What’s new with TypeScript 4.0 beta

What’s new with TypeScript 4.0 beta

TypeScript 4.0 improves type inference and now allows to correctly type functions dealing with tuples. First of all, it now supports generics when defining tuple types, allowing to use generic types defined on a function for tuple elements.

TypeScript 3.9 has been released last month, and the first beta of TypeScript 4.0 is already in beta!

In this article I’ll go over everything that’s been announced with the first beta and I’ll also cover the additional things I’ve discovered by looking at the 4.0 iteration plan.

I’ll only cover the language features. I might write additional posts to cover what’s also coming regarding editor productivity, performance and bug fixes.

Please keep in mind that this is article is based on beta release notes and a roadmap, it doesn’t mean that everything will actually be part of the final release as is.

Class property inference from constructors (part of the beta)

Currently, when tsc is configured innoImplicitAny mode, the following TS code doesn’t compile:

Now that this PR has been merged and thus, as of TS 4.0, the code above will compile and TypeScript will infer the type of x to be string | boolean.

This is one more case where TypeScript’s type inference will help us out!

javascript typescript

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