解析Cloud Code Promise错误处理

Question

I'm kind of new to JS and really new to Promises.

Let's say I have a chain of promises:

//var challenge is created before all this

isUserInCooldown().then( function(hoursRemaining) 
{
    if (hoursRemaining > 0) 
    {
        return Parse.Promise.error("You can't challenge 'cuz you're on cooldown.");
    }

    var objectsToSave = [];

    //DO SOME STUFF TO THE OBJECTS

    return Parse.Object.saveAll(objectsToSave);

}).then( function(list)
{
    //DO SOME STUFF TO CHALLENGE

    return challenge.save(null);

}).then( function(challenge) 
{
    return Parse.Promise.as(challenge);
},
function(error) 
{
    if (theSaveAllFailed) { return "Couldn't save all"; }
    if (theSaveFailed) { return "Couldn't save the challenge"; }
    //etc.
});

I am refactoring a bunch of code that used callbacks and each error: function(error) {} returned a custom error message. I want to be able to pass a custom error message depending on where the chain broke down.

I figure it has something to do with fail() or reject() but I haven't figured out how.

I would like to know:

1: How can I return custom error messages like I want to?

2: Am I using promises correctly (based on what you see here?)

Thanks!

Answer 1

1. If you want to override the error message of a promise rejection, you can use .fail to add handlers as you suspected.

2. While your promises would function as written, they could be tidied up:

   • isUserInCooldown should include the hoursRemaining > 0 check so it doesn't pollute other functions.
   • If challenge.save doesn't depend on objectsToSave , firing them both off simultaneously and using Parse.Promise.when would be faster.
   • You never call response.success or response.error as cloud functions require.

What you did here is pointless, as I repeat to prove a point:

.then(function(challenge){
  return Parse.Promise.as(challenge);
}).then(function(challenge){
  return Parse.Promise.as(challenge);
}).then(function(challenge){
  return Parse.Promise.as(challenge);
});

Ah, much better:

isUserInCooldown().then( function() {
  return Parse.Object.saveAll(objectsToSave).fail(function(){
    return "Couldn't save all";
  });
}).then( function(){
  return challenge.save().fail(function(){
    return "Couldn't save the challenge";
  });
}).then(response.success, response.error)

Or, if you're not using cloud functions:

.fail(function(error) {
  console.log(error); //logs "Couldn't save all" or "Couldn't save the challenge"
});