You are currently viewing JavaScript Cheat Sheet 10.8: JavaScript Const

JavaScript Cheat Sheet 10.8: JavaScript Const

JavaScript Const : A Comprehensive Guide for Beginners

Welcome to the world of programming! If you’re new to the exciting realm of computer science and software development, you’ve come to the right place. In this tutorial, we’ll be exploring the basics of programming in JavaScript, with a specific focus on the const keyword. By the end of this tutorial, you’ll have a solid understanding of what const does, how to use it, and why it’s an essential part of JavaScript.

1. Introduction

Why const is Important

In JavaScript, variables are essential for storing and managing data. The const keyword plays a crucial role in controlling how data is stored and manipulated. It is used for declaring variables whose values should remain constant, meaning they cannot be reassigned after their initial assignment.

This tutorial will take you through the basics of JavaScript and then delve into the specifics of using the const keyword effectively.

2. JavaScript Basics

Before we dive into const, it’s important to understand some fundamental concepts of JavaScript:


Variables are containers that store data values. In JavaScript, you can declare variables using the var, let, or const keywords.

Data Types

JavaScript supports various data types, including numbers, strings, booleans, objects, arrays, and more. The data type of a variable determines what kind of data it can hold.


Operators are symbols used for performing operations on variables and values. JavaScript supports various operators such as arithmetic, comparison, and logical operators.

3. Understanding const

Declaration and Initialization

The const keyword is used to declare a constant variable. When you declare a variable with const, you must initialize it with a value. Once assigned, the value of a const variable cannot be changed.

const pi = 3.14159;

Reassigning Values

Attempting to reassign a const variable will result in an error. This is one of the key characteristics of const.

const age = 25;
age = 26; // This will throw an error

Scope and Hoisting

const variables are block-scoped, which means they are only accessible within the block where they are defined. This is different from var, which has function scope.

function example() {
  if (true) {
    const x = 10;
    console.log(x); // 10
  console.log(x); // ReferenceError: x is not defined

4. Sample Codes and Outputs

Let’s explore some code examples to see const in action.

Code 1: Basic const Declaration

const temperature = 25;
console.log(temperature); // 25

In this code, we declare a const variable temperature and initialize it with the value 25. When we log temperature, it correctly outputs 25.

Code 2: Reassigning const Value

const score = 100;
score = 110; // This will throw an error

Here, we attempt to reassign the value of score, which is declared as a const. As expected, it throws an error, indicating that you cannot reassign const variables.

Code 3: const in Function Scope

function example() {
  if (true) {
    const localVar = "I am a local variable";
    console.log(localVar); // "I am a local variable"
  console.log(localVar); // ReferenceError: localVar is not defined

In this code, we see how const variables are block-scoped. localVar is only accessible within the if block where it is defined.

Code 4: const in Block Scope

if (true) {
  const blockVar = "I am in a block";
  console.log(blockVar); // "I am in a block"

console.log(blockVar); // ReferenceError: blockVar is not defined

In this example, blockVar is defined within a block (inside the if statement), and it is not accessible outside of that block.


In this tutorial, we’ve explored the const keyword in JavaScript and its role in declaring constant variables. Understanding the fundamental concepts of variables, data types, and operators in JavaScript is crucial to mastering this powerful language.

To summarize, const is used to create variables with values that cannot be changed after their initial assignment. It is block-scoped, making it a safe choice for ensuring data integrity in your code.

When working with JavaScript, it’s important to choose the right variable declaration keyword (var, let, or const) based on your needs. const should be your choice when you want to create unchangeable, constant values in your code.

By mastering the const keyword and other JavaScript fundamentals, you’ll be well on your way to becoming a proficient programmer.

Best Practices

  • Use const when you have values that should not change.
  • Use let for variables that may change over time.
  • Use var sparingly, as it has broader scope and can lead to unintended consequences.
  • Always initialize const variables when you declare them.
  • Keep your code organized and use meaningful variable names to improve readability.

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