Add transform that strips private properties in build types script (#49060)

Summary:

We want to hide private properties from JS public API interface. The stripPrivateProperties transform removes all private nodes of type ObjectTypeProperty, Property, PropertyDefinition and MethodDefinition. There is also a change in transforms reducer that incorporates `print` function from hermes-transform which modifies the code base on the transformed ast (transformed.mutatedCode seems to be a code before the transform operation).

## Changelog:
[Internal] -  Added transform that strips private properties in build-types script

Reviewed By: huntie

Differential Revision: D68892853

Author: coado

scripts/build/build-types/transforms/__tests__/stripPrivateProperties-test.js

@@ -0,0 +1,40 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ * @oncall react_native
+ */
+
+const stripPrivateProperties = require('../stripPrivateProperties.js');
+const {parse, print} = require('hermes-transform');
+
+const prettierOptions = {parser: 'babel'};
+
+async function translate(code: string): Promise<string> {
+  const parsed = await parse(code);
+  const result = await stripPrivateProperties(parsed);
+  return print(result.ast, result.mutatedCode, prettierOptions);
+}
+
+describe('stripPrivateProperties', () => {
+  test('should strip private properties', async () => {
+    const code = `const Foo = {
+      foo: 'foo',
+      bar() {},
+      _privateFoo: 'privateFoo',
+      _privateBar() {},
+    }`;
+    const result = await translate(code);
+    expect(result).toMatchInlineSnapshot(`
+      "const Foo = {
+        foo: \\"foo\\",
+        bar() {},
+      };
+      "
+    `);
+  });
+});

scripts/build/build-types/transforms/stripPrivateProperties.js

@@ -18,7 +18,26 @@ import type {ParseResult} from 'hermes-transform/dist/transform/parse';
 const {transformAST} = require('hermes-transform/dist/transform/transformAST');
 
 const visitors /*: TransformVisitor */ = context => ({
-  // TODO
+  ObjectTypeProperty(node) /*: void */ {
+    if (node.key.type === 'Identifier' && node.key.name.startsWith('_')) {
+      context.removeNode(node);
+    }
+  },
+  Property(node) /*: void */ {
+    if (node.key.type === 'Identifier' && node.key.name.startsWith('_')) {
+      context.removeNode(node);
+    }
+  },
+  PropertyDefinition(node) /*: void */ {
+    if (node.key.type === 'Identifier' && node.key.name.startsWith('_')) {
+      context.removeNode(node);
+    }
+  },
+  MethodDefinition(node) /*: void */ {
+    if (node.key.type === 'Identifier' && node.key.name.startsWith('_')) {
+      context.removeNode(node);
+    }
+  },
 });
 
 async function stripPrivateProperties(

scripts/build/build-types/translateSourceFile.js

@@ -15,7 +15,7 @@ import type {TransformASTResult} from 'hermes-transform/dist/transform/transform
 */
 
 const translate = require('flow-api-translator');
-const {parse} = require('hermes-transform');
+const {parse, print} = require('hermes-transform');
 
 /*::
 type TransformFn = (ParseResult) => Promise<TransformASTResult>;
@@ -55,10 +55,14 @@ async function applyTransforms(
   return transforms.reduce((input, transform) => {
     return input.then(async result => {
       const transformed = await transform(result);
+      const code = transformed.astWasMutated
+        ? await print(transformed.ast, transformed.mutatedCode, prettierOptions)
+        : transformed.mutatedCode;
+
       return {
         ...result,
         ast: transformed.ast,
-        code: transformed.mutatedCode,
+        code,
       };
     });
   }, Promise.resolve(source));