What is the difference between a function expression and a function declaration in JavaScript?

Table of Contents

• Introduction
• Key Differences
  • 1. Syntax
  • 2. Hoisting
  • 3. Usage
• Conclusion

Introduction

In JavaScript, functions can be defined in two main ways: as function declarations and function expressions. Both allow you to create reusable blocks of code, but they differ in how they are declared, how they behave with hoisting, and where they are typically used.

Key Differences

1. Syntax

• Function Declaration: A function declaration is a straightforward way to define a named function in JavaScript. It starts with the function keyword followed by the function name.

• Function Expression: A function expression involves defining a function as part of an expression, such as assigning it to a variable. Function expressions can be anonymous (without a name) or named.

2. Hoisting

• Function Declaration: Function declarations are hoisted to the top of their scope. This means you can call the function before its actual definition in the code.

• Function Expression: Function expressions are not hoisted in the same way. The variable used to hold the function is hoisted, but the function itself is not initialized until the code reaches that line. If you try to call the function before it is defined, you'll get an error.

3. Usage

• Function Declaration: Function declarations are often used when you need a named function that can be called anywhere in the scope, thanks to hoisting.
• Function Expression: Function expressions are useful when you need a function that is defined dynamically, such as passing functions as arguments, using them in callbacks, or when you want to create anonymous functions.

Conclusion

The key differences between function expressions and function declarations in JavaScript revolve around syntax, hoisting, and usage. Function declarations are hoisted and can be called before they are defined, while function expressions are not fully hoisted and must be defined before calling. Understanding these differences will help you use the right function type based on your needs.