Add parsing support for ES6 class private properties

PiperOrigin-RevId: 722828272

Author: Closure Team

src/com/google/javascript/jscomp/CodeGenerator.java

@@ -706,7 +706,7 @@ protected void add(Node node, Context context, boolean printComments) {
 
             // Add the property name.
             if (!node.isQuotedStringKey()
-                && TokenStream.isJSIdentifier(name)
+                && (TokenStream.isJSIdentifier(name) || node.isPrivateIdentifier())
                 &&
                 // do not encode literally any non-literal characters that were
                 // Unicode escaped.

src/com/google/javascript/jscomp/RewritePrivateClassProperties.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2025 The Closure Compiler Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.javascript.jscomp;
+
+import static com.google.javascript.jscomp.TranspilationUtil.cannotConvertYet;
+
+import com.google.errorprone.annotations.CanIgnoreReturnValue;
+import com.google.javascript.jscomp.parsing.parser.FeatureSet;
+import com.google.javascript.jscomp.parsing.parser.FeatureSet.Feature;
+import com.google.javascript.rhino.Node;
+
+/** Transpiles away usages of private properties in ES6 classes. */
+final class RewritePrivateClassProperties extends AbstractPeepholeTranspilation {
+
+  private final AbstractCompiler compiler;
+
+  RewritePrivateClassProperties(AbstractCompiler compiler) {
+    this.compiler = compiler;
+  }
+
+  @Override
+  FeatureSet getTranspiledAwayFeatures() {
+    return FeatureSet.BARE_MINIMUM.with(Feature.PRIVATE_CLASS_PROPERTIES);
+  }
+
+  @Override
+  @CanIgnoreReturnValue
+  Node transpileSubtree(Node n) {
+    if (n.isPrivateIdentifier()) {
+      cannotConvertYet(compiler, n, "private class properties");
+    }
+    return n;
+  }
+}

src/com/google/javascript/jscomp/TranspilationPasses.java

@@ -197,6 +197,9 @@ public static void addTranspilationPasses(PassListBuilder passes, CompilerOption
                         compiler,
                         compiler.getOptions().getBrowserFeaturesetYearObject(),
                         compiler.getOptions().getOutputFeatureSet()));
+                if (compiler.getOptions().needsTranspilationOf(Feature.PRIVATE_CLASS_PROPERTIES)) {
+                  peepholeTranspilations.add(new RewritePrivateClassProperties(compiler));
+                }
                 if (compiler.getOptions().needsTranspilationOf(Feature.OPTIONAL_CATCH_BINDING)) {
                   peepholeTranspilations.add(new RewriteCatchWithNoBinding(compiler));
                 }