Bloomberg TS5.5-beta feedback

Acknowledgement

• I acknowledge that issues using this template may be closed without further explanation at the maintainer's discretion.

Comment

We are in the process of evaluating the impact of TS 5.5 beta on Bloomberg code. Below are preliminary findings.

Overall this looks to be a low-impact release. The following changes were observed and seem to either be benign in nature or else improvements:

Stricter type-checking of partial objects

We observed one project with a new type error in 5.5-beta, seemingly due to an improvement in type-checking for partials.

A minimal reproduction is here.

Improved narrowing of Array.isArray result

We observed for one project a new type error from trying to compare a string with an array after an Array.isArray check. The generics involved make a minimal reproduction infeasible but the essence of the code is:

if (Array.isArray(foo)) {
     If (foo !== "something") // raises a new error for some complex foo
}

Exports are hoisted to the top of a module

Module exports seem to be hoisted to the top of modules now. No runtime impact observed.

Object.defineProperty(exports, "__esModule", { value: true });
+exports.foo = foo;
 function foo() {
     // …
 }
-exports.foo = foo;

The issue names no repository files or tests; start with the linked TypeScript 5.5 beta minimal reproduction and compare the reported type-checking behavior with the release notes. Then investigate the Array.isArray narrowing example and emitted export ordering, with done meaning the findings are confirmed as expected behavior or actionable regressions.