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JavaScript Default Parameters

Sensible Values When the Caller Forgets

JavaScript Default Parameters

Give a parameter a default value with `=`. If the caller doesn't pass that argument, the default kicks in.

3 min read Level 1/5 #default-params#parameters#functions
What you'll learn
  • Set default parameter values
  • Know that defaults trigger on `undefined`, not on other falsy values
  • Use earlier parameters in later defaults

Default parameters give a parameter a fallback value, used when the caller doesn’t pass anything (or passes undefined).

Default parameter script.js 
function greet(name = "friend") {
  console.log(`Hello, ${name}!`);
}

greet("Ada");      // "Hello, Ada!"
greet();           // "Hello, friend!"
greet(undefined);  // "Hello, friend!"
▶ Preview: console

null Doesn’t Trigger Defaults

Defaults kick in only on undefined (or “no argument”). Passing null keeps null.

null vs undefined script.js 
function format(value = "no value") {
  console.log(value);
}

format();           // "no value"
format(undefined);  // "no value"
format(null);       // null         ← null is a real value
format(0);          // 0            ← so is 0
format("");         // ""           ← and ""
▶ Preview: console

Earlier Parameters in Later Defaults

Default expressions can reference parameters declared earlier in the list.

Cascading defaults script.js 
function rectangle(width, height = width) {
  return width * height;
}

console.log(rectangle(5, 10)); // 50
console.log(rectangle(5));     // 25  ← square, height defaults to width
▶ Preview: console

Defaults Can Be Expressions

The default isn’t limited to literals — it can be any expression.

Expression defaults script.js 
let nextId = 1;

function makeUser(name, id = nextId++) {
  return { id, name };
}

console.log(makeUser("Ada"));  // { id: 1, name: 'Ada' }
console.log(makeUser("Lin"));  // { id: 2, name: 'Lin' }
console.log(makeUser("Tim", 99)); // { id: 99, name: 'Tim' }  ← default skipped
▶ Preview: console

The default is evaluated only when needed, not at function definition time.

When NOT To Use Defaults

If “no value” should be an error (a required parameter), don’t silently substitute a default — throw an explicit error so the bug shows up loudly:

A 'required' helper script.js 
function required(name) {
  throw new Error(`Missing required parameter: ${name}`);
}

function fetchUser(id = required("id")) {
  console.log("fetching user", id);
}

fetchUser(42);
// "fetching user 42"

// fetchUser();
// Error: Missing required parameter: id
▶ Preview: console

Try It Yourself

Exercise

A greeting with a default

Difficulty 1/5~2 min
Write a function `greet(name)` where `name` defaults to `"friend"`. It should return the string `` `Hello, ${name}!` ``. Show that it works both with and without an argument by calling `greet("Ada")` and `greet()` and logging both.
solution.js
// declare greet here

console.log(greet("Ada"));
console.log(greet());
4tests will run
💡 Show hint
Default parameter syntax: `function greet(name = "friend") { … }`.
✅ Show solution
function greet(name = "friend") {
  return `Hello, ${name}!`;
}

console.log(greet("Ada"));
console.log(greet());

Up Next

What about variable-length argument lists? Rest and spread.

JavaScript Rest & Spread →