add langchain js sample

docs/en/samples/pre_post_processing/js/langchain/agent.js

@@ -0,0 +1,162 @@
+// Copyright 2026 Google LLC
+//
+// 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.
+
+import { ToolboxClient } from "@toolbox-sdk/core";
+import { ChatVertexAI } from "@langchain/google-vertexai";
+import { AgentExecutor, createToolCallingAgent } from "langchain/agents";
+import { ChatPromptTemplate } from "@langchain/core/prompts";
+import { tool } from "@langchain/core/tools";
+import { fileURLToPath } from "url";
+import process from "process";
+
+const systemPrompt = `
+  You're a helpful hotel assistant. You handle hotel searching, booking and
+  cancellations. When the user searches for a hotel, mention it's name, id,
+  location and price tier. Always mention hotel ids while performing any
+  searches. This is very important for any operations. For any bookings or
+  cancellations, please provide the appropriate confirmation. Be sure to
+  update checkin or checkout dates if mentioned by the user.
+  Don't ask for confirmations from the user.
+`;
+
+/**
+ * Pre-processing: Enforce Business Rules
+ * @param {string} name 
+ * @param {object} args 
+ * @returns {string|null} Error message if blocked, null otherwise
+ */
+function checkBusinessRules(name, args) {
+  console.log(`POLICY CHECK: Intercepting '${name}'`);
+
+  if (name === "update-hotel" && args.checkin_date && args.checkout_date) {
+    try {
+      const start = new Date(args.checkin_date);
+      const end = new Date(args.checkout_date);
+      const duration = (end - start) / (1000 * 60 * 60 * 24); // days
+
+      if (duration > 14) {
+        console.log("BLOCKED: Stay too long");
+        return "Error: Maximum stay duration is 14 days."; 
+      }
+    } catch (e) {
+      // Ignore invalid dates
+    }
+  }
+  return null;
+}
+
+/**
+ * Post-processing: Enrich Response
+ * @param {string} name 
+ * @param {any} result 
+ * @returns {any} Enriched result
+ */
+function enrichResponse(name, result) {
+  let content = result;
+  if (typeof result === 'object' && result !== null && result.content) {
+      content = result.content;
+  }
+
+  if (name === "book-hotel" && typeof content === 'string' && !content.includes("Error")) {
+      const loyaltyBonus = 500;
+      const enrichedContent = `Booking Confirmed!\n You earned ${loyaltyBonus} Loyalty Points with this stay.\n\nSystem Details: ${content}`;
+      
+      if (typeof result === 'object' && result !== null) {
+          result.content = enrichedContent;
+          return result;
+      }
+      return enrichedContent;
+  }
+  return result;
+}
+
+/**
+ * Wraps a tool to add pre- and post-processing logic.
+ * @param {import("@langchain/core/tools").StructuredTool} toolInstance
+ */
+function wrapToolWithBusinessLogic(toolInstance) {
+  const originalInvoke = toolInstance.invoke.bind(toolInstance);
+
+  toolInstance.invoke = async (input, config) => {
+    // 1. Pre-processing
+    const validationError = checkBusinessRules(toolInstance.name, input);
+    if (validationError) {
+      return validationError;
+    }
+
+    // 2. Execution
+    const result = await originalInvoke(input, config);
+
+    // 3. Post-processing
+    return enrichResponse(toolInstance.name, result);
+  };
+
+  return toolInstance;
+}
+
+/**
+ * Helper to run a single turn.
+ */
+async function runTurn(agentExecutor, input) {
+  console.log(`\nUSER: '${input}'`);
+  const result = await agentExecutor.invoke({ input });
+  console.log("-" .repeat(50));
+  console.log("Final Client Response:");
+  console.log(`AI: ${result.output}`);
+}
+
+async function main() {
+  const client = new ToolboxClient("http://127.0.0.1:5000");
+  
+  try {
+    const rawTools = await client.loadToolset("my-toolset");
+    
+    // Convert & Wrap tools
+    const tools = rawTools
+      .map(t => tool(t, {
+          name: t.getName(),
+          description: t.getDescription(),
+          schema: t.getParamSchema()
+      }))
+      .map(wrapToolWithBusinessLogic);
+
+    const model = new ChatVertexAI({
+      model: "gemini-2.5-flash",
+      temperature: 0,
+    });
+
+    const prompt = ChatPromptTemplate.fromMessages([
+      ["system", systemPrompt],
+      ["placeholder", "{chat_history}"],
+      ["human", "{input}"],
+      ["placeholder", "{agent_scratchpad}"],
+    ]);
+
+    const agent = createToolCallingAgent({ llm: model, tools, prompt });
+    const agentExecutor = new AgentExecutor({ agent, tools, verbose: false });
+
+    // Turn 1: Booking
+    await runTurn(agentExecutor, "Book hotel with id 3.");
+    
+    // Turn 2: Policy Violation
+    await runTurn(agentExecutor, "Update my hotel with id 3 with checkin date 2025-01-18 and checkout date 2025-02-10");
+
+  } catch (error) {
+    console.error("Error running agent:", error);
+  }
+}
+
+if (process.argv[1] === fileURLToPath(import.meta.url)) {
+  main();
+}

docs/en/samples/pre_post_processing/js/langchain/package.json

@@ -0,0 +1,19 @@
+{
+  "name": "langchain",
+  "version": "1.0.0",
+  "description": "LangChain.js sample for pre/post processing",
+  "type": "module",
+  "main": "agent.js",
+  "scripts": {
+    "start": "node agent.js"
+  },
+  "dependencies": {
+    "@langchain/core": "^0.3.0",
+    "@langchain/google-vertexai": "^0.1.0",
+    "@toolbox-sdk/core": "^0.2.1",
+    "langchain": "^0.3.0",
+    "zod": "^3.23.8"
+  },
+  "author": "",
+  "license": "ISC"
+}