safe.js

const isObject = obj => obj && typeof obj === 'object';
const hasKey = (obj, key) => key in obj;

const safe = obj =>
  new Proxy(obj, {
    get: (target, name) => {
      if (!hasKey(target, name)) {
        return 'undefined';
      }
      if (!isObject(target[name])) {
        return target[name];
      }
      return safe(target[name]);
    }
  });

const test = { a: { b: 'x' } };
const testSafe = safe(test);

// console.log(test.a.b);   // x
console.log(testSafe.a.b);  // x
// console.log(test.c.d);   // TypeError
console.log(testSafe.c.d);  // undefined