type Flags = {
  aLongName: boolean;
  anotherReallyLongOne: number;
  someOtherOnes: boolean;
  [other: string]: number | boolean;
}

function f(flags: Flags) {
  flags.aLongName; // ok
  flags.anUnknownName; // ok
  flags.aLongname; // ok, but misspelled
  flags['aLongName']; // ok
  flags['anUnknownName']; // ok
  flags['aLongname']; // ok, but misspelled
}

function f(flags: Flags) {
  flags.aLongName; // ok
  flags.anUnknownName; // error, unknown
  flags.aLongname; // error, misspelled
  flags['aLongName']; // ok
  flags['anUnknownName']; // ok
  flags['aLongname']; // ok, but misspelled
}