How could join two YAML document?

[gixlg]

Hi all,
I have a input YAML like this one (input.yml):

lev1:
  lev2:
    A: "a"
    ### Comment ###
    B: "b"
    C: "c"
    ### Something ###
    ### Another comment ###
    D: "d"
    ### blablabla ###

and a Yaml snipet like this other (new.yml)

### New staff ###
E: 'e'
F: 'f'
### End new staff ###

I want to obtain an output document joining these two YAMLs, adding the snipet under lev2.
The desired result is this one:

lev1:
  lev2:
    A: "a"
    ### Comment ###
    B: "b"
    C: "c"
    ### Something ###
    ### Another comment ###
    D: "d"
    ### blablabla ###
    ### New staff ###
    E: 'e'
    F: 'f'
    ### End new staff ###

I've tried in this way:

const doc = YAML.parseDocument(fs.readFileSync('input.yml', 'utf8'))
const newStaff = YAML.parseDocument(fs.readFileSync('new.yml', 'utf8'));

let lev2 = doc.get('lev1').get('lev2');
lev2.add(newStaff)
console.log(String(doc));

but in output I got this:

lev1:
  lev2:
    A: "a"
    ### Comment ###
    B: "b"
    C: "c"
    ### Something ###
    ### Another comment ###
    D: "d"
    E: e
      F: f: null
    ### blablabla ###

not what I expected.

Why is that?
How can I do to get the expected result?

[eemeli]

A couple of things going on here.

1. Semantically, a YAML document can't contain another YAML document. What you're really wanting to do is to merge the root level mapping of the newStaff document with the value of lev2.
2. The add() method only adds one value to the collection, so in this case you're adding a pair with the document as a key and an empty value to lev2.
3. The stringifier somehow missed the newline in the inner document's string value, which is surprising. That's a bug that I ought to fix, but it won't really help you.

To get the results you want, iterate through newStaff.contents.items and add() each separately to lev2. You'll also need to add the trailing comment separately, if you want to keep it.

[gixlg]

Got it thanks.
I hoped that there was a way to avoid adding iteratively every single field.
Isn't exist a merge functionality for my needs, right?

Thanks Eemeli!

[eemeli]

If you really want to avoid iteration, a collection's items is just an array, so you could also apply native JS operations for concatenating them.