index.js

ons.ready(() => {
  resolveVariable(['Tab1Controller', 'Tab2Controller'])
    .then(() => {
      // 変数定義済み
    });
});

const resolveVariable = (ary) => {
  return new Promise((res, rej) => {
    setTimeout(() => {
      let bol = true;
      for (let index in ary) {
        if (!(ary[index] in window)) {
          bol = false;
          return resolveVariable(ary)
        }
      }
      if (bol) res();
    }, 100);
  })
}