Chapter 2 · Lesson 1

Declaring Variables

Variables are containers for storing data values. Think of them as labeled boxes where you can put information and retrieve it later.

JavaScript provides three keywords for declaring variables:

let - The modern way to declare variables that can change

Block-scoped (only accessible within their code block)
Can be reassigned but not redeclared in the same scope

const - For values that should not be reassigned

Block-scoped like let
Must be initialized when declared
Cannot be reassigned (but objects/arrays can be modified)

var - The legacy way (avoid in modern code)

Function-scoped (not block-scoped)
Can be redeclared and reassigned
Has "hoisting" quirks that can cause bugs

Naming Conventions:

Use camelCase for variables: myVariableName
Use UPPER_SNAKE_CASE for constants: MAX_VALUE
Be descriptive: userEmail instead of e
Start with a letter, underscore, or dollar sign

Code Examples

Using let and constjavascript

// let - for values that will change
let score = 0;
console.log("Initial score:", score);

score = 10;  // Reassignment is OK
console.log("Updated score:", score);

// const - for values that shouldn't change
const PI = 3.14159;
const MAX_USERS = 100;
console.log("PI:", PI);

// const with objects - contents CAN change
const user = { name: "Alice", age: 25 };
user.age = 26;  // This is allowed!
console.log("User:", user);

// But reassigning the whole object fails
// user = { name: "Bob" };  // Error!

Use const by default, and only use let when you know the value needs to change. This makes your code more predictable.

Variable Scopejavascript

// Block scope demonstration
let globalVar = "I'm global";

if (true) {
  let blockVar = "I'm in a block";
  console.log(globalVar);   // Works - global is accessible
  console.log(blockVar);    // Works - we're inside the block
}

console.log(globalVar);     // Works
// console.log(blockVar);   // Error! blockVar doesn't exist here

// Function scope
function myFunction() {
  let functionVar = "I'm in a function";
  console.log(functionVar); // Works
}

myFunction();
// console.log(functionVar); // Error! functionVar doesn't exist here

Scope determines where variables are accessible. Block-scoped variables (let/const) are only available within their curly braces.

Quick Quiz

1. Which keyword should you use for a value that won't change?

2. What happens if you try to use a let variable outside its block?

3. Can you modify properties of a const object?

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