Add Unsubscribe link in emails using Google Apps Script

Provide your subscribers with an option to opt-out of mailing lists by adding unsubscribe link using Google Apps Script.

This blog was originally published on RavSam’s blog. We publish our articles on Dev after a week.

When setting up our email marketing campaigns or newsletters, one thing that is often forgot is the Unsubscribe link. Not providing an option to unsubscribe from the mailing list can land our emails into spam. In this blog, we will look at how we can add an Unsubscribe link in our emails sent using Google Apps Script.

1. Setting up a Spreadsheet
2. Writing a Hash Function
3. Writing Email Template
4. Writing Unsubscribe Code
Results

1. Setting up a Spreadsheet

The first task is to set up a Spreadsheet.

Create a new Google Spreadsheet and name the sheet as emails.
Add the following fields in the top row of our spreadsheet.

2. Writing a Hash Function

To provide a secure way to unsubscribe, we need to create a unique token for each of our subscribers. Google Apps Script provides us with utility functions to create a hash of a string using the MD5 hashing algorithm. The following function is used to create a hash of the string provided as a parameter.

function getMD5Hash(value) {
  const digest = Utilities.computeDigest(Utilities.DigestAlgorithm.MD5, value, Utilities.Charset.UTF_8);

  let hash = '';

  for (i = 0; i < digest.length; i++) {
    let byte = digest[i];
    if (byte < 0) byte += 256;
    let bStr = byte.toString(16);
    if (bStr.length == 1) bStr = '0' + bStr;
    hash += bStr;
  }

  return hash;
}

Since no two strings in the world have the same hash, this is the right way to provide unsubscribe tokens to our subscribers in our marketing campaigns or newsletters. However, there is a security problem here. If anyone knows the email of our subscriber, he can easily compute the hash and unsubscribe the subscriber from our email list. So, to make the hash impossible to guess, we can add some randomness to our email string. We can create a random string and append it to our original email string. The following snippet of code will help us to achieve our purpose.

function getMD5Hash(value) {
  value = value + generateRandomString(8); // added this

  const digest = Utilities.computeDigest(Utilities.DigestAlgorithm.MD5, value, Utilities.Charset.UTF_8);

  let hash = '';

  for (i = 0; i < digest.length; i++) {
    let byte = digest[i];
    if (byte < 0) byte += 256;
    let bStr = byte.toString(16);
    if (bStr.length == 1) bStr = '0' + bStr;
    hash += bStr;
  }

  return hash;
}

function generateRandomString(length) {
  const randomNumber = Math.pow(36, length + 1) - Math.random() * Math.pow(36, length);
  const string = Math.round(randomNumber).toString(36).slice(1);
  return string;
}

Google Spreadsheet with Unsubscribe Hashes

3. Writing Email Template

Let us create a basic HTML template for testing our Google Apps Script for the unsubscribing feature. Our email template contains the link for unsubscribing. We have also provided two parameters, email and unsubscribe_hash. When the subscriber will tap this link, it will send a GET request to our Google Apps Script deployed as a Web App.

<!DOCTYPE html>
<html>
  <head>
    <base target="_top">
  </head>
  <body>
    <h1>We are testing our unsubscribe feature</h1>
    <a href="{{WEBAPP_URL}}?email={{EMAIL}}&unsubscribe_hash={{TOKEN}}">Unsubscribe</a>
  </body>
</html>

Make sure to replace the values in curly braces.

4. Writing Unsubscribe Code

The final step to bring our workflow together is to write a code that handles our unsubscribe functionality. In our Main.gs, let us add the following code to handle the GET request as we discussed earlier:

function doGet(e) {
  const email = e.parameter['email'];
  const unsubscribeHash = e.parameter['unsubscribe_hash'];
  const success = unsubscribeUser(email, unsubscribeHash);
  if (success) return ContentService.createTextOutput().append('You have unsubscribed');
  return ContentService.createTextOutput().append('Failed');
}

The above script is pretty self-explanatory. First of all, we retrieve the email and unsubscribe_hash from the query parameters and pass them to our unsubscribeUser function. Based on the output of our function, we return an appropriate response.

Let us write the code for unsubscribeUser:

function unsubscribeUser(emailToUnsubscribe, unsubscribeHash) {  

  // get the active sheet which contains our emails
  let sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('emails');

  // get the data in it
  const data = sheet.getDataRange().getValues();

  // get headers
  const headers = data[0];

  // get the index of each header
  const emailIndex = headers.indexOf('email');

  const unsubscribeHashIndex = headers.indexOf('unsubscribe_hash');

  const subscribedIndex = headers.indexOf('subscribed');

  // iterate through the data, starting at index 1
  for (let i = 1; i < data.length; i++) {
    const row = data[i];
    const email = row[emailIndex];
    const hash = row[unsubscribeHashIndex];

    // if the email and unsubscribe hash match with the values in the sheet
    // then update the subscribed value to 'no'
    if (emailToUnsubscribe === email && unsubscribeHash === hash) {
      sheet.getRange(i+1, subscribedIndex+1).setValue('no');
      return true;
    }
  }
}

In the above function, we simply iterate our Google Sheet and check for the details for every subscriber. If the subscriber’s email and unsubscribe hash match with those sent as a query parameter, we unsubscribe the subscriber by updated the value in the sheet.

Results

Let us send a test email to our subscriber specified in the Google Sheet.

Email with Unsubscribe link — An email with Unsubscribe link

We can see that we have received our email with an option to Unsubscribe. Let us unsubscribe and check back our sheet.

Google Sheet with updated data about subscriber — Google Sheet with updated data about the subscriber

Oo Yea! We can see that the value for the subscribed field has changed to no. Using this workflow, we can provide our subscribers with an option to opt-out of our mailing list for newsletters or maybe marketing emails.

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Add Unsubscribe link in emails using Google Apps Script

Ravgeet Dhillon

Provide your subscribers with an option to opt-out of mailing lists by adding unsubscribe link using Google Apps Script.

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