.then of Promise.all not executing - node.js

Question

I'm using firestore to retrieve data which has the following DS.
I have a Company collection which contains a subcollection Branches
So I'm trying to retrieve to list all the Companies with its Branches
Code:
exports.findAll = function (req, res) {
getcompanies().
then((companies) => {
console.log("Main "+ companies) // info: Main TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined
return res.json(companies);
})
.catch((err) => {
console.log('Error getting documents', err);
});
}
function getCompanies(){
var companiesRef = db.collection('companies');
return companiesRef.get()
.then((snapshot) => {
let companies = [];
return Promise.all(
snapshot.forEach(doc => {
let company = {};
company.id = doc.id;
company.company = doc.data();
var branchesPromise = getBranchesForCompanyById(company.id);
return branchesPromise.then((branches) => {
company.branches = branches;
companies.push(company);
if(snapshot.size === companies.length){
console.log("companies - Inside" + JSON.stringify(companies)); //This prints all companies with its branches
}
return Promise.resolve(companies);
})
.catch(err => {
console.log("Error getting sub-collection documents", err);
return Promise.reject(err);
})
})
)
.then(companies => {
console.log("Outside " + companies) // This is never executed
return companies;
})
.catch(err => {
return err;
});
})
.catch(err => {
return err;
});
}
function getBranchesForCompanyById(id){
var branchesRef = db.collection('companies').doc(id).collection('branches');
let branches = [];
return branchesRef.get()
.then(snapshot => {
snapshot.forEach(brnch => {
let branch = {};
branch.id = brnch.id;
branch.branch = brnch.data();
branches.push(branch);
})
return branches;
})
.catch(err => {
return err;
})
}
I've all the data needed at this point.
console.log("companies - Inside" + JSON.stringify(companies)); //This prints all companies with its branches
But the then of Promise.all is never executed. So getting this error -
info: Main TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined
console.log("Main "+ companies) // info: Main TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined
I feel I have followed all the rules specified here: https://stackoverflow.com/a/31414472/2114024 with respect to nested promises, not sure where I'm missing the point.
Thanks in advance!

Answer 1

I see at least 2 problems:
forEach likely doesn't return anything, and you send the result of forEach into Promise.all().
If Promise.all() throws an exception, some of your catch handlers just grab the error and return it. Returning it turns it into a non-exception.
You also really don't have to add a catch to every Promise chain, as long as you feed the result of a Promise chain back into another promise chain, you probably only need 1 catch block.
Also one of your then() functions should not be nested as deeply. Just move it a level up, that's the point of promises.

Answer 2

In your code, you can use map instead of forEach. Promise.all accept an array of promises but forEach does not return an array
return Promise.all(
snapshot.map(doc => {
let company = {};
company.id = doc.id;
company.company = doc.data();
var branchesPromise = getBranchesForCompanyById(company.id);
return branchesPromise.then((branches) => {
company.branches = branches;
companies.push(company);
if (snapshot.size === companies.length) {
console.log("companies - Inside" + JSON.stringify(companies)); //This prints all companies with its branches
}
return Promise.resolve(companies);
})
.catch(err => {
console.log("Error getting sub-collection documents", err);
return Promise.reject(err);
})
})
)

Answer 3

Based on inputs from Evert and Rahul, thanks to both of you, I have resolved the problem here.
I handled all the error in the catch block
Promise.all was not returning anything so I converted the forEach to map.
So this is my updated code, which solves the problem:
exports.findAll = function (req, res) {
getcompanies().
then((companies) => {
console.log("Main " + companies) // Prints all companies with its branches
return res.json(companies);
})
.catch((err) => {
console.log('Error getting documents', err);
return res.status(500).json({ message: "Error getting the all companies" + err });
});
}
function getCompanies() {
var companiesRef = db.collection('companies');
return companiesRef.get()
.then((snapshot) => {
let companies = [];
return Promise.all(
snapshot.docs.map(doc => {
let company = {};
company.id = doc.id;
company.company = doc.data();
var branchesPromise = getBranchesForCompanyById(company.id);
return branchesPromise.then((branches) => {
company.branches = branches;
companies.push(company);
if (snapshot.size === companies.length) {
console.log("companies - Inside" + JSON.stringify(companies));
return companies;
}
})
.catch(err => {
console.log("Error getting sub-collection documents", err);
throw new Error(err);
})
})
)
.then(companies => {
console.log("Outside " + companies); // Executed now
return companies[companies.length - 1];
})
.catch(err => {
throw new Error(err);
});
})
.catch(err => {
throw new Error(err);
});
}
function getBranchesForCompanyById(id) {
var branchesRef = db.collection('companies').doc(id).collection('branches');
let branches = [];
return branchesRef.get()
.then(snapshot => {
snapshot.forEach(brnch => {
let branch = {};
branch.id = brnch.id;
branch.branch = brnch.data();
branches.push(branch);
})
return branches;
})
.catch(err => {
throw new Error(err);
})
}