How to store a list in firebase firestore? - node.js

Question

I am trying to store a list with firebase firestore, but I always get a encoding error:
UnhandledPromiseRejectionWarning: Error: Cannot encode value: a,b,c
I have tried the following code snippets for node js:
const data = {
a: "hello world",
b: ["a", "b", "c"]
};
const res = await db.collection('data').doc('one').set(data);
and
const data = {
a: "hello world",
b: new Array(["a", "b", "c"])
};
const res = await db.collection('data').doc('one').set(data);
and
const data = {
a: "hello world",
b: []
};
const res = await db.collection('data').doc('one').set(data);
of which these all result in the same encoding error.
I have also tried to store a list as the following the blog post here: https://firebase.googleblog.com/2014/04/best-practices-arrays-in-firebase.html
{
0: "a",
1: "b",
2: "c"
}
but this stored it as a object not a list and I need to store it as a list.
I can create new documents with lists in the firebase console, but i cant do it programmatically. I am using node js.
Thank you for aany help you can offer.

Answer 1

So, I found a solution after looking at the firebase docs a bit more.
This page : https://firebase.google.com/docs/firestore/manage-data/add-data told me that I could use array union to do this. Here is the working code sample:
const data = {
a: "hello world",
b: admin.firestore.FieldValue.arrayUnion("a")
};
const res = await db.collection('data').doc('one').set(data);