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Learning TypeScript 2.x - Second Edition
book

Learning TypeScript 2.x - Second Edition

by Remo H. Jansen
April 2018
Beginner content levelBeginner
536 pages
13h 21m
English
Packt Publishing
Content preview from Learning TypeScript 2.x - Second Edition

Covariant checking in callback parameters

TypeScript 2.4 changed the way the type system behaves internally to improve the error detection in nested callbacks and promises:

TypeScript's checking of callback parameters is now covariant concerning immediate signature checks. Previously it was bivariant, which could sometimes let incorrect types through. Basically, this means that callback parameters and classes that contain callbacks are checked more carefully, so TypeScript will require stricter types in this release. This is particularly true of Promises and Observables due to the way in which their APIs are specified.

In TypeScript versions before 2.4, the following example was considered valid, and no errors were thrown:

declare function ...
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Publisher Resources

ISBN: 9781788391474Supplemental Content