I'm trying to follow the 'when' example on Parse JavaScript SDK: Parse.Promise with the following code:
GetFromDB2 = function(id) {
var DB2 = Parse.Object.extend("DB2");
var q = new Parse.Query(DB2);
q.get(id, {
success: function(res) {
return Parse.Promise.as(10);
},
error: function(res, err) {
console.log( err);
}
});
}
GetData = function() {
var DB1 = Parse.Object.extend("DB1");
var query = new Parse.Query(DB1);
query.equalTo("param", "close");
query.find().then(function(results) {
var promises = [];
_.each(results, function(res) {
promises.push(GetFromDB2(res.get("user")));
});
Parse.Promise.when(promises).then(function() {
console.log(arguments); // expect: [10, 10, ...]
})
});
};
The length of the array arguments is correct but not sure why its values are undefined.
As written, GetFromDB2()
returns undefined
. To deliver a value, it must return either a value or a promise.
At its simplest, to deliver 10
, you could write :
function GetFromDB2(id) {
return 10;
}
But to be asynchronous, and still deliver 10
, you need to return a promise that will resolve to 10
:
function GetFromDB2(id) {
return Parse.Promise.as(10);
}
Or the .get(id)
query you really want :
function GetFromDB2(id) {
var DB2 = Parse.Object.extend('DB2');
var q = new Parse.Query(DB2);
return q.get(id).then(function(res) {
return 10;
});
//omit `.then(...)` entirely to deliver `res`
}
Now GetData()
can be written as follows :
function GetData() {
var DB1 = Parse.Object.extend('DB1');
var query = new Parse.Query(DB1);
query.equalTo('param', 'close');
return query.find().then(function(results) {
var promises = _.map(results, function(res) {
return GetFromDB2(res.get('user'));
});
Parse.Promise.when(promises).then(function() {
console.log(arguments); // expect: [10, 10, ...]
}, function(err) {
console.log(err);
return err;
});
});
};
Notes:
promises = _.map(...)
is more elegant than _.each(...)
plus `promises.push(...). GetData()
allows a greater range of possible errors to be handled, eg an error arising from query.find()
.
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