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Default Parameters

Default Parameters

Introduction

Default parameters allow you to specify default values for function parameters. If an argument is undefined, the default value is used.

Syntax

function greet(name = 'Guest') {
  return `Hello, ${name}!`;
}

Example

function multiply(a, b = 1) {
  return a * b;
}

console.log(multiply(5, 2)); // 10
console.log(multiply(5));    // 5 (b defaults to 1)

Defaults Can Be Expressions

function createElement(type = 'div', content = getDefaultContent()) {
  // ...
}

function getDefaultContent() {
  return 'Default content';
}

Important Notes

// Defaults only apply for undefined, NOT for null
function greet(name = 'Guest') {
  return `Hello, ${name}!`;
}

greet(undefined); // 'Hello, Guest!'
greet(null);      // 'Hello, null!'

Summary

Default parameters use = in function signatures
Only trigger for undefined arguments
Can be expressions or function calls
Make functions more flexible and safer

Further Reading

MDN: Default Parameters