ssjs.guide SFMC

Debugging

Practical techniques for debugging SSJS code — from basic Write() tracing to structured error logging and silent error detection.

Debugging SSJS is challenging: there is no browser DevTools, no console, and SFMC’s native error messages are often unhelpful. This page covers proven techniques for finding and fixing SSJS issues.

1. Write() Debugging

The simplest approach — output values directly to the page:

var value = Platform.Function.Lookup("Config", "apiKey", "key", "main");
Write("DEBUG apiKey: " + value + "<br>");

Structured Debug Output

function debug(label, value) {
    Write("<pre style='background:#111;color:#0f0;padding:8px;font-size:12px'>"
        + label + ": " + Stringify(value)
        + "</pre>");
}

debug("requestMethod", Platform.Request.Method);
debug("queryParam id", Platform.Request.GetQueryStringParameter("id"));
debug("lookupResult", Platform.Function.LookupRows("Orders", "Status", "pending"));

Conditional Debug Mode

var DEBUG = Platform.Request.GetQueryStringParameter("debug") === "1";

function debugWrite(msg, data) {
    if (DEBUG) {
        Write("<pre>" + msg + ": " + Stringify(data) + "</pre>");
    }
}

// Access: /page?debug=1
debugWrite("payload", requestBody);

2. Error Page Analysis

When SFMC shows a generic error page ("Sorry, we encountered a problem"), the error is logged in SFMC’s native logs. To read them:

  • Email Studio → CloudPages → (select page) → Activity — shows page execution errors
  • Automation Studio → Activity — shows script activity errors in automations

However, these logs are often truncated. Prefer logging to a DE (see Error Logging).

3. Testing Execution Contexts

Different execution contexts have different constraints. Test in the right context:

Context URL Pattern Notes
CloudPage pub.s10.exacttarget.com/... Full Platform.Request access
JSON Resource pub.s10.exacttarget.com/...-json Set Content-Type application/json
Automation No URL Use Email Studio Activity logs
Email Preview in SFMC Limited subset of functions

4. ParseJSON Error Diagnosis

The most common SSJS error is TypeError: Cannot read property 'X' of undefined on a ParseJSON result. The root cause is usually a null or undefined HTTP response.

var rawBody = Platform.Request.GetPostData();
Write("RAW BODY: [" + rawBody + "]<br>"); // is it empty?

var parsed = Platform.Function.ParseJSON(rawBody + ""); // + "" prevents 500
Write("PARSED TYPE: " + typeof parsed + "<br>");
Write("PARSED VALUE: " + Stringify(parsed) + "<br>");

5. Try/Catch with Write

Wrap suspicious code in try/catch and output the error:

try {
    var result = someRiskyOperation();
    Write("Success: " + Stringify(result));
} catch(e) {
    Write("<pre style='color:red'>");
    Write("Error: " + e.message + "\n");
    Write("Stack: " + e.stack + "\n");
    Write("</pre>");
}

6. Diagnosing Blank Pages

A blank CloudPage with no output is almost always a silent runtime error. Common causes:

  • Platform.Load not called before Core library use
  • ParseJSON called on null/undefined without + ""
  • switch statement with default not executing (see Known Bugs)
  • DataExtension.Rows.Retrieve() called without filter on CloudPage

Diagnosis pattern:

Write("1: Script started<br>");
Platform.Load("core", "1.1.5");
Write("2: Core loaded<br>");

var de = DataExtension.Init("MyDE");
Write("3: DE initialized<br>");

var rows = de.Rows.Retrieve({
    Property: "Active",
    SimpleOperator: "equals",
    Value: "true"
});
Write("4: Retrieved " + rows.length + " rows<br>");

7. Automation Studio Debugging

Since automations have no URL, add a DE-based log:

function logStep(step, message) {
    try {
        Platform.Function.InsertData(
            "AutomationLog",
            ["RunId", "Step", "Message", "Timestamp"],
            [runId, step, message, Platform.Function.Now()]
        );
    } catch(e) {}
}

logStep("start", "Automation script started");
// ... code ...
logStep("de_init", "DE rows: " + rows.length);
// ... code ...
logStep("complete", "Finished successfully");

8. Email Context Debugging

In email sends, use AMPscript variable output for debugging — SSJS Write() still works:

Write("<!-- DEBUG: email=" + emailAddr + " subKey=" + subscriberKey + " -->");

Check the “View Email” preview in Content Builder to see the rendered HTML including debug comments.

9. Silent Error Patterns

Some SSJS operations fail silently (no error, no output). Always validate return values:

// InsertData returns the new row count — check it
var inserted = Platform.Function.InsertData("Log",
    ["Event", "Timestamp"],
    ["pageview", Platform.Function.Now()]
);
if (inserted === 0) {
    Write("Warning: InsertData returned 0 rows inserted");
}

// Lookup returns "" on no match — not null
var val = Platform.Function.Lookup("DE", "Field", "Key", "value");
if (val === "") {
    Write("No record found");
}

See Also

See Also