Fix trailing comma support in import type attributes

src/compiler/parser.ts

@@ -4562,16 +4562,17 @@ namespace Parser {
             else {
                 parseErrorAtCurrentToken(Diagnostics._0_expected, tokenToString(SyntaxKind.WithKeyword));
             }
-            parseExpected(SyntaxKind.ColonToken);
-            attributes = parseImportAttributes(currentToken as SyntaxKind.WithKeyword | SyntaxKind.AssertKeyword, /*skipKeyword*/ true);
-            if (!parseExpected(SyntaxKind.CloseBraceToken)) {
-                const lastError = lastOrUndefined(parseDiagnostics);
-                if (lastError && lastError.code === Diagnostics._0_expected.code) {
-                    addRelatedInfo(
-                        lastError,
-                        createDetachedDiagnostic(fileName, sourceText, openBracePosition, 1, Diagnostics.The_parser_expected_to_find_a_1_to_match_the_0_token_here, "{", "}"),
-                    );
-                }
+            parseExpected(SyntaxKind.ColonToken);
+            attributes = parseImportAttributes(currentToken as SyntaxKind.WithKeyword | SyntaxKind.AssertKeyword, /*skipKeyword*/ true);
+            parseOptional(SyntaxKind.CommaToken); // Allow trailing comma after import attributes
+            if (!parseExpected(SyntaxKind.CloseBraceToken)) {
+                const lastError = lastOrUndefined(parseDiagnostics);
+                if (lastError && lastError.code === Diagnostics._0_expected.code) {
+                    addRelatedInfo(
+                        lastError,
+                        createDetachedDiagnostic(fileName, sourceText, openBracePosition, 1, Diagnostics.The_parser_expected_to_find_a_1_to_match_the_0_token_here, "{", "}"),
+                    );
+                }
             }
         }
         parseExpected(SyntaxKind.CloseParenToken);

tests/baselines/reference/importTypeTrailingComma.errors.txt

@@ -0,0 +1,44 @@
+importTypeTrailingComma.ts(5,17): error TS2792: Cannot find module './nonexistent'. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?
+importTypeTrailingComma.ts(8,17): error TS2792: Cannot find module './nonexistent'. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?
+importTypeTrailingComma.ts(11,17): error TS2792: Cannot find module './nonexistent'. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?
+importTypeTrailingComma.ts(14,17): error TS2792: Cannot find module './nonexistent'. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?
+importTypeTrailingComma.ts(17,24): error TS2792: Cannot find module './nonexistent'. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?
+importTypeTrailingComma.ts(20,17): error TS2792: Cannot find module './nonexistent'. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?
+importTypeTrailingComma.ts(20,42): error TS1464: Type import attributes should have exactly one key - 'resolution-mode' - with value 'import' or 'require'.
+
+
+==== importTypeTrailingComma.ts (7 errors) ====
+    // Test that trailing commas are allowed in import type attributes
+    // This should work consistently with dynamic imports
+
+    // Trailing comma inside with object (already worked)
+    type A = import("./nonexistent", { with: { type: "json", } })
+                    ~~~~~~~~~~~~~~~
+!!! error TS2792: Cannot find module './nonexistent'. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?
+
+    // Trailing comma after with object (this is the fix)
+    type B = import("./nonexistent", { with: { type: "json" }, })
+                    ~~~~~~~~~~~~~~~
+!!! error TS2792: Cannot find module './nonexistent'. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?
+
+    // No trailing comma for comparison
+    type C = import("./nonexistent", { with: { type: "json" } })
+                    ~~~~~~~~~~~~~~~
+!!! error TS2792: Cannot find module './nonexistent'. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?
+
+    // Assert syntax with trailing comma  
+    type D = import("./nonexistent", { assert: { type: "json" }, })
+                    ~~~~~~~~~~~~~~~
+!!! error TS2792: Cannot find module './nonexistent'. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?
+
+    // typeof with trailing comma
+    type E = typeof import("./nonexistent", { with: { type: "json" }, })
+                           ~~~~~~~~~~~~~~~
+!!! error TS2792: Cannot find module './nonexistent'. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?
+
+    // Multiple properties in with object with trailing comma after
+    type F = import("./nonexistent", { with: { type: "json", "resolution-mode": "import" }, })
+                    ~~~~~~~~~~~~~~~
+!!! error TS2792: Cannot find module './nonexistent'. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?
+                                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS1464: Type import attributes should have exactly one key - 'resolution-mode' - with value 'import' or 'require'.

tests/baselines/reference/importTypeTrailingComma.js

@@ -0,0 +1,27 @@
+//// [tests/cases/compiler/importTypeTrailingComma.ts] ////
+
+//// [importTypeTrailingComma.ts]
+// Test that trailing commas are allowed in import type attributes
+// This should work consistently with dynamic imports
+
+// Trailing comma inside with object (already worked)
+type A = import("./nonexistent", { with: { type: "json", } })
+
+// Trailing comma after with object (this is the fix)
+type B = import("./nonexistent", { with: { type: "json" }, })
+
+// No trailing comma for comparison
+type C = import("./nonexistent", { with: { type: "json" } })
+
+// Assert syntax with trailing comma  
+type D = import("./nonexistent", { assert: { type: "json" }, })
+
+// typeof with trailing comma
+type E = typeof import("./nonexistent", { with: { type: "json" }, })
+
+// Multiple properties in with object with trailing comma after
+type F = import("./nonexistent", { with: { type: "json", "resolution-mode": "import" }, })
+
+//// [importTypeTrailingComma.js]
+// Test that trailing commas are allowed in import type attributes
+// This should work consistently with dynamic imports

tests/baselines/reference/importTypeTrailingComma.symbols

@@ -0,0 +1,30 @@
+//// [tests/cases/compiler/importTypeTrailingComma.ts] ////
+
+=== importTypeTrailingComma.ts ===
+// Test that trailing commas are allowed in import type attributes
+// This should work consistently with dynamic imports
+
+// Trailing comma inside with object (already worked)
+type A = import("./nonexistent", { with: { type: "json", } })
+>A : Symbol(A, Decl(importTypeTrailingComma.ts, 0, 0))
+
+// Trailing comma after with object (this is the fix)
+type B = import("./nonexistent", { with: { type: "json" }, })
+>B : Symbol(B, Decl(importTypeTrailingComma.ts, 4, 61))
+
+// No trailing comma for comparison
+type C = import("./nonexistent", { with: { type: "json" } })
+>C : Symbol(C, Decl(importTypeTrailingComma.ts, 7, 61))
+
+// Assert syntax with trailing comma  
+type D = import("./nonexistent", { assert: { type: "json" }, })
+>D : Symbol(D, Decl(importTypeTrailingComma.ts, 10, 60))
+
+// typeof with trailing comma
+type E = typeof import("./nonexistent", { with: { type: "json" }, })
+>E : Symbol(E, Decl(importTypeTrailingComma.ts, 13, 63))
+
+// Multiple properties in with object with trailing comma after
+type F = import("./nonexistent", { with: { type: "json", "resolution-mode": "import" }, })
+>F : Symbol(F, Decl(importTypeTrailingComma.ts, 16, 68))
+

tests/baselines/reference/importTypeTrailingComma.types

@@ -0,0 +1,48 @@
+//// [tests/cases/compiler/importTypeTrailingComma.ts] ////
+
+=== importTypeTrailingComma.ts ===
+// Test that trailing commas are allowed in import type attributes
+// This should work consistently with dynamic imports
+
+// Trailing comma inside with object (already worked)
+type A = import("./nonexistent", { with: { type: "json", } })
+>A : any
+>  : ^^^
+>type : any
+>     : ^^^
+
+// Trailing comma after with object (this is the fix)
+type B = import("./nonexistent", { with: { type: "json" }, })
+>B : any
+>  : ^^^
+>type : any
+>     : ^^^
+
+// No trailing comma for comparison
+type C = import("./nonexistent", { with: { type: "json" } })
+>C : any
+>  : ^^^
+>type : any
+>     : ^^^
+
+// Assert syntax with trailing comma  
+type D = import("./nonexistent", { assert: { type: "json" }, })
+>D : any
+>  : ^^^
+>type : any
+>     : ^^^
+
+// typeof with trailing comma
+type E = typeof import("./nonexistent", { with: { type: "json" }, })
+>E : any
+>  : ^^^
+>type : any
+>     : ^^^
+
+// Multiple properties in with object with trailing comma after
+type F = import("./nonexistent", { with: { type: "json", "resolution-mode": "import" }, })
+>F : any
+>  : ^^^
+>type : any
+>     : ^^^
+

tests/cases/compiler/importTypeTrailingComma.ts

@@ -0,0 +1,21 @@
+// @module: esnext
+// Test that trailing commas are allowed in import type attributes
+// This should work consistently with dynamic imports
+
+// Trailing comma inside with object (already worked)
+type A = import("./nonexistent", { with: { type: "json", } })
+
+// Trailing comma after with object (this is the fix)
+type B = import("./nonexistent", { with: { type: "json" }, })
+
+// No trailing comma for comparison
+type C = import("./nonexistent", { with: { type: "json" } })
+
+// Assert syntax with trailing comma  
+type D = import("./nonexistent", { assert: { type: "json" }, })
+
+// typeof with trailing comma
+type E = typeof import("./nonexistent", { with: { type: "json" }, })
+
+// Multiple properties in with object with trailing comma after
+type F = import("./nonexistent", { with: { type: "json", "resolution-mode": "import" }, })