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function optionalChaining(obj, chain) {
  return chain
    .split('.')
    .reduce(function(acc, val) {
      return acc ? acc[val] : undefined;
    }, obj);
}
var user = { address: { street: 'No.969 West WenYi Road', }, a: { b: { c: 2 } }, }
var ret = optionalChaining(user, 'address');
console.log(ret)