(err) {
    // process error here
});

If there's more you wanted to do in your loop, you would have to disclose that actual code before we could advise further on how to best do that.

---

If you want to repeat some operation until some condition, then just put that operation in a function, test the condition in the .then() handler and if you want to repeat, then just call the function again.

function doSomething() {
    // some async operation that returns a promise
}

function next() {
    return doSomething.then(function(result) {
        if (result < someValue) {
             // run the operation again
             return next();
        } else {
             return result;
        }
    });
}

next().then(function(result) {
      // process final result here
}).catch(function(err) {
    // process error here
});

---

EDIT: With ES7, you can use async and await . It won't really be a while loop, but it will obviate the need for a while loop because you don't have to "poll" the asynchronous value at all.

async function f() {
   let value = await somePromise;
   // put code here to execute after the promise
   console.log(value);
}

The internal execution of function f() will be paused until the promise resolves or rejects. Note that the caller of f() will not be paused. f() will return a promise immediately as soon as the await line is executed. That promise itself will resolve and reject when the internal execution of f() finally finishes.

Answer 2

Yes it is!

What you're looking for are generators: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_Generators .

I prefer using Observables eg RxJS for more complex stuff.