Credit to Brady from the YNAB team. Assembled from here: https://support.youneedabudget.com/t/k9rxc9/using-google-apps-script-with-the-api

ynab_to_google_sheets.js

function onOpen() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet();
  var entries = [{
    name : "Refresh Categories",
    functionName : "refresh_ynab_categories"
  }];
  sheet.addMenu("YNAB", entries);
};

function refresh_ynab_categories() {
  SpreadsheetApp.getActiveSpreadsheet().getRange('A1').setValue(new Date().toTimeString());
}

function get_ynab_categories(accessToken, budgetId) {
  const groups = fetch_ynab_data(accessToken, "budgets/" + budgetId + "/categories").category_groups;

  const columns = ["Name", "Budget", "Activity", "Balance"];
  const rows = [];

  for (var group_idx = 0; group_idx < groups.length; group_idx++) {
    // Add the group
    var group = groups[group_idx];
    // Skip internal and hidden categories
    if (['Internal Master Category', 'Hidden Categories'].indexOf(group.name) >= 0) continue;
    rows.push([group.name]);

    // Add the categories
    for (var category_idx = 0; category_idx < group.categories.length; category_idx++) {
      var category = group.categories[category_idx];
      var name = "      " + category.name; // Indent categories a bit so they are offset from groups
      // Calculate currency amounts from mulliunits
      var budgeted = category.budgeted / 1000.0;
      var activity = category.activity / 1000.0;
      var balance = category.balance / 1000.0;
      rows.push([name, budgeted, activity, balance]);
    }
  }

  return [columns].concat(rows);
}


function get_ynab_accounts(accessToken, budgetId) {
  const transactions = fetch_ynab_data(accessToken, "budgets/" + budgetId + "/accounts").accounts;

  const headers = ["Name", "Type", "Budget", "Closed", "Balance"];
  const rows = transactions.map(function(t){
   const balance = t.balance / 1000;
   return [t.name, t.type, t.on_budget, t.closed, balance]
  });

  return [headers].concat(rows);
}

function get_ynab_categories_for_month(accessToken, budgetId, month) {
  const groups = fetch_ynab_data(accessToken, "budgets/" + budgetId + "/categories").category_groups;
  const categories = fetch_ynab_data(accessToken, "budgets/" + budgetId + "/months/" + month).month.categories;

  // Organize groups by id on groups_by_id
  const groups_by_id = [];
  for (var group_idx = 0; group_idx < groups.length; group_idx++) {
    var group = groups[group_idx];
    // Clear categories array since we'll replace with categories from a particular month
    group.categories = [];
    groups_by_id[group.id] = group;
  }

  // Collate categories back to their group
  for (var category_idx = 0; category_idx < categories.length; category_idx++) {
    var category = categories[category_idx];
    if (groups_by_id[category.category_group_id]){
      groups_by_id[category.category_group_id].categories.push(category);
    }
  }

  const columns = ["Name", "Budgeted", "Activity", "Balance"];
  const rows = [];

  for (var group_idx = 0; group_idx < groups.length; group_idx++) {
    // Add the group
    var group = groups[group_idx];
    // Skip internal and hidden categories
    if (['Internal Master Category', 'Hidden Categories'].indexOf(group.name) >= 0) continue;
    rows.push([group.name]);

    // Add the categories
    for (var category_idx = 0; category_idx < group.categories.length; category_idx++) {
      var category = group.categories[category_idx];
      var name = "      " + category.name; // Indent categories a bit so they are offset from groups
      // Calculate currency amounts from mulliunits
      var budgeted = category.budgeted / 1000.0;
      var activity = category.activity / 1000.0;
      var balance = category.balance / 1000.0;
      rows.push([name, budgeted, activity, balance]);
    }
  }

  return [columns].concat(rows);
}


function get_ynab_transactions(accessToken, budgetId) {
  const transactions = fetch_ynab_data(accessToken, "budgets/" + budgetId + "/transactions").transactions;

  const headers = ["ID","Account Name", "Date", "Payee", "Category", "Amount"];
  const rows = transactions.map(function(t){
   const amount = t.amount / 1000;
   return [t.id,t.account_name, t.date, t.payee_name, t.category_name, amount]
  });

  return [headers].concat(rows);
}

function get_ynab_transactions_month(accessToken, budgetId, month) {
  const data = fetch_ynab_data(accessToken, "budgets/" + budgetId +  "/transactions?since_date="+ month).transactions;
  const headers = ["Type", "Account Name", "Date", "Payee", "Category", "Amount"];
  const transactions = [];
  const subtransactions = [];
  data.forEach(function(t){
   const amount = t.amount / 1000;
   transactions.push(["Transaction", t.account_name, t.date, t.payee_name, t.category_name, amount]);
   t.subtransactions.forEach(function(st){
     subtransactions.push(["SubTransaction", t.account_name, t.date, st.payee_id, st.category_id, st.amount]);
   });
  });

  return [headers].concat(transactions, subtransactions);
}

function get_ynab_scheduled_transactions(accessToken, budgetId) {
  const transactions = fetch_ynab_data(accessToken, "budgets/" + budgetId + "/scheduled_transactions").scheduled_transactions;

  const headers = ["Account Name", "Date","Frequency", "Payee", "Category", "Amount"];
  const rows = transactions.map(function(t){
   const amount = t.amount / 1000;
   return [t.account_name, t.date_next,t.frequency, t.payee_name, t.category_name, amount]
  });

  return [headers].concat(rows);
}

function fetch_ynab_data(accessToken, path){
  const url = "https://api.youneedabudget.com/v1/" + path;
  const options = {
    "headers": {
      "Authorization": "Bearer " + accessToken
    }
  };
  const response = UrlFetchApp.fetch(url, options);
  const data = JSON.parse(response.getContentText()).data;
  return data;
}