Control Flow

if / else

var score = 85;

if (score >= 90) {
    Write("A");
} else if (score >= 80) {
    Write("B");
} else if (score >= 70) {
    Write("C");
} else {
    Write("F");
}

Omitting braces works for single statements but is not recommended:

if (valid) doSomething(); // works but fragile

Truthy / Falsy Checks

var value = Platform.Request.GetQueryStringParameter("id");

// Preferred: !! coercion or direct truthiness check
if (!value) {
    // value is "", null, undefined, 0, or false
    Platform.Response.Redirect("/error");
}

// Explicit empty string check
if (value === "" || value === null) {
    // strictly no value
}

SFMC functions often return "" (empty string) rather than null/undefined when data is missing. Use !value or value === "" accordingly.

Boolean Coercion Patterns

// Convert to boolean explicitly
var hasValue = !!value;
var isActive = !!subscriber.active;

// Short-circuit for defaults
var name = Platform.Request.GetQueryStringParameter("name") || "Subscriber";

// Safe property access (no optional chaining in SSJS)
var city = (person && person.address && person.address.city) || "Unknown";

switch

The switch statement works but has a known SFMC engine bug:

The default case may not execute reliably. If none of the case values match and execution reaches default, the engine may skip it.

var status = "pending";

switch (status) {
    case "active":
        Write("Active");
        break;
    case "inactive":
        Write("Inactive");
        break;
    default:
        // ⚠️ This may NOT execute in SSJS
        Write("Unknown status");
}

Safe pattern: Replace default with an explicit final else:

// Safer approach — use if/else for the fallback
var output = "";
if (status === "active") {
    output = "Active";
} else if (status === "inactive") {
    output = "Inactive";
} else {
    output = "Unknown status"; // Always works
}
Write(output);

Or check explicitly before the switch:

var validStatuses = { active: true, inactive: true };
if (!validStatuses[status]) {
    Write("Unknown status");
} else {
    switch (status) {
        case "active":   Write("Active");   break;
        case "inactive": Write("Inactive"); break;
    }
}

Fall-through

switch fall-through (omitting break) works as in standard JavaScript:

switch (level) {
    case "admin":
    case "superuser":
        // Both "admin" and "superuser" fall through here
        Write("Full access");
        break;
    case "user":
        Write("Limited access");
        break;
}

Ternary

The ternary operator condition ? thenValue : elseValue is fully supported:

var greeting = hour < 12 ? "Good morning" : "Good afternoon";
var label = count === 1 ? "item" : "items";
var cssClass = isActive ? "active" : "inactive";

Avoid deeply nested ternaries — they become unreadable quickly.

Short-Circuit Evaluation

Use && and || for concise conditional execution:

// Execute only if condition is true
isDebug && Write("<pre>" + Stringify(data) + "</pre>");

// Default value
var timeout = config.timeout || 30;

// Safe property access
var name = user && user.profile && user.profile.firstName;

Throw / Early Return

Use throw or early returns to guard against invalid states:

function processSubscriber(sk) {
    if (!sk) {
        throw new Error("SubscriberKey is required");
    }

    var email = Platform.Function.Lookup("Subscribers", "Email", "SubscriberKey", sk);
    if (!email) {
        return null; // early return — nothing to process
    }

    // ... rest of function
    return email;
}

try {
    var result = processSubscriber(sk);
    if (result) {
        Write(result);
    }
} catch (e) {
    Write("Error: " + e.message);
}