Support `Object.defineProperty` for expando properties and CommonJS exports in JS files #2513

ahejlsberg · 2026-01-15T02:25:35Z

In this PR:

In JS files, permit expando properties to be defined with Object.defineProperty.
In JS files, permit CommonJS exports to be defined with Object.defineProperty applied to exports or module.exports.
Fix circularity errors and unwanted implicit-any errors by deferring resolution of expando object property types.
Fix error on typeof module.exports when used to define types in JSDoc comments in JS files.
Cache creation of CommonJS module symbols in getModuleSymbol.
Don't emit CommonJSExport nodes in JS files (those are synthetic reparsed nodes intended just for checking).
Refactor declaration emit to support CommonJS exports defined with Object.defineProperty.
Port Cache started nonexistent property error checks to prevent reentrancy in the check TypeScript#60683 to fix stack overflow in test.
Support chained assignments in reparsing.
Revised handling of CommonJS assignment declarations with undefined initializers.

The revised handling of CommonJS assignment declarations now ignores undefined initializers when they occur as the initial assignment declaration of an export with multiple declarations. This provides for the common pattern of first initializing CommonJS exports to undefined and then subsequently assigning the actual exported value.

Fixes #2527.

jakebailey · 2026-01-15T02:52:43Z

...ubmodule/fourslash/findRenameLocations/renameTemplateLiteralsDefinePropertyJs.baseline.jsonc

-//     [`prop`]: 1
+// let obj = {};
+// 
+// Object.defineProperty(obj, `prop`, { value: 0 });


Seems like Strada's rename would also update the defineProperty. Not sure how possible that is now.

I'll take a look.

Now fixed, except initiating a rename in a string literal doesn't work, but that's a general issue for Corsa that we should fix in a separate PR.

jakebailey · 2026-01-15T02:54:16Z

...ata/baselines/reference/submodule/conformance/jsDeclarationsExportDefinePropertyEmit.js.diff

+        y: typeof module.exports.b;
+                  ~~~~~~
+!!! error TS2580: Cannot find name 'module'. Do you need to install type definitions for node? Try `npm i --save-dev @types/node`.
+    }): void;
+    declare const _exported_6: typeof hh;
+    export { _exported_6 as "h" };
+    declare const _exported_7: () => void;
+    export { _exported_7 as "i" };
+    declare const _exported_8: () => void;
+    export { _exported_8 as "ii" };
+    declare const _exported_9: () => void;
+    export { _exported_9 as "jj" };
+    declare const _exported_10: () => void;
+    export { _exported_10 as "j" };
+    


The exports here seem to be causing module to not exist, or something? Definitely odd.

It's a declaration emit issue. The old declaration emitter wouldn't attempt to reference module.exports (it's only a CommonJS thing and can't be referenced in a .d.ts file), but instead would inline the actual type.

jakebailey · 2026-01-15T02:56:44Z

testdata/baselines/reference/submodule/compiler/jsExpandoObjectDefineProperty.errors.txt.diff

+    var chrome = {}
+    Object.defineProperty(chrome, 'devtools', { value: {}, enumerable: true })
+    chrome.devtools.inspectedWindow = {}


I suppose this is consistentish, but it is weird that if I write:

var chrome = {} chrome.devtools = {} chrome.devtools.inspectedWindow = {}

You get an error on the second line too, so expandos are sort of a way around a check.

We could probably fix this.

This works in a JS file (but not in a TS file):

var chrome = {} chrome.devtools = {} chrome.devtools.inspectedWindow = {}

So you could argue that using Object.defineProperty for the middle one should work too. Which is the case in Strada.

Honestly, having expando objects that are rooted in Object.defineProperty seems like a totally esoteric feature. I'm going to hold off on implementing that.

Copilot

Pull request overview

This PR adds support for Object.defineProperty in JavaScript files, specifically for:

Expando properties (properties added to regular objects)
CommonJS exports defined via Object.defineProperty on exports or module.exports

The changes fix circularity errors and implicit-any errors by deferring type resolution, fix errors when using typeof module.exports in JSDoc, cache module symbols, prevent emission of CommonJSExport nodes in JS files, and refactor declaration emit.

Changes:

Support for Object.defineProperty to define CommonJS exports and expando properties in JS files
Fixes for error handling around typeof module.exports in JSDoc comments
Removal of erroneous errors about missing module and exports in JS files
Improved type inference for properties defined with Object.defineProperty

Reviewed changes

Copilot reviewed 149 out of 149 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
checkExportsObjectAssignProperty.*.diff	Tests now correctly recognize exports defined via Object.defineProperty, removing false positive errors about undefined module/exports
assignmentToVoidZero*.diff	Fixes type handling for void 0 assignments
typeFromPropertyAssignment*.diff	Improved symbol and type resolution for property assignments
jsDeclarations*.diff	Removes spurious export var statements in emitted JS files
checkOtherObjectAssignProperty.*.diff	Properly recognizes properties defined via Object.defineProperty on exports
checkObjectDefineProperty.*.diff	Correctly infers types from Object.defineProperty calls
defaultPropertyAssignedClassWithPrototype.*.diff	Better type tracking for expando properties
nodeModulesCJSEmit*.diff	Removes incorrect export var statements in CommonJS emit
Various other conformance tests	Fixes related to CommonJS export handling

jakebailey · 2026-01-16T21:56:53Z

Are you planning on putting more into this PR or is it ready?

ahejlsberg · 2026-01-16T23:23:05Z

Are you planning on putting more into this PR or is it ready?

The PR is ready now. The tests reveal issues in declaration emit, namely:

Use of typeof module.exports.xxx (which isn't permitted in .ts files),
Emitting multiple export declare var foo: Foo for multiple assignments to exports.foo, and
Emitting export declare var foo: Foo instead of export const foo: Foo for CommonJS exports.

We can address these in separate PRs.

Co-authored-by: Copilot <[email protected]>

ahejlsberg added 2 commits January 14, 2026 16:05

Object.defineProperty for expandos and CommonJS exports in JS files

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ahejlsberg requested review from DanielRosenwasser, RyanCavanaugh, Copilot, jakebailey and sandersn and removed request for Copilot January 15, 2026 02:25

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Merge branch 'main' into object-define-property

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ahejlsberg added 2 commits January 14, 2026 18:07

Fix stack overflow by porting #60683

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Copilot AI review requested due to automatic review settings January 15, 2026 04:09

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ahejlsberg added 6 commits January 15, 2026 06:50

Don't widen literal types in CommonJS exports

5a646ff

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55f2298

Support Object.defineProperty in find-all-refs

87f617f

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477f3b1

Support chained assignments in reparsing

69c05c9

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896577b

jakebailey mentioned this pull request Jan 16, 2026

tsgo incorrectly emits both ESM and CJS exports when compiling .js files containing exports.xxx statements #2527

Closed

ahejlsberg added 2 commits January 16, 2026 13:11

Revise handling of declaring CommonJS exports with undefined values

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ahejlsberg mentioned this pull request Jan 16, 2026

Multiple differences in declaration emit for CommonJS exports #2533

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jakebailey approved these changes Jan 17, 2026

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ahejlsberg added this pull request to the merge queue Jan 17, 2026

Merged via the queue into main with commit ebba558 Jan 17, 2026
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ahejlsberg deleted the object-define-property branch January 17, 2026 17:08

Copilot AI mentioned this pull request Jan 18, 2026

Update CHANGES.md to reflect #2513 #2541

Merged

gabritto mentioned this pull request Jan 27, 2026

Support Object.defineProperty in document symbols #2592

Open

github-merge-queue bot pushed a commit that referenced this pull request Jan 27, 2026

Update CHANGES.md to reflect #2513 (#2541)

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Co-authored-by: Copilot <[email protected]>

Support Object.defineProperty for expando properties and CommonJS exports in JS files #2513

Support Object.defineProperty for expando properties and CommonJS exports in JS files #2513

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Pull request overview

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Support `Object.defineProperty` for expando properties and CommonJS exports in JS files #2513

Support `Object.defineProperty` for expando properties and CommonJS exports in JS files #2513

ahejlsberg commented Jan 15, 2026 •

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