How to get an array of object from firestore in appscript - database

Question

I have a set of documents in Firestore in this format. Questions array will 10 questions.
I want to get the data of questions field: one row for one question
I do I code in the appscript to perform this
This is my code so far (for one document only)
function test(){
const firestore = getFirestore();
var query = firestore.getDocument("QuestionCollection/test").fields;
var data = {};
data.subject = query.subject;
data.questions= query.questions;
const sheet = SpreadsheetApp.getActiveSheet();
for(i = 0 ; i < 10 (no. of question); i++){
const row = [data.questions[i].answer, data.questions[i].difficulty];
sheet.appendRow(row);
}
}
Error:
TypeError: Cannot read property 'answer' of undefined

Answer 1

Modification points:
When I saw your sample data in the image, it seems that the array length of questions is 2. But at the for loop, the end index of loop is 9. I think that by them, such error occurs.
When you want to put the value of "Serial", it is required to add the value for putting to Spreadsheet.
In your script, appendRow is used in a loop. In this case, the process cost becomes high.
When above points are reflected to your script, it becomes as follows.
Modified script:
Your for loop is modified as follows.
From:
for(i = 0 ; i < 10 (no. of question); i++){
const row = [data.questions[i].answer, data.questions[i].difficulty];
sheet.appendRow(row);
}
To:
var values = [];
for (var i = 0; i < data.questions.length; i++) {
const row = [i + 1, data.questions[i].answer, data.questions[i].difficulty];
values.push(row);
}
sheet.getRange(sheet.getLastRow() + 1, 1, values.length, values[0].length).setValues(values);
For the end index of loop, the array length is used.
Reference:
setValues(values)

Answer 2

You shouldn't query the .fields property directly (because your data isn't converted properly). Assuming you're using v28+ of the library, your code should look something like this:
function test(){
const firestore = getFirestore();
const query = firestore.getDocument("QuestionCollection/test").obj; // Don't use .fields here
const sheet = SpreadsheetApp.getActiveSheet();
const values = [["Serial", "Answer", "Difficulty"]]; // Init 2D array with Header row
// Loop through each question in the array and extract necessary values to construct Data rows
for (const question of query.questions){
values.push([query.questions.indexOf(question) + 1, question.answer, question.difficulty]);
}
// Replace 1, 1 below with coords of "Serial" header cell
const range = sheet.getRange(1, 1, values.length, values[0].length);
range.setValues(values);
// sheet.getRange(subjRow, subjCol).setValue(query.subject); // Add location for Subject data
}
I saw that you wanted "Serial" to represent "Question number", so I added that column to the header and data rows.
As Tanaike mentioned, there's a huge performance hit for writing to the spreadsheet in a loop, so it's better if you set up a 2D array of values to write all at once using range.setValues(array2D). Ideally, you'll want to minimize the calls to the Spreadsheet API.
Disclaimer: I'm an active contributor to the FirestoreGoogleAppsScript library.