const - Creating Constants

const is used to create constant variables that never change their value.

What is const?

const creates variables whose values cannot be reassigned after they're created. Think of them as permanent marker - once you set the value it cannot be changed.

Copyconst name = "Alice"

// Trying to change a const variable will throw an error
name = "Bob" // ❌ TypeError: Assignment to constant variable

Differences From let

1. Must Be Given a Value When Created

   Unlike let, you cannot create a const variable without giving it a value:

   Copy// ✅ Good - const with initial value
   const name = "Alice"
   const age = 45
   
   const message // ❌ SyntaxError: Missing initializer in const declaration
   

2. Cannot Be Reassigned

   Once created, you cannot change the value of a const variable:

   Copyconst name = "Alice"
   
   name = "Bob" // ❌ TypeError: Assignment to constant variable
   

const vs let Decision Guide

I use const by default unless I know the value will change. This helps prevent accidental changes to important values.

Use const when:

• The value never changes
• It's a global constant like a configuration or setting

Use let when:

• The value will change as your program runs
• You're counting or tracking something

Global Constants

In larger applications, you might define global constants that are used throughout your codebase:

Copyconst APP_NAME = "My Awesome App"
const VERSION = "1.0.0"
const SUPPORT_EMAIL = "support@example.com"

These global constant values are often defined in uppercase with underscores separating words.