Provide Fallbacks
Default parameters set a value when an argument is undefined.
They reduce boilerplate checks inside your function.
JavaScript Tutorial
JavaScript Default Parameters
Default parameters let you assign fallback values when arguments are missing.
They simplify functions and reduce manual checks inside the body.
Why We Need It
Functions often accept optional inputs. Defaults make those cases clean and predictable.
They also make function signatures more self-documenting.
Syntax
function name(param = defaultValue) { ... }Basic Example
1. Basic default
function greet(name = "Guest") {
return "Hello " + name;
}
console.log(greet());Missing arguments use the default value.
Real World Example
2. Number default
function addTax(price, rate = 0.18) {
return price + price * rate;
}
console.log(addTax(100));Defaults work with any type.
Multiple Use Cases
Safe Configuration
Defaults are helpful for configuration objects and optional inputs.
They keep function signatures clean and easy to read.
Evaluation Rules
Default values are evaluated at call time, not at function creation time.
You can even use earlier parameters to compute defaults.
More Examples
3. Computed default
function makeId(prefix, id = prefix + "-" + Date.now()) {
return id;
}
console.log(makeId("user"));Defaults can depend on earlier parameters.
4. Undefined only
function label(status = "pending") {
return status;
}
console.log(label(null)); // null, not default
console.log(label(undefined)); // uses defaultDefaults only apply when the argument is undefined.
Comparison
Without
function greet(name) {
if (name === undefined) {
name = "Guest";
}
return "Hello " + name;
}With
function greet(name = "Guest") {
return "Hello " + name;
}Common Mistakes and Fixes
Assuming null uses default
Defaults apply only to undefined, not null.
Overusing defaults
Use defaults for optional values, not required inputs.
Complex defaults
Keep defaults simple or move logic into the function body.
Interview Questions
When do default parameters apply?
When the argument is undefined.
Do defaults apply to null?
No, null is treated as a real value.
Can defaults be expressions?
Yes, they are evaluated at call time.
Practice Problem
Practice: Create a function that formats currency with a default symbol.
// TODO: function format(amount, symbol = "$")
One Possible Solution
function format(amount, symbol = "$") {
return symbol + amount.toFixed(2);
}
console.log(format(10));Frequently Asked Questions
When do defaults apply?
Only when the argument is undefined.
Can defaults reference other params?
Yes, as long as the referenced param appears earlier.
Do defaults work with destructuring?
Yes, you can provide defaults in destructured params.
Try It Yourself
Try calling the function with and without arguments.