Overview

Server-Side JavaScript (SSJS) is a scripting language built into Salesforce Marketing Cloud that allows you to interact with the platform's data and services programmatically. Unlike client-side JavaScript, SSJS runs on the SFMC server — it executes before the page or email is delivered to the subscriber.

SSJS is available in three contexts:

📘 SSJS vs AMPscript

Both can personalise emails and interact with Data Extensions, but SSJS is better suited for complex logic, loops, and HTTP calls, while AMPscript is lighter and executes faster in high-volume email sends. See the comparison section below.

SSJS vs AMPscript

CapabilitySSJSAMPscript
Email personalisation✓✓ (preferred)
CloudPages / landing pages✓ (preferred)✓
HTTP / REST API calls✓Limited
Complex loops & logic✓Limited
Write to Data Extensions✓✓
Execution speed at scaleSlowerFaster
SyntaxJavaScript-likeProprietary tag language

A common pattern is to use AMPscript for email content personalisation and SSJS for CloudPage logic, form handling, and API integrations.

Script blocks

SSJS code lives inside <script> tags with the runat attribute. The value controls where the code executes:

SSJS — basic script block
<script runat="server">
  Platform.Load("Core", "1");

  var firstName = Variable.GetValue("@FirstName");
  Write("Hello, " + firstName + "!");
</script>

The runat values you'll use most:

⚠️ Always load a library

SSJS requires you to explicitly load a library before using its functions. Forgetting Platform.Load() is one of the most common causes of SSJS errors.

Core library

The Core library provides general-purpose utilities — string manipulation, date functions, encoding, and variable access. Load it with:

Load Core library
Platform.Load("Core", "1");

Commonly used Core functions:

Core — common usage
Platform.Load("Core", "1");

// Write output to the page
Write("<p>Hello World</p>");

// Get AMPscript variable value
var firstName = Variable.GetValue("@FirstName");

// Set an AMPscript variable from SSJS
Variable.SetValue("@greeting", "Welcome back, " + firstName);

// String utilities
var upper   = String.Trim("  hello  ");  // "hello"
var encoded = Platform.Function.Base64Encode("text");

// Date utilities
var today = Platform.Function.Now();
var formatted = Platform.Function.Format(today, "yyyy-MM-dd");

Platform library

The Platform library gives you access to SFMC-specific objects — Data Extensions, subscriber records, tracking data, and more.

Load Platform library
Platform.Load("Core", "1");
// Platform objects are available after loading Core

Key Platform objects:

Working with Data Extensions

SSJS can read, write, update, and upsert rows in any Data Extension your running context has access to.

DE — lookup a row
Platform.Load("Core", "1");

var de = DataExtension.Init("CustomerPreferences");
var row = de.Rows.Lookup(["EmailAddress"], [Variable.GetValue("@EmailAddress")]);

if (row.length > 0) {
  var preferredChannel = row[0]["PreferredChannel"];
  Write(preferredChannel);
}
DE — upsert a row
Platform.Load("Core", "1");

var de = DataExtension.Init("FormSubmissions");
var result = de.Rows.Add({
  EmailAddress: Request.GetFormField("email"),
  Name:         Request.GetFormField("name"),
  SubmittedAt:  Platform.Function.Now()
});

if (result === "Created" || result === "Updated") {
  // Success
}
DE — retrieve multiple rows
Platform.Load("Core", "1");

var de = DataExtension.Init("Products");
var rows = de.Rows.Retrieve({
  Property:  "CategoryID",
  SimpleOperator: "equals",
  Value: "42"
});

for (var i = 0; i < rows.length; i++) {
  Write("<li>" + rows[i]["ProductName"] + "</li>");
}

HTTP requests

The HTTPS object lets SSJS call external REST APIs — useful for pulling in live data, posting form results to third-party systems, or triggering webhooks.

HTTP GET request
Platform.Load("Core", "1");

var url = "https://api.example.com/v1/products/42";
var header = ["Authorization", "Bearer YOUR_TOKEN"];
var result = HTTPS.Get(url, [header]);

if (result.StatusCode === 200) {
  var data = Platform.Function.ParseJSON(result.Response[0]);
  Write(data.productName);
}
HTTP POST request (JSON body)
Platform.Load("Core", "1");

var url = "https://api.example.com/v1/submissions";
var headers = [["Content-Type", "application/json"], ["Authorization", "Bearer YOUR_TOKEN"]];
var payload = Platform.Function.Stringify({
  email: Request.GetFormField("email"),
  name:  Request.GetFormField("name")
});

var result = HTTPS.Post(url, "application/json", payload, headers);

if (result.StatusCode === 200 || result.StatusCode === 201) {
  Redirect("https://yoursite.com/thank-you");
}
💡 Store API tokens in Data Extensions

Never hardcode API tokens in SSJS code. Instead, store them in a locked Data Extension (accessible only from SSJS/AMPscript) and retrieve them at runtime using DataExtension.Init().

CloudPages & landing pages

SSJS is the primary scripting language for CloudPages. Common use cases:

CloudPage — form submission handler
<script runat="server" executioncontexttype="post">
Platform.Load("Core", "1");

var email = Request.GetFormField("email");
var name  = Request.GetFormField("name");

if (email && name) {
  var de = DataExtension.Init("LeadCapture");
  de.Rows.Add({
    EmailAddress: email,
    FullName:     name,
    SubmittedAt:  Platform.Function.Now(),
    Source:       "CloudPage-LeadForm"
  });
  Variable.SetValue("@formStatus", "success");
} else {
  Variable.SetValue("@formStatus", "error");
}
</script>

%%[
IF @formStatus == "success" THEN
]%%
  <p>Thanks — we'll be in touch!</p>
%%[
ELSE
]%%
  <p>Please fill in all fields.</p>
%%[
ENDIF
]%%

Debugging

SSJS errors can be opaque. These techniques help narrow down issues:

Debug — try/catch with Write
Platform.Load("Core", "1");

try {
  var de = DataExtension.Init("MyDE");
  var rows = de.Rows.Retrieve();
  Write("Row count: " + rows.length);
} catch (e) {
  Write("Error: " + Stringify(e));
}

Best practices

✅ SSJS checklist

Follow these patterns to keep SSJS reliable and maintainable: