index.js

const a_class = ncmb.DataStore('A_class');
const b_class = ncmb.DataStore('B_class');
//         ↓ ここ
ons.ready(async function() {
    const result = await a_class.fetchAll();
    for (let row of result) {
      const result2 = await b_class
        .relatedTo(row, 'tests')
        .fetchAll();
      objectB.push(result2.count);
    }
})();