Using the docs but completely confused... mongoose Model#save - node.js

Question

I am trying to return specific status codes such as, 409 Conflict. I have used the Model#save docs
Edit: I am not trying to solve the error, it is deliberate.
According to the docs, there is three parameters on the callback: err, product, and numAffected.
EDIT: I wrote this code wrong and I edited. Either way, I got an excellent answer from Ryan.
app.post('/skill', (req, res) => {
const skill = new Skill({some_duplicate_object});
skill.save((err, product, numAffected) => {
console.log("Error: ", err);
});
NOT my console.log, in the Mocha test cli, I get an error:
(node:19760) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 5): ValidationError: Path `name` is required.
By playing around and shear luck, I did this: This is NOT in the mongoose docs and is my main reason for writing this post.
app.post('/skill', (req, res) => {
const skill = new Skill({});
skill.save()
.then((err, product, numAffected) => {
console.log("Nothing displayed here");
}, (err) => {
console.log(err.errors);
});
Even though this is not in the docs, it shows the error I want. As someone that is REALLY trying to use official docs more, I find it so hard to understand what is really going on. Why does this work and if it is in the docs, where would this info be?
{ name:
{ MongooseError: Path `name` is required.
at ValidatorError (/home/codeamend/Coding/projects/portfolio/work/CodeAmend.Com/backend/node_modules/mongoose/lib/error/validator.js:24:11)
at validate (/home/codeamend/Coding/projects/portfolio/work/CodeAmend.Com/backend/node_modules/mongoose/lib/schematype.js:706:13)
at /home/codeamend/Coding/projects/portfolio/work/CodeAmend.Com/backend/node_modules/mongoose/lib/schematype.js:752:11
at Array.forEach (native)
at SchemaString.SchemaType.doValidate (/home/codeamend/Coding/projects/portfolio/work/CodeAmend.Com/backend/node_modules/mongoose/lib/schematype.js:712:19)
at /home/codeamend/Coding/projects/portfolio/work/CodeAmend.Com/backend/node_modules/mongoose/lib/document.js:1408:9
at _combinedTickCallback (internal/process/next_tick.js:73:7)
at process._tickCallback (internal/process/next_tick.js:104:9)
message: 'Path `name` is required.',
name: 'ValidatorError',
properties:
{ type: 'required',
message: 'Path `{PATH}` is required.',
validator: [Function],
path: 'name',
value: undefined },
kind: 'required',
path: 'name',
value: undefined,
reason: undefined } }
Extra info:
"devDependencies": {
"chai": "^3.5.0",
"mocha": "^2.4.5",
"request": "^2.81.0",
"supertest": "^3.0.0"
},
"dependencies": {
"body-parser": "^1.17.1",
"express": "^4.15.2",
"mongoose": "^4.9.2"
}

Answer 1

Your problem is two fold, and both of the errors you are getting are telling you exactly what is wrong. The source of your troubles is lack of understanding (not trying to pick on you). This is kind of the same as if you were a beginner at electronics and I told your that a tube is biased improperly and you said "I don't understand" - well, you need to learn about tubes then because I just described to you the exact problem with your circuit.
1. Promise error - UnhandledPromiseRejectionWarning: Unhandled promise rejection.... This cannot be more descript. Promises are either 1) resolved or 2) rejected. If you fail to "handle" the rejected case, you will get an error. Handling a rejected promise can happen one of two ways:
// pass a 2nd function to `.then()`:
somePromise.then(function (result) {
// promise was "resolved" successfully
}, function (err) {
// Promise was "rejected"
});
// use `.catch()` - this is preferred in my opinion:
somePromise.then(function (result) {
// promise was "resolved" successfully
}).catch(function (err) {
// Promise was "rejected"
});
Using either of the above scenarios means you "handled" the promise rejection correctly.
2. Database validation - Path 'name' is required.. This literally means that you have a required path called "name" which is required (meaning, it must have a value). So looking at your code it's pretty obvious:
const skill = new Skill({});
skill.save() //-> results in error
This is fixed by adding all required data before saving:
const skill = new Skill({ name: "Jason" });
skill.save() //-> yay, no error

Answer 2

You're not handling the promise rejection.
Change this:
.then((err, product, numAffected) => {
console.log("Error: ", err);
});
to this:
.then((result) => {
console.log('Result:', result);
}).catch((error) => {
console.log('Error:', error);
Of course change what happens in the callbacks to whatever you need.
See this answer for more general info on unhandled rejection:
Should I refrain from handling Promise rejection asynchronously?