TypeScript 5.9 is here, and it’s packed with some pretty neat features. This update is now officially out for everyone to use, after a few beta and release candidate versions earlier this year. If you’ve been waiting to upgrade, now’s a good time. You can update your project by running the simple command npm install -D typescript.

Deferred Module Evaluation: More Control Over Loading

One of the biggest highlights in TypeScript 5.9 is support for a new JavaScript feature called deferred module evaluation. This uses the new import defer syntax, which means you can import modules without immediately running their code or initializing dependencies. Instead, the module only executes when it’s actually needed for the first time. This can be a game-changer for performance, especially if you have modules that are slow to load or only needed in certain situations.

Think of it like delaying a heavy task until it’s absolutely necessary. This helps your app start faster because it doesn’t have to run all the code upfront. It’s particularly helpful if you want to conditionally load modules based on platform or user actions, making your app more efficient and responsive.

Expanded Hover Info and Other Improvements

Another nice addition is a preview feature for hover tips. When you hover over variables or code snippets in editors like Visual Studio Code, you’ll now see a small + button. Clicking this expands the tooltip to show more detailed type information, and clicking – collapses it back. This makes it easier to peek into complex types without cluttering your screen. Plus, developers can now set a maximum length for these hover tooltips, so they don’t get too long and unwieldy. You can change this in VS Code with the js/ts.hover.maximumLength setting.

Beyond this, TypeScript 5.9 also improves performance by caching intermediate instantiations. This means if TypeScript has already started working on a specific type, it can reuse that work rather than redo it. Developers working with newer modules will also appreciate the support for --module node20, which aligns with Node.js 20 and sets the target to ES2023. Additionally, the tsc --init command now produces a cleaner, leaner tsconfig.json file by skipping commented-out settings, making configuration simpler.

Looking Ahead: Faster TypeScript and New Compiler Projects

These features come about two months after the release of TypeScript Native Previews, which is a project aiming to build a native TypeScript compiler based on the Go language. The goal here is to make TypeScript faster and reduce memory usage, especially during builds. Microsoft plans to include this native compiler in a future TypeScript 7 release, which could significantly speed up development workflows.

All in all, TypeScript 5.9 offers developers more control, better performance, and smoother editing experiences. It’s a solid upgrade that addresses some common pain points and prepares the ground for future improvements. Whether you’re working on a small project or a large app, these tools can help you write cleaner, faster, and more efficient code.

Inspired by

• https://www.infoworld.com/article/4020579/typescript-5-9-brings-deferred-module-evaluation-expandable-hovers.html

Sources

• gmpg.org
• microsoft.com
• microsoft.com
• microsoft.com
• github.com