feat(transformers): Add beforeDeclarations transformers

src/compiler/transformer.ts

@@ -183,7 +183,11 @@ function getScriptTransformers(compilerOptions: CompilerOptions, customTransform
 
 function getDeclarationTransformers(customTransformers?: CustomTransformers) {
     const transformers: TransformerFactory<SourceFile | Bundle>[] = [];
+
+    addRange(transformers, customTransformers && map(customTransformers.beforeDeclarations, wrapDeclarationTransformerFactory));
+
     transformers.push(transformDeclarations);
+
     addRange(transformers, customTransformers && map(customTransformers.afterDeclarations, wrapDeclarationTransformerFactory));
     return transformers;
 }

src/compiler/types.ts

@@ -4867,6 +4867,8 @@ export interface CustomTransformers {
     before?: (TransformerFactory<SourceFile> | CustomTransformerFactory)[];
     /** Custom transformers to evaluate after built-in .js transformations. */
     after?: (TransformerFactory<SourceFile> | CustomTransformerFactory)[];
+    /** Custom transformers to evaluate before built-in .d.ts transformations. */
+    beforeDeclarations?: (TransformerFactory<Bundle | SourceFile> | CustomTransformerFactory)[];
     /** Custom transformers to evaluate after built-in .d.ts transformations. */
     afterDeclarations?: (TransformerFactory<Bundle | SourceFile> | CustomTransformerFactory)[];
 }

src/testRunner/unittests/transform.ts

@@ -55,6 +55,22 @@ describe("unittests:: TransformAPI", () => {
         return (file: T) => file;
     }
 
+    function transformInitializerToDefaultJSDoc<T extends ts.SourceFile | ts.Bundle>(context: ts.TransformationContext) {
+        return (file: T) => {
+            if (!ts.isSourceFile(file)) {
+                return file;
+            }
+            const visitor: ts.Visitor = (node: ts.Node): ts.VisitResult<ts.Node> => {
+                if (ts.isPropertyDeclaration(node) && node.initializer) {
+                    return ts.addSyntheticLeadingComment(node, /*kind*/ ts.SyntaxKind.MultiLineCommentTrivia, /*text*/ `* @default \`${node.initializer.getText()}\``, /*hasTrailingNewLine*/ true);
+                }
+                return ts.visitEachChild(node, visitor, context);
+            };
+
+            return ts.visitNode(file, visitor) as T;
+        };
+    }
+
     function replaceIdentifiersNamedOldNameWithNewName2(context: ts.TransformationContext) {
         const visitor = (node: ts.Node): ts.Node => {
             if (ts.isIdentifier(node) && node.text === "oldName") {
@@ -393,6 +409,25 @@ describe("unittests:: TransformAPI", () => {
         });
     });
 
+    testBaseline("transformDeclarationFileBefore", () => {
+        return baselineDeclarationTransform(
+            `
+                export class Test { 
+                    public foo: string = "bar" 
+                }
+            `,
+            {
+                transformers: {
+                    beforeDeclarations: [transformInitializerToDefaultJSDoc],
+                },
+                compilerOptions: {
+                    newLine: ts.NewLineKind.CarriageReturnLineFeed,
+                    declaration: true,
+                },
+            },
+        );
+    });
+
     // https://github.com/microsoft/TypeScript/issues/33295
     testBaseline("transformParameterProperty", () => {
         return transpileModule("", {

tests/baselines/reference/api/typescript.d.ts

@@ -6058,6 +6058,8 @@ declare namespace ts {
         before?: (TransformerFactory<SourceFile> | CustomTransformerFactory)[];
         /** Custom transformers to evaluate after built-in .js transformations. */
         after?: (TransformerFactory<SourceFile> | CustomTransformerFactory)[];
+        /** Custom transformers to evaluate before built-in .d.ts transformations. */
+        beforeDeclarations?: (TransformerFactory<Bundle | SourceFile> | CustomTransformerFactory)[];
         /** Custom transformers to evaluate after built-in .d.ts transformations. */
         afterDeclarations?: (TransformerFactory<Bundle | SourceFile> | CustomTransformerFactory)[];
     }

tests/baselines/reference/transformApi/transformsCorrectly.transformDeclarationFileBefore.js

@@ -0,0 +1,4 @@
+export declare class Test {
+    /** @default `"bar"`*/
+    foo: string;
+}