TypeScript 4.6, the next iteration of Microsoft’s strongly typed superset of JavaScript, has moved to a beta release, with enhancements for coding and control flow analysis. The beta was announced on January 21.One improvement relaxes coding restrictions by allowing code in constructors before super(). In JavaScript classes, it is mandatory to call super() prior to referring to this. TypeScript also enforced this but was a bit too strict, according to a bulletin on TypeScript 4.6. It had been an error to contain any code at the beginning of a constructor if its containing class had property initializers. This made it “cheap” to check that super() was called before this was referenced, but resulted in valid code being rejected. TypeScript 4.6 relaxes this check. It also permits other code to run before super() while ensuring that super() occurs at the top level before references to this.The beta can be accessed through NuGet or via NPM:npm install typescript@betaTypeScript 4.6 is due as a release candidate on February 8 and as a production release on February 22. Elsewhere in TypeScript 4.6:
With control flow analysis for dependent parameters, TypeScript can narrow parameters that depend on others, in some situations where TypeScript infers from a signature in a rest parameter.
Improved recursion depth checks offer better relationship checking for recursive types.
TypeScript now can infer to indexed access types that immediately index into a mapped object type.
The set of syntax and binding errors in JavaScript files has been expanded. These will be seen if developers open JavaScript files in an editor such as Visual Studio or Visual Studio Code, or if they run JavaScript code through the TypeScript compiler.
A trace analyzer tool has been published, to help teams running into build performance issues.
Object rest expressions now drop members that appear to be unspreadable or generic objects. This is a breaking change.
The predecessor TypeScript 4.5 release was published last November, with Promise improvements.
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