JavaScript Intermediate Day 1: Scope & Closures

Goal of this Day

Today you will learn:

• Global vs Local scope
• Block scope with let and const
• Closures and why they are important

By the end, you will understand how variables live and behave in JavaScript functions.

Step 1: Scope

Scope defines where a variable is accessible.

• Global scope: variable is accessible everywhere
• Local scope: variable is accessible only inside a function
• Block scope: with let and const, variables are limited to the block they are defined in

Step 2: Closures

A closure is a function that “remembers” the variables from the scope it was created in, even after that scope has finished.

function outer() {
  let count = 0;

  return function inner() {
    count++;
    return count;
  };
}

const counter = outer();
console.log(counter()); // 1
console.log(counter()); // 2

Explanation:

• The inner function has access to count even after outer finished executing
• This allows us to create private variables

Practice

function outer() {
  let count = 0;

  return function inner() {
    count++;
    return count;
  };
}

const counter = outer();
console.log(counter()); // 1
console.log(counter()); // 2

Task

Create a counter function using closure that:

• Starts from 0
• Increments by 1 each time it is called
• Remembers its value between calls

Example:

function createCounter() {
  let count = 0;

  return function() {
    count++;
    return count;
  };
}

const myCounter = createCounter();
console.log(myCounter()); // 1
console.log(myCounter()); // 2
console.log(myCounter()); // 3