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What's new in TypeScript 3.9 RC

April 29, 2020 by Tomek

TypeScript is a superset of JavaScript which provides a lot of benefits like autocompletion, type checking, classes, interfaces, and many more. Many developers have fallen in love with it since first use and consider is a one and only choice when writing modern web apps.

Yesterday Microsoft dev team announced the release candidate version of TypeScript 3.9 RC. This version focuses mostly on improvements in performance and stability improvements like:

  • speeding up the compiler
  • improved editing experience
  • removal of some bug fixes & crashes

Performance improvements

The new version provides some serious performances improvements. After detecting poor compilation speed with material-ui packages, the series of fixed were made to improve cases involving large unions, intersections, conditional & mapped types. It is said that version 3.9 achieved a ~40% reduction in material-ui’s compile-time.

Reduced material-ui compile-time

Source: undraw.co

Improvements in Inference and Promise.all

Recent versions of TypeScript have had some changes to the way Promise.all and Promise.race functions are declared which generated few regressions linked to mixing null//undefined values. In 3.9 that was fixed along with some improvements to the inference process.

CommonJS Auto-Imports in JavaScript

A major improvement are polished auto-imports in JavaScript files using CommonJS modules. TypeScript 3.8 by default assumed that you wanted an ECMAScript-style import regardless of your file. In version 3.9 RC TypeScript automatically detects the types of imports you’re using in your project to keep your file’s style consistent.

TypeScript 3.9 offers improved auto-imports in JavaScript

Source: devblogs.miscrosoft.com

The awaited Type is gone (as for now)

A new type of operator called awaited with a goal to accurately design the unwrapping of Promise in JavaScript is still in progress. The dev team decided to pull the feature out of our main branch as awaited is still in progress and requires some more work before they can roll it out to everyone with confidence.


Quite a lot right? And these are only some of the changes! If you would like to read more about the new version the Microsoft dev blog contains the whole list of features and improvements available inTypScript 3.9 RC.


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Tomek Poniatowicz
Written by Tomek Poniatowicz GraphQL enthusiast. Magic the Gathering fanatic. Pizza monster. email me:[email protected]Twitter Shield
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