TypeScript 3.3 RC is here!

Today, Microsoft announced the general availability of TypeScript 3.3 RC in a blog post. This version does not contain any major or breaking changes.

Better behavior while calling union types

When there is a union type A | B, TypeScript now allows users to access all of the properties common to both A and B. For example, the intersection of members. You can get a property from a union type only if it’s known to be present in every union type. When every type has only one signature with identical parameters, things work. Such a restriction was too much and have errors in some areas. So, in TypeScript 3.3, the following code as shown in the blog will work:

type Fruit = "apple" | "orange";

type Color = "red" | "orange";

type FruitEater = (fruit: Fruit) => number;     // eats and ranks the fruit

type ColorConsumer = (color: Color) => string;  // consumes and describes the colors

declare let f: FruitEater | ColorConsumer;

f("orange"); // It works! Returns a 'number | string'.

f("apple");  // error - Argument of type '"apple"' is not assignable to parameter of type '"orange"'.

f("red");    // error - Argument of type '"red"' is not assignable to parameter of type '"orange"'.

The parameters of the above signatures are ‘intersected’ to create a new signature. When the impossible intersections are gone, what remains is "orange" & "orange" which is just "orange". That is not to say there are no restrictions. The new behavior is active only when only one type in the union has multiple overloads and a generic signature. The forEach method will now be callable, but there may be some issues under noImplicitAny.

The --build mode’s --watch flag leverages incremental file watching as well in TypeScript 3.3. This can result in significantly faster builds with --build --watch. Reportedly, there was over 50% reduced build times.

Future of ESLint support in TypeScript

Announcing ‘TypeScript Roadmap’ for January 2019- June 2019

Introducing ReX.js v1.0.0 a companion library for RegEx written in TypeScript