Why doesn't null trigger the default?

By design, defaults only activate for undefined (missing argument). null is considered an intentional value meaning 'explicitly nothing.' This matches how destructuring defaults work too.

Should defaults go at the end?

Usually yes — you can't skip a parameter. If you need to skip middle params, pass undefined explicitly: fn(1, undefined, 3). For many optional values, use an options object pattern instead.

Do defaults work with arrow functions?

Yes, identically: const greet = (name = 'World') => `Hello, ${name}!`; All function types support default parameters.

Performance impact of defaults?

Negligible. Default expressions are only evaluated when needed (when the argument is undefined). They're not pre-computed. Use them freely.

JavaScript Default Parameters

Fallback values for missing arguments. No more undefined checks.

Module 9 · Modern JavaScript (ES6+)

ES2015 and later additions transformed how JavaScript is written. let and const, template literals, modules, spread/rest, optional chaining, and nullish coalescing are now standard in every modern codebase.

Learn Default Parameters in JavaScript — covering Basics, Expressions as Defaults, Gotchas, Patterns with interactive visual examples and step-by-step explanations.

About this module

ES2015 (ES6) and the releases that followed added the syntax modern JavaScript is written in today. let and const replaced var, template literals replaced string concatenation, and import/export replaced global script tags.

This module covers the ES6+ features you will see in every codebase and job interview. Each chapter shows the old pattern alongside the modern equivalent so you can read legacy code and write idiomatic JavaScript.

Why this matters

Understanding default parameters is essential for every JavaScript developer. It shows up in frontend UI code, backend APIs, and framework internals — and getting it wrong leads to bugs that are hard to trace. This chapter builds intuition with visuals so the behavior sticks.

Key topics covered

Basics
Expressions as Defaults
Gotchas
Patterns

Where you'll use it

Building interactive web applications with default parameters
Processing and transforming data in frontend and backend code
Writing cleaner, more maintainable JavaScript modules
Debugging and reviewing code during development

Basics

Assign default values directly in the parameter list.

// Old way — manual checks:
function greet(name) {
  name = name || "World"; // ⚠️ buggy with falsy values
  return "Hello, " + name + "!";
}

// ES6 — default parameters:
function greet(name = "World") {
  return `Hello, ${name}!`;
}

greet("Alice"); // "Hello, Alice!"
greet();        // "Hello, World!"
greet(undefined); // "Hello, World!" (undefined triggers default)

// Multiple defaults:
function createUser(name = "Anonymous", role = "viewer", active = true) {
  return { name, role, active };
}

createUser();              // { name: "Anonymous", role: "viewer", active: true }
createUser("Alice");       // { name: "Alice", role: "viewer", active: true }
createUser("Bob", "admin"); // { name: "Bob", role: "admin", active: true }

Expressions as Defaults

Defaults can be any expression — they're evaluated at call time.

// Function calls as defaults:
function getTimestamp() {
  return Date.now();
}

function log(message, time = getTimestamp()) {
  console.log(`[${time}] ${message}`);
}

log("start"); // [1717593600000] start
log("end");   // [1717593601000] end  ← different time!

// Default is evaluated fresh each call, not once at definition

Default expressions are lazily evaluated — they run every time the function is called without that argument.

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Gotchas

Edge cases to watch for.

// null does NOT trigger default (only undefined does):
function test(x = 42) { return x; }
test(undefined); // 42 (default used)
test(null);      // null (NOT the default!)
test(0);         // 0 (NOT the default!)
test("");        // "" (NOT the default!)
test(false);     // false (NOT the default!)

// Only missing or explicitly undefined → default

// arguments object doesn't reflect defaults:
function show(a = 1, b = 2) {
  console.log(arguments.length); // actual args passed
}
show();     // arguments.length = 0 (not 2!)
show(10);   // arguments.length = 1

// Default params create their own scope (rare edge case):
let x = 1;
function f(a = x, b = () => a) {
  let x = 2;  // doesn't affect default 'a'
  return b(); // returns the parameter 'a', not local 'x'
}
f(); // 1

Patterns

Useful patterns with default parameters.

// 1. Required parameter (throw on missing):
function required(name) {
  throw new Error(`Missing required param: ${name}`);
}

function createUser(name = required("name"), email = required("email")) {
  return { name, email };
}
createUser(); // Error: Missing required param: name

// 2. Options object with defaults:
function fetchData(url, {
  method = "GET",
  headers = {},
  timeout = 5000,
  retry = 3,
} = {}) {
  // All options have defaults, and the whole object defaults to {}
  return { url, method, headers, timeout, retry };
}
fetchData("/api/users"); // all defaults applied
fetchData("/api", { method: "POST", timeout: 10000 });

// 3. Factory with configurable defaults:
function createLogger(prefix = "APP", level = "info") {
  return (message) => console.log(`[${prefix}:${level}] ${message}`);
}
const log = createLogger("AUTH", "debug");
log("User logged in"); // [AUTH:debug] User logged in

FAQ

Common questions about default parameters.

// Old way — manual checks: function greet(name) { name = name || "World"; // ⚠️ buggy with falsy values return "Hello, " + name + "!"; } // ES6 — default parameters: function greet(name = "World") { return `Hello, ${name}!`; } greet("Alice"); // "Hello, Alice!" greet(); // "Hello, World!" greet(undefined); // "Hello, World!" (undefined triggers default) // Multiple defaults: function createUser(name = "Anonymous", role = "viewer", active = true) { return { name, role, active }; } createUser(); // { name: "Anonymous", role: "viewer", active: true } createUser("Alice"); // { name: "Alice", role: "viewer", active: true } createUser("Bob", "admin"); // { name: "Bob", role: "admin", active: true }

// Function calls as defaults: function getTimestamp() { return Date.now(); } function log(message, time = getTimestamp()) { console.log(`[${time}] ${message}`); } log("start"); // [1717593600000] start log("end"); // [1717593601000] end ← different time! // Default is evaluated fresh each call, not once at definition

// null does NOT trigger default (only undefined does): function test(x = 42) { return x; } test(undefined); // 42 (default used) test(null); // null (NOT the default!) test(0); // 0 (NOT the default!) test(""); // "" (NOT the default!) test(false); // false (NOT the default!) // Only missing or explicitly undefined → default // arguments object doesn't reflect defaults: function show(a = 1, b = 2) { console.log(arguments.length); // actual args passed } show(); // arguments.length = 0 (not 2!) show(10); // arguments.length = 1 // Default params create their own scope (rare edge case): let x = 1; function f(a = x, b = () => a) { let x = 2; // doesn't affect default 'a' return b(); // returns the parameter 'a', not local 'x' } f(); // 1

// 1. Required parameter (throw on missing): function required(name) { throw new Error(`Missing required param: ${name}`); } function createUser(name = required("name"), email = required("email")) { return { name, email }; } createUser(); // Error: Missing required param: name // 2. Options object with defaults: function fetchData(url, { method = "GET", headers = {}, timeout = 5000, retry = 3, } = {}) { // All options have defaults, and the whole object defaults to {} return { url, method, headers, timeout, retry }; } fetchData("/api/users"); // all defaults applied fetchData("/api", { method: "POST", timeout: 10000 }); // 3. Factory with configurable defaults: function createLogger(prefix = "APP", level = "info") { return (message) => console.log(`[${prefix}:${level}] ${message}`); } const log = createLogger("AUTH", "debug"); log("User logged in"); // [AUTH:debug] User logged in

About this chapter

About this module

Why this matters

Key topics covered

Where you'll use it

Basics

Expressions as Defaults

Gotchas

Patterns

FAQ

About this chapter

About this module

Why this matters

Key topics covered

Where you'll use it

Basics

Expressions as Defaults

Gotchas

Patterns

FAQ