Add support for list.lastIndexOf in Soy and Array#lastIndexOf in WizToSoy

java/src/com/google/template/soy/basicfunctions/BasicFunctions.java

@@ -56,6 +56,7 @@ public static ImmutableList<SoySourceFunction> functions() {
         new ListFlatMethod(),
         new ListIncludesFunction(),
         new ListIndexOfFunction(),
+        new ListLastIndexOfFunction(),
         new ListReverseMethod(),
         new ListSliceMethod(),
         new ListUniqMethod(),

java/src/com/google/template/soy/basicfunctions/BasicFunctionsRuntime.java

@@ -167,6 +167,21 @@ public static int listIndexOf(
     return indexInSubList == -1 ? -1 : indexInSubList + clampedStartIndex;
   }
 
+  /** Returns the last index of value in list or -1. */
+  public static int listLastIndexOf(
+      List<? extends SoyValueProvider> list, SoyValue value, NumberData startIndex) {
+    int clampedStartIndex = clampListIndex(list, startIndex);
+    if (clampedStartIndex == list.size()) {
+      clampedStartIndex = list.size() - 1;
+    }
+    for (int i = clampedStartIndex; i >= 0; i--) {
+      if (list.get(i).resolve().equals(value)) {
+        return i;
+      }
+    }
+    return -1;
+  }
+
   /** Joins the list elements by a separator. */
   @Nonnull
   public static String join(List<? extends SoyValueProvider> list, String separator) {

java/src/com/google/template/soy/basicfunctions/ListLastIndexOfFunction.java

@@ -0,0 +1,94 @@
+/*
+ * Copyright 2026 Google Inc.
+ *
+ * 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.template.soy.basicfunctions;
+
+import com.google.template.soy.data.SoyValue;
+import com.google.template.soy.data.restricted.NumberData;
+import com.google.template.soy.plugin.java.restricted.JavaPluginContext;
+import com.google.template.soy.plugin.java.restricted.JavaValue;
+import com.google.template.soy.plugin.java.restricted.JavaValueFactory;
+import com.google.template.soy.plugin.java.restricted.SoyJavaSourceFunction;
+import com.google.template.soy.plugin.javascript.restricted.JavaScriptPluginContext;
+import com.google.template.soy.plugin.javascript.restricted.JavaScriptValue;
+import com.google.template.soy.plugin.javascript.restricted.JavaScriptValueFactory;
+import com.google.template.soy.plugin.javascript.restricted.SoyJavaScriptSourceFunction;
+import com.google.template.soy.plugin.python.restricted.PythonPluginContext;
+import com.google.template.soy.plugin.python.restricted.PythonValue;
+import com.google.template.soy.plugin.python.restricted.PythonValueFactory;
+import com.google.template.soy.plugin.python.restricted.SoyPythonSourceFunction;
+import com.google.template.soy.shared.restricted.Signature;
+import com.google.template.soy.shared.restricted.SoyMethodSignature;
+import com.google.template.soy.shared.restricted.SoyPureFunction;
+import java.lang.reflect.Method;
+import java.util.List;
+
+/**
+ * Soy function for checking the last index an item is contained in a list.
+ *
+ * <p>Usage: {@code list.lastIndexOf(item)}
+ *
+ * <ul>
+ *   <li>list: The list in which to look for the item.
+ *   <li>item: The item to search for in the list.
+ * </ul>
+ */
+@SoyMethodSignature(
+    name = "lastIndexOf",
+    baseType = "list<any>",
+    value = {
+      @Signature(parameterTypes = "any", returnType = "int"),
+      @Signature(
+          parameterTypes = {"any", "float|int"},
+          returnType = "int")
+    })
+@SoyPureFunction
+public class ListLastIndexOfFunction
+    implements SoyJavaSourceFunction, SoyJavaScriptSourceFunction, SoyPythonSourceFunction {
+
+  @Override
+  public JavaScriptValue applyForJavaScriptSource(
+      JavaScriptValueFactory factory, List<JavaScriptValue> args, JavaScriptPluginContext context) {
+    return factory.callNamespaceFunction("soy", "soy.$$listLastIndexOf", args);
+  }
+
+  @Override
+  public PythonValue applyForPythonSource(
+      PythonValueFactory factory, List<PythonValue> args, PythonPluginContext context) {
+    return factory.global("runtime.list_lastindexof").call(args);
+  }
+
+  // lazy singleton pattern, allows other backends to avoid the work.
+  private static final class Methods {
+    static final Method LIST_CONTAINS_FN =
+        JavaValueFactory.createMethod(
+            BasicFunctionsRuntime.class,
+            "listLastIndexOf",
+            List.class,
+            SoyValue.class,
+            NumberData.class);
+  }
+
+  @Override
+  public JavaValue applyForJavaSource(
+      JavaValueFactory factory, List<JavaValue> args, JavaPluginContext context) {
+    return factory.callStaticMethod(
+        Methods.LIST_CONTAINS_FN,
+        args.get(0),
+        args.get(1),
+        args.size() == 3 ? args.get(2) : factory.constant(Integer.MAX_VALUE));
+  }
+}

java/src/com/google/template/soy/passes/ResolveExpressionTypesPass.java

@@ -72,6 +72,7 @@
 import com.google.template.soy.basicfunctions.ListFlatMethod;
 import com.google.template.soy.basicfunctions.ListIncludesFunction;
 import com.google.template.soy.basicfunctions.ListIndexOfFunction;
+import com.google.template.soy.basicfunctions.ListLastIndexOfFunction;
 import com.google.template.soy.basicfunctions.ListReverseMethod;
 import com.google.template.soy.basicfunctions.ListSliceMethod;
 import com.google.template.soy.basicfunctions.ListUniqMethod;
@@ -2165,7 +2166,8 @@ private void finishMethodCallNode(MethodCallNode node, boolean nullSafe) {
             node.setType(returnType);
           }
         } else if (sourceFunction instanceof ListIncludesFunction
-            || sourceFunction instanceof ListIndexOfFunction) {
+            || sourceFunction instanceof ListIndexOfFunction
+            || sourceFunction instanceof ListLastIndexOfFunction) {
           node.setType(sourceMethod.getReturnType());
           if (node.getParam(0) instanceof RecordLiteralNode) {
             errorReporter.report(node.getParam(0).getSourceLocation(), RECORD_LITERAL_NOT_ALLOWED);

javascript/soyutils_usegoog.js

@@ -2067,6 +2067,17 @@ const $$listIndexOf = function(list, val, startIndex = 0) {
   return indexInSublist === -1 ? -1 : indexInSublist + clampedStartIndex;
 };
 
+/**
+ * Returns the last index of val in list or -1
+ * @param {!ReadonlyArray<?>} list
+ * @param {*} val
+ * @param {number=} startIndex
+ * @return {number}
+ */
+const $$listLastIndexOf = function(list, val, startIndex = list.length - 1) {
+  return list.lastIndexOf(val, startIndex);
+};
+
 /**
  * Reverses a list and returns it. The original list passed is unaffected.
  * @param {!ReadonlyArray<T>} list
@@ -3104,6 +3115,7 @@ exports = {
   $$truncate,
   $$listContains,
   $$listIndexOf,
+  $$listLastIndexOf,
   $$listReverse,
   $$listUniq,
   $$listFlat,

python/runtime.py

@@ -427,6 +427,17 @@ def list_indexof(l, item, start_index=0):
   return -1
 
 
+def list_lastindexof(l, item, start_index=None):
+  """Equivalent getting the last index of `item in l` but using soy's equality algorithm."""
+  if start_index is None or start_index >= len(l):
+    start_index = len(l) - 1
+  clamped_start_index = clamp_list_start_index(l, start_index)
+  for i in range(clamped_start_index, -1, -1):
+    if strict_eq(l[i], item):
+      return i
+  return -1
+
+
 def strict_eq(first, second):
   """An equality function that handles JS primitive types as primitives."""
   if (