错误的Promise返回值

Question

I'm trying to make a function returns whether an id is the validId. However, it turns out that sometimes, even if await isValidId(230) is true , validId is not 230.

I assume this is because promises are always resolved asynchronously. Is that right? How am I supposed to design this function then?

let validId = 230;
let isValidId = function(id){
  return new Promise(resolve => {
    //async code
    resolve(validId === id);
  });
}

if (await isValidId(230)){
  //validId is not necessary 230
}

Here's a more complete example.

let playerAskedToLogOut = false;
let playersOnline = ['MyUsername'];
let canPlayerUseItem = async function(player,itemId){      
  let hasItem = await Database.playerHasItem(player, itemId);
  let isStillOnline = playersOnline.includes(player);
  playerAskedToLogOut = true;  //normally called by another function
  return hasItem && isStillOnline;    
};

setInterval(() => {
  useItems();
  logOutPlayers();
}, 100);

let useItems = async function(){      
  if (await canPlayerUseItem('MyUsername', 'hammer')){
    //the player is not online anymore. Yet, await canPlayerUseItem returned true
  }
}
let logOutPlayers = function(){
  if(playerAskedToLogOut)
    playersOnline = []
}

Answer 1

The code in the question has a syntax error: It's using await in a non- async function, which doesn't work. (You've edited the code again to make canPlayerUseItem async )

First: There's no purpose at all in using new Promise within an async function. The function creates a promise for you automatically. So that code (as of this writing, you keep changing it) really should be just:

let canPlayerUseItem = async function(player,itemId){
  let hasItem = await Database.playerHasItem(player, itemId);
  let isStillOnline = playersOnline.includes(player);
  return hasItem && isStillOnline;
};

The only reason canPlayerUseItem 's promise would resolve with true would be if hasItem comes back truthy and playersOnline contains player as of when we do that check, which is after we've waited for playerHasItem .. If the player is not online when you call canPlayerUserItem but is online when the playerHasItem check has completed, it will resolve with true .

If you want to do the playersOnline check before waiting for the playerHasItem call:

let canPlayerUseItem = async function(player,itemId){
  if (playersOnline.includes(player)) {
    return false;
  }
  return await Database.playerHasItem(player, itemId);
};

Or if you want to check both (but they could have gone offline, then come back online, in-between):

let canPlayerUseItem = async function(player,itemId){
  if (playersOnline.includes(player)) {
    return false;
  }
  let hasItem = await Database.playerHasItem(player, itemId);
  let isStillOnline = playersOnline.includes(player);
  return hasItem && isStillOnline;
};

---

With your latest edit, you've suggested that something is removing the player from the online array after the promise resolution. That suggests to me that you're asking about how you can know the player is still online in the code using canPlayerUseItem .

Answer: You can't. Nothing you do in canPlayerUseItem can protect you from the race condition inherent in this:

if (await canPlayerUseItem(player, itemId)) {
    // They player may now be offline
}

...because it's possible for the player to sign out after canPlayerUseItem has done its check but before your code consuming that result runs. This is the nature of asynchronous code. The above is basically this:

1. flag = canPlayerUseItem(player, itemId)
2. Yield until end of this event loop iteration
3. if (flag) ... (at which point the player may no longer be logged in)

If you need to do that check, you'll have to do it yourself outside the asynchronous function:

if (await Database.playerHasItem(player, itemId) && playersOnline.includes(player)) {
    // As of this event loop, the player is online, and had the item
    // when we checked a moment ago
}

which is:

1. flag = Database.playerHasItem(player, itemId)
2. Yield until the end of this event loop iteration
3. if (flag && playersOnline.includes(player) ...