JavascriptHow-ToBeginner · 3 min read

How to Declare Constant in JavaScript: Syntax and Examples

In JavaScript, you declare a constant using the const keyword followed by the constant name and its value. Constants cannot be reassigned after declaration, ensuring their value stays the same throughout the program.

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Syntax

To declare a constant in JavaScript, use the const keyword, then the name of the constant, an equals sign, and the value you want to assign. The value must be assigned immediately and cannot be changed later.

const: keyword to declare a constant
name: the identifier for the constant
=: assignment operator
value: the fixed value assigned to the constant

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const PI = 3.14;

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Example

This example shows how to declare a constant and use it in a calculation. Trying to change the constant value will cause an error.

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const MAX_USERS = 100;
console.log('Maximum users allowed:', MAX_USERS);

// Uncommenting the next line will cause an error
// MAX_USERS = 200;

Output

Maximum users allowed: 100

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Common Pitfalls

One common mistake is trying to reassign a constant after it is declared, which causes an error. Another is declaring a constant without initializing it, which is not allowed.

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/* Wrong: Reassigning a constant */
const SPEED_LIMIT = 60;
// SPEED_LIMIT = 80; // Error: Assignment to constant variable.

/* Wrong: Declaring without initialization */
// const TAX_RATE; // Error: Missing initializer in const declaration

/* Right: Declare and assign immediately */
const TAX_RATE = 0.07;

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Quick Reference

Concept	Description
const	Keyword to declare a constant
Initialization	Must assign a value when declaring
Reassignment	Not allowed after declaration
Scope	Block-scoped, like let
Use case	Values that should not change

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Key Takeaways

Use const to declare constants that cannot be reassigned.

Always assign a value when declaring a constant with const.

Constants are block-scoped, similar to variables declared with let.

Trying to change a constant's value causes a runtime error.

Use constants to protect values that should remain fixed throughout your code.