JavaScript Function Declaration vs Function Expression: Differences, Pros, and Cons

When maintaining or writing JavaScript code, you may notice two common ways to define functions: function declarations and function expressions. Understanding the differences, benefits, and limitations of each is essential for writing clean, maintainable code. This guide explains the distinctions and when to use each method.

JavaScript Function Declaration vs Function Expression Differences Pros and Cons

coldshadow44 on 2025-10-13

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2025-10-14

In JavaScript, there are two main ways to define functions:

Function Expression

var functionOne = function() {

// Some code

};

A function expression is defined only when the code execution reaches that line.
You cannot call the function before it is defined; doing so will result in a TypeError.

// TypeError: functionOne is not a function

functionOne();

var functionOne = function() {

console.log("Hello!");

};

Function Declaration

function functionTwo() {

// Some code

}

A function declaration is hoisted, meaning it is defined as soon as its surrounding script or function runs.
You can safely call it before the function appears in the code.

// Outputs: "Hello!"

functionTwo();

function functionTwo() {

console.log("Hello!");

}

Example: Function Declaration in a Block (Strict Mode)

'use strict';

{

function functionThree() {

console.log("Hello!");

}

functionThree(); // ReferenceError: functionThree is not defined

Summary

Use function declarations when you want hoisting and readability.
Use function expressions when you want to define functions conditionally, assign them to variables, or control when they become available.
Both are powerful, but understanding hoisting and scope differences helps avoid subtle bugs.

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