TypeScript 5.9 Beta introduces several key improvements including a streamlined tsc --init that generates cleaner configuration files with modern defaults, support for ECMAScript's import defer syntax for deferred module evaluation, a stable --module node20 option for Node.js v20 compatibility, enhanced DOM API documentation with summary descriptions, expandable hover tooltips in editors, configurable hover length limits, and performance optimizations for type instantiation caching and file existence checks.