How to replace nested .then() function in bookshelf - node.js

Question

I am new to node.js and I am having trouble with removing multiple nested .then() inside of a bookshelf.js function.
The code below is what I am working on right now. And it works fine for now but I need to add more than 20+ nested .then() to finish the project. Therefore, I am trying to replace .then() to some other method before it gets crazy.
Any help or tip would be appreciated.
getItem: function (req, res, next) {
DepartmentCollection.forge()
.fetch({
debug: true
})
.then(function(collection) {
new GlossaryTerm({'GlossaryTermID': req.params.id}).fetch({
withRelated: ['department'],
debug: true
})
.then(function(model) {
if ("undefined" === typeof collection) { console.log("step 2: variable is undefined") }
else { console.log("step 2: variable is defined") };
res.render('glossary/glossary-term-detail',{domain:'GlossaryTerm', title: 'Glossary Term Detail',
data: model, department_data: collection });
}).catch(function(error) {
console.log(error);
res.send('An error occured');
});
}).catch(function(error) {
console.log(error);
res.send('An error occured');
});
}

Answer 1

Usually you do not need to nest thenables. The result of a thenable becomes the input for the next one. So most times you can chain them instead of nesting:
getItem: function (req, res, next) {
// Creating a closure for storing intermediate then() results
let collection;
// Making getItem() a promise itself
return DepartmentCollection
.forge()
.fetch({
debug: true
})
.then(function(fetched_collection) {
collection = fetched_collection;
if ("undefined" === typeof collection) {
console.log("step 2: variable is undefined")
} else {
console.log("step 2: variable is defined")
}
return new GlossaryTerm({'GlossaryTermID': req.params.id})
.fetch({
withRelated: ['department'],
debug: true
});
})
.then(function(model) {
res.render('glossary/glossary-term-detail', {
domain:'GlossaryTerm',
title: 'Glossary Term Detail',
data: model,
department_data: collection });
})
.catch(function(error) {
console.log(error);
res.send('An error occured');
});
}
An exception is when the logic must be split into more chains that provide a result using different ways. In these cases it is advisable to move the complex logic to another function (returning a promise):
so this
function complex() {
return f1()
.then(x => {
if (!x) {
return f2()
.then(f3);
}
return f3();
})
.then(...);
}
becomes this:
function complex() {
function f4(x) {
if (!x) {
return f2()
.then(f3);
}
return f3();
}
return f1()
.then(f4)
.then(...);
}

Answer 2

I would highly recommend the co library.
npm install co --save
Then you can make these calls 'appear' synchronous and avoid some of this pyramid of doom type problem.
const co = require('co');
co(function* () {
const resultOne = yield asyncCallOne();
const resultTwo = yield asyncCallTwo();
try {
const resultThree = yield asyncCallThree();
} catch (error) {
console.log(error);
}
// and so on, and so on.
});