Auto-Reply Gmail Emails Using Google Apps Script (JavaScript)

Auto-Reply Gmail Emails Using Google Apps Script (JavaScript)

Automating email responses is a game-changer for productivity. Whether you’re managing customer support, internal communication, or just want cleaner inbox management, Google Apps Script (GAS) offers a powerful and accessible way to build smart auto-replies directly into your Gmail account using JavaScript.

In this blog post, we’ll walk through building an auto-reply system in Gmail using Google Apps Script. We’ll detect specific keywords in incoming emails and send contextual replies automatically. You’ll learn how to set up triggers, filter messages, send dynamic replies, and manage performance efficiently.

1. Understanding Google Apps Script and Gmail Integration

Google Apps Script is a JavaScript-based language built into Google Workspace. It allows you to automate tasks across Gmail, Sheets, Drive, etc. For this project, we’ll use the Gmail service in GAS to access messages, analyze their content, and reply where relevant.

Start by opening https://script.google.com and creating a new project:

// File: Code.gs
function autoReplyToEmails() {
  var threads = GmailApp.getInboxThreads(0, 20); // Check last 20 threads
  for (var i = 0; i < threads.length; i++) {
    var messages = threads[i].getMessages();
    var lastMsg = messages[messages.length - 1];
    if (!lastMsg.isReplied()) {
      var body = lastMsg.getPlainBody().toLowerCase();
      if (body.includes("pricing") || body.includes("quote")) {
        lastMsg.reply("Hi there! Here is our pricing guide: [link]. Let us know if you need anything else.");
      }
    }
  }
}

This function checks the latest threads, looks for keywords, and replies when appropriate.

2. Setting Up Time-Driven Triggers

We don’t want to run this manually every time. We'll set up a time-driven trigger so the script runs periodically (e.g., every 10 minutes).

// Run this once to set the trigger
function createTimeDrivenTrigger() {
  ScriptApp.newTrigger("autoReplyToEmails")
    .timeBased()
    .everyMinutes(10)
    .create();
}

Save the script and run createTimeDrivenTrigger() once from the Apps Script editor. This will automatically invoke autoReplyToEmails() every 10 minutes.

3. Filtering Emails with More Context and Accuracy

To minimize false positives, let’s only target messages from external senders and older than 2 minutes (to avoid race conditions).

function autoReplyToEmails() {
  var threads = GmailApp.search("is:inbox -from:me newer_than:1d");
  for (var i = 0; i < threads.length; i++) {
    var messages = threads[i].getMessages();
    var lastMsg = messages[messages.length - 1];
    if (!lastMsg.isReplied()) {
      var from = lastMsg.getFrom();
      if (!from.includes("@yourdomain.com")) {
        var body = lastMsg.getPlainBody().toLowerCase();

        if (body.includes("support") || body.includes("help")) {
          lastMsg.reply("Thanks for reaching out! Our support team will respond shortly.");
        } else if (body.includes("pricing") || body.includes("quote")) {
          lastMsg.reply("You can view our current pricing here: https://example.com/pricing.");
        }
      }
    }
  }
}

We added Gmail Search operators and improved the filter logic. This update ensures better targeting and avoids replying to your own messages.

4. Customizing Responses Dynamically

Let’s personalize our replies using name extraction from the email’s “From” field. This improves engagement and feels more human.

function extractName(fromField) {
  var match = fromField.match(/^(.*?)\s*

function getOrCreateLabel(name) {
  var label = GmailApp.getUserLabelByName(name);
  return label || GmailApp.createLabel(name);
}

function autoReplyToEmails() {
  var label = getOrCreateLabel("Auto Replied");
  var threads = GmailApp.search("is:inbox -from:me -label:auto-replied newer_than:1d");

  for (var i = 0; i < threads.length; i++) {
    var messages = threads[i].getMessages();
    var lastMsg = messages[messages.length - 1];
    var body = lastMsg.getPlainBody().toLowerCase();
    var from = lastMsg.getFrom();
    var name = extractName(from);
    
    var reply = null;
    if (body.includes("demo")) {
      reply = `Hi ${name},\n\nYou can schedule a demo with us here: https://example.com/demo.`;
    }
    if (reply) {
      lastMsg.reply(reply);
      threads[i].addLabel(label);
    }
  }
}