Node Javascript class ReferenceError: X is not defined

Why am I getting an undefined error when the function is defined in my class?

That's the problem - you defined a class method, not a function. It would have to be called as this.#getClient (if it was static as well).

Really this should not be a class at all. There's no state, you never use an instance for anything (it would be empty). Export an async function addObject, and make those helper "methods" to local functions in the module:

const MongoClient = require("mongodb").MongoClient;
const ConnectionDetails = require("./ConnectionDetails").ConnectionDetails;
const Recipe = require("./recipe").Recipe;
var ObjectId = require('mongodb').ObjectID;

async function getClient(uriString){
    const client = new MongoClient(uriString);
    await client.connect();
    return client;
}

function getDatabase(client, dbString){
    const db = client.db(dbString);
    return db;
}

function getCollection(client, collectionString){
    return client.collection(collectionString)
}

exports.addObject = async function (object, connectionDetails){
    if(!connectionDetails instanceof ConnectionDetails){
        throw "connetionDetails must be of type ConnectionDetails but is " + typeof(connectionDetails);
    }

    if(!object instanceof Recipe){
        throw "object must be of type Recipe but is " + typeof(object);
    }

    const mongoClient = getClient(connectionDetails.getUri());
    const database = getDatabase(client, connectionDetails.getDatabase());
    const collection = getCollection(client, connectionDetails.getCollection());
    return await collection.insertOne(object);
}