了解 JavaScript 新特性：Optional Chaining

// Error prone-version, could throw.const nameLength = db.user.name.length;
// Less error-prone, but harder to read.let nameLength;if (db && db.user && db.user.name)  nameLength = db.user.name.length;

const nameLength =  (db    ? (db.user      ? (db.user.name        ? db.user.name.length        : undefined)      : undefined)    : undefined);

// Still checks for errors and is much more readable.const nameLength = db?.user?.name?.length;

// Extends the interface with an optional method, which is present// only for admin users.const adminOption = db?.user?.validateAdminAndGetPrefs?.().option;

// Extends the capabilities of the static property access// with a dynamically generated property name.const optionName = 'optional setting';const optionLength = db?.user?.preferences?.[optionName].length;

// If the `usersArray` is `null` or `undefined`,// then `userName` gracefully evaluates to `undefined`.const userIndex = 42;const userName = usersArray?.[userIndex].name;

const object = { id: 123, names: { first: 'Alice', last: 'Smith' }};
{ // With lodash:  const firstName = _.get(object, 'names.first');  // → 'Alice'
  const middleName = _.get(object, 'names.middle', '(no middle name)');  // → '(no middle name)'}
{ // With optional chaining and nullish coalescing:  const firstName = object?.names?.first ?? '(no first name)';  // → 'Alice'
  const middleName = object?.names?.middle ?? '(no middle name)';  // → '(no middle name)'}

// `age` is incremented only if `db` and `user` are defined.db?.user?.grow(++age);

// An optional chain may be followed by another optional chain.const firstNameLength = db.users?.[42]?.names.first.length;

// `db.user` is deleted only if `db` is defined.delete db?.user;