I have a shallow understanding of JavaScript Promise and promise chain.
Say, I have a method as shown below. It's written is TypeScript, but could be modified to match JavaScript ES6
private InsertPersonInDB(p : Person) {
return this.db.find({ //<- would this return?
selector: {objType: 'Person'},
fields: ['_id'],
sort: ['_id']
}).then( result => {
let allpersondIds : string[] = [];
(result.docs).forEach(rec => {
allpersondIds.push(rec._id);
});
return allpersondIds;
}).then ( allpersonIdsInDB => {
var id = this.getIdfromPersonName(person.personName, allpersonIdsInDB);
person._id = id;
return this.db.post(person) //<- or would this return?
}
}
//Calling function
for(let person of this.persons) {
InsertPersonInDB(person).then(result => {
console.log(result)
//Some UI updates
}).catch(err => {
console.log(err)
//Some UI updates notifying user about failure
});
}
Here, I have two return, first is
return this.db.find
which find function is a promise
and end of the then chain returns
return this.db.post(person)
even post function is a promise.
Here, I have three questions
1) When this function executes, what would return?
2) If the function immediately returns the promise, when would the chain thens execute?
2) What are better approaches to refactored the promise chain in a layered application. E.g. Few chain then needs to be executed in the service, other needs to be executed in UI, how do I structure my promises code?