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TypeScript is a superset of JavaScript that compiles to clean JavaScript output.
https://www.typescriptlang.org
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Bloomberg TS5.5-beta feedback #58523

Open murtyjones opened 5 months ago

murtyjones commented 5 months ago

Acknowledgement

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:

Change
 Impacts Release notes Packages affected
Stricter type-checking of partial objects Type checker This may be caused by improved type predicates 1
Improved narrowing of Array.isArray result Type checker This may be caused by improved type predicates 1
Exports are hoisted to the top of a module Emitted JS Not announced Most

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;
Andarist commented 5 months ago

Stricter type-checking of partial objects

My educated guess is that the observed change was caused by the work done in https://github.com/microsoft/TypeScript/pull/57946 and https://github.com/microsoft/TypeScript/pull/57549

Improved narrowing of Array.isArray result

I don't think type predicate inference could change this anyhow. That change focuses on inferring some functions as type predicates. Array.isArray was already a type predicate - it's declaration is defined in a lib file.