TypeScript 5.9 RC: import defer, Minimal tsconfig, and Major Performance Gains

TypeScript 5.9 RC Released

On July 25, 2025, the TypeScript team published the Release Candidate of TypeScript 5.9, marking a significant step toward the final release with major enhancements across configuration, module loading, editor tooling, and compiler performance (devblogs.microsoft.com).

Minimal tsconfig by Default

The default output of tsc --init has been overhauled to produce a lean, opinionated tsconfig.json with modern defaults—module: "nodenext", target: "esnext", strictness flags, JSX support set to react-jsx, and more—saving developers from manual configuration and enforcing best practices out of the box (devblogs.microsoft.com).

Deferred Module Evaluation with import defer

TypeScript 5.9 adds support for ECMAScript’s import defer syntax, enabling deferred execution of modules until first access. This static, namespace-only import mechanism defers side effects and initializations, cutting startup costs in large applications and improving performance in serverless or CLI environments (devblogs.microsoft.com).

Stable Node 20 Module Resolution

A new --module node20 option stabilizes Node.js module resolution semantics to match Node v20, implicitly setting target: es2023 and providing a predictable, future-proof configuration for server-side applications without the caveats of legacy nodenext mode (devblogs.microsoft.com).

Richer Editor Experience

In-editor tooling sees a leap forward with inline MDN-sourced documentation for DOM APIs, previewable expandable hovers for complex types, and a configurable maximum hover length—eliminating context switches when exploring unfamiliar or deeply nested types (devblogs.microsoft.com).

Key Compiler Optimizations

Under the hood, the compiler now caches intermediate type instantiations to avoid redundant work in generic-heavy code, and eliminates unnecessary closure allocations during file existence checks, both contributing to measurable speedups and reduced memory usage in large codebases (devblogs.microsoft.com).

Source: Announcing TypeScript 5.9 RC