Added

• simpler ECMAScript import (#1589)
• Option.preset() allows specifying value/arg for option when used without option-argument (especially optional, but also boolean option) (#1652)
• .executableDir() for custom search for subcommands (#1571)
• throw with helpful message if pass Option to .option() or .requiredOption() (#1655)
• .error() for generating errors from client code just like Commander generated errors, with support for .configureOutput (), .exitOverride(), and .showHelpAfterError() (#1675)
• .optsWithGlobals() to return merged local and global options (#1671)

Changed

• Breaking: Commander 9 requires Node.js v12.20.0 or higher
• update package-lock.json to lockfile@2 format (#1659)
• showSuggestionAfterError is now on by default (#1657)
• Breaking: default value specified for boolean option now always used as default value (see .preset() to match some previous behaviours) (#1652)
• default value for boolean option only shown in help if true/false (#1652)
• use command name as prefix for subcommand stand-alone executable name (with fallback to script name for backwards compatibility) (#1571)
• allow absolute path with executableFile (#1571)
• removed restriction that nested subcommands must specify executableFile (#1571)
• TypeScript: allow passing readonly string array to .choices() (#1667)
• TypeScript: allow passing readonly string array to .parse(), .parseAsync(), .aliases() (#1669)

Fixed

• option with optional argument not supplied on command line now works when option already has a value, whether from default value or from previous arguments (#1652)

Removed

• Breaking: removed internal fallback to require.main.filename when script not known from arguments passed to .parse()
  (can supply details using .name(), and .executableDir() or executableFile) (#1571)