多个 javascript promises.then and.catch 打错了

Question

I think that I am not understanding something properly here as it's very strange behaviour. If I call queryFindPlayer it should be falling into the.then which it does if queryFindContract function is not there but when it is there like below it seems to fall to the queryFindPlayer catch and add a new player.

queryFindPlayer(models, ConsoleId, UserId, SeasonId, LeagueId).then(players => {
    const player = players[0];
    queryFindContract(db, player.Team.id, UserId, SeasonId, LeagueId).then(contracts => {
      console.log("player has a contract to a team");
    }).catch(e => {
      console.log("failed to find player");
    });
}).catch(e => {
  queryAddPlayer(models, UserId, TeamId).then(player => {
    console.log("added player");   
  }).catch(addPlayerError => {
    console.log("failed to add player, shouldn't happen");
  });          
});

Answer 1

If queryfindPlayer() resolves so you start execution of the .then() handler, but then you end up in the queryFindPlayer().catch() handler, that can occur for one of the following reasons:

1. If your code threw an exception before calling queryFindContract() such as if const player = players[0] threw an error of if queryFindContract() wasn't defined.

2. If your code threw an exception evaluating the arguments to pass queryFindContract() such as player.Team.id throws or any of the other variables you're passing don't exist.

3. If queryFindContract() throws synchronously before it returns its promise.

4. If queryFindContract() doesn't return a promise and thus queryfindContract().then() would throw an exception.

All of these will cause a synchronous exception to be thrown in the queryFindPlayer.then() handler which will cause it to go to the queryFindPlayer.catch() handler. It never gets to the queryFindContract().catch() handler because queryFindContract() either never got to execute or because it never got to finish and return its promise.

---

You can most likely see exactly what is causing your situation by just adding

console.log(e)

at the start of both .catch() handlers. For clarity, also add a descriptive string before the e . such as:

console.log("qfc", e);

and

console.log("qfp", e);

I pretty much always log rejections, even if expecting them sometimes because you can also get rejections for unexpected reasons such as programming errors and you want to be able to see those immediately and not get confused by them.

Answer 2

Thanks for the help, I was not using the exception handler how intended.

queryGetContractById(models, id).then(c => {
  return queryFindPlayer(models, ConsoleId, UserId, SeasonId, LeagueId).then(players => {
    if(players.length != 0) {
      return queryFindContract(models, players[0].Team.id, UserId, SeasonId, LeagueId).then(contracts => {
        if(contracts.length != 0) {
          console.log("player has a contract to a team");
        } else {
          queryUpdatePlayersTeam(models, players[0].id, TeamId);
        }
      });
    } else {
      return queryAddPlayer(models, UserId, TeamId).then(player => {
        console.log("added player");   
      });
    }
  });
}).catch(e => {
  console.log("failed", e);   
});

In my promise, I rejected if there was no data returned and resolving if there was. I can see how this was wrong and I think this is now right.