Enable `strictNullChecks` for `traverse` #17499

liuxingbaoyu · 2025-08-30T12:40:13Z

Q	A
Fixed Issues?	`Fixes #1, Fixes #2`
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

babel-bot · 2025-08-30T12:43:18Z

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/60040

liuxingbaoyu · 2025-08-30T12:44:22Z

packages/babel-traverse/src/path/index.ts

  skipKeys: Record<string, boolean> | null = null;
-  parentPath: NodePath_Final = null;
+  parentPath: NodePath_Final | null = null;
  container: t.Node | Array<t.Node> | null = null;
-  listKey: string | null = null;
+  listKey: string | null | undefined = null;
  key: string | number | null = null;
  node: t.Node | null = null;
  type: t.Node["type"] | null = null;


Perhaps we could define more properties as non-null, such as container, like we did with parentPath.

I'm a bit hesitant to do the same with scope.parent.

liuxingbaoyu · 2025-08-30T12:46:41Z

packages/babel-traverse/src/path/index.ts

+interface NodePath<
+  N extends t.Node | null,
+  T extends t.Node["type"] = NonNullable<N>["type"],
+> extends InstanceType<typeof NodePath_Final>,


NodePath<null> was introduced here, but it was not used more in path.get because it would be very cumbersome to use if we used strict null and non-null.
Suggestions welcome!

It seems the ecosystem has moved to a stricter null, I'll double-check.

liuxingbaoyu · 2025-08-30T12:48:52Z

packages/babel-traverse/src/path/lib/removal-hooks.ts

      self.replaceWith({
        type: "BlockStatement",
+        directives: [],
        body: [],
-      } as t.BlockStatement);
+      } satisfies t.BlockStatement);
      return true;


This is a very good fix.

Right, though looking back I think we could just use the blockStatement builder, since the @babel/types is already a dependency of @babel/traverse.

pkg-pr-new · 2025-08-30T13:03:28Z

Open in StackBlitz

commit: 2b67663

packages/babel-traverse/src/scope/index.ts

JLHwung

This PR looks good to me in general. Nice work.

JLHwung · 2025-09-29T18:08:32Z

packages/babel-traverse/src/path/inference/inferer-reference.ts

  binding: Binding,
  path: NodePath,
-  name: string,
+  name: string | undefined,


The name here is passed from a name property of an Identifier, I think it should be always a string unless the AST is malformed.

babel/packages/babel-traverse/src/path/inference/inferer-reference.ts

Line 23 in c36fa6a

return getTypeAnnotationBindingConstantViolations(

babel/packages/babel-traverse/src/path/inference/inferer-reference.ts

Line 55 in c36fa6a

const testType = getConditionalAnnotation(binding, path, name);

Can we remove the optional marker here?

babel/packages/babel-traverse/src/path/inference/inferer-reference.ts

Line 211 in c36fa6a

name?: string,

JLHwung · 2025-09-29T18:11:32Z

packages/babel-traverse/src/path/inference/inferer-reference.ts

  if (types.length) {
    return {
+      // @ts-expect-error FIXME: may return undefined
      typeAnnotation: createUnionType(types),


The createUnionType will return something as long as types are either all Flow types or TS types. The parser will not mix flow types with ts types, so if there are any outliers, they should be considered malformed AST. In this case can we use the definite marker here?

JLHwung · 2025-09-29T18:13:21Z

packages/babel-traverse/src/path/lib/hoister.ts

          // We also move past any constant violations.
          for (const violationPath of binding.constantViolations) {
+            // @ts-expect-error comparing undefined and number
            if (this.getAttachmentParentForPath(violationPath).key > path.key) {


The getAttachmentParentForPath now always return a NodePath, does TS throw an error because we are comparing string to number?

It seems that it is because key may be null.

liuxingbaoyu · 2025-09-29T22:02:50Z

I pushed fa9e572 (#17499) to fix issues I encountered while upgrading the ecosystem.

liuxingbaoyu · 2025-09-29T22:23:43Z

I'm marking this as a bug fix because this fixes compatibility issues with strictNullChecks.

nicolo-ribaudo · 2025-10-03T10:41:42Z

packages/babel-traverse/src/scope/index.ts

+  block!: t.Pattern | t.Scopable;

-  inited;
+  declare inited;


Can we avoid declare here? We want them to be defined as own properties.

Co-authored-by: Huáng Jùnliàng <[email protected]>

liuxingbaoyu commented Aug 30, 2025

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liuxingbaoyu force-pushed the strictNullChecks-traverse branch from bf1c9f0 to e0bcaa6 Compare September 12, 2025 12:32

JLHwung reviewed Sep 22, 2025

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packages/babel-traverse/src/scope/index.ts Outdated Show resolved Hide resolved

JLHwung approved these changes Sep 29, 2025

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liuxingbaoyu force-pushed the strictNullChecks-traverse branch from bceb22f to d423e2f Compare September 29, 2025 22:01

liuxingbaoyu added the PR: Bug Fix (next major) 🐛 A type of pull request used for our changelog categories for next major release label Sep 29, 2025

nicolo-ribaudo reviewed Oct 3, 2025

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liuxingbaoyu and others added 9 commits October 3, 2025 16:00

strictNullChecks

d00fd47

fix

072d181

review

c0f6ec7

Update packages/babel-traverse/src/scope/index.ts

16ad14d

Co-authored-by: Huáng Jùnliàng <[email protected]>

improve

8e975b7

review

32f2a88

fix lint

8e6ea37

fix

cea7130

Avoid declare

2b67663

nicolo-ribaudo force-pushed the strictNullChecks-traverse branch from ce2662e to 2b67663 Compare October 3, 2025 14:03

nicolo-ribaudo approved these changes Oct 3, 2025

View reviewed changes

nicolo-ribaudo merged commit e579cb0 into babel:main Oct 3, 2025
73 of 74 checks passed

github-actions bot added the outdated A closed issue/PR that is archived due to age. Recommended to make a new issue label Jan 3, 2026

github-actions bot locked as resolved and limited conversation to collaborators Jan 3, 2026

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Enable strictNullChecks for traverse #17499

Enable strictNullChecks for traverse #17499

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liuxingbaoyu commented Aug 30, 2025

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liuxingbaoyu commented Sep 29, 2025

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Enable `strictNullChecks` for `traverse` #17499

Enable `strictNullChecks` for `traverse` #17499

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