Here's a sample Node.js Cloud Function called processData that handles real-time data processing for inventory management. In this example, we assume that the inventory data is stored in a Firestore database and the Cloud Function is triggered by a Cloud Pub/Sub message containing the product ID and the change in quantity.

function.js

const { Firestore } = require('@google-cloud/firestore');
const firestore = new Firestore();

const INVENTORY_COLLECTION = 'inventory';

async function updateInventory(productId, quantityChange) {
  const productRef = firestore.collection(INVENTORY_COLLECTION).doc(productId);

  try {
    await firestore.runTransaction(async (transaction) => {
      const productSnapshot = await transaction.get(productRef);

      if (!productSnapshot.exists) {
        console.error(`Product not found: ${productId}`);
        return;
      }

      const currentQuantity = productSnapshot.data().quantity;
      const newQuantity = currentQuantity + quantityChange;

      if (newQuantity < 0) {
        console.error(`Insufficient stock for product: ${productId}`);
        return;
      }

      transaction.update(productRef, { quantity: newQuantity });
      console.log(`Updated inventory for product ${productId}: ${newQuantity}`);
    });
  } catch (error) {
    console.error(`Error updating inventory: ${error}`);
  }
}

exports.processData = async (message, context) => {
  const messageData = JSON.parse(Buffer.from(message.data, 'base64').toString());
  const productId = messageData.productId;
  const quantityChange = messageData.quantityChange;

  if (!productId || typeof quantityChange !== 'number') {
    console.error('Invalid message data');
    return;
  }

  await updateInventory(productId, quantityChange);
};

package.json

{
  "name": "inventory-management",
  "version": "1.0.0",
  "dependencies": {
    "@google-cloud/firestore": "^4.13.1"
  }
}