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JavaScript Notes · Lesson 3.2

Variables: let, const, var

CODE SCOPE JAVASCRIPT SERIESLESSON 3.2 OF 3.1–3.26

Labelled boxes for your data: naming rules, let for changing values, const with its lock, and why old var is avoided — const first.

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Free PDF of this lesson — save it, print it, revise offline. The video is the same lesson on the Code Scope YouTube channel.

Introduction

Your code needs to remember things: a price, a name, a count of cups. JavaScript remembers values in variables. Think of a variable as a labelled box: the label is written outside the box, and the value sits inside it. When your code says the label, JavaScript looks inside the box.

JavaScript gives you three words to make a box: let, const, and the old var. They are not equal. In this lesson you will learn the naming rules, see how let values change, feel the lock on const, and learn the simple decision rule professionals use: const first.

(Roman Urdu: Variable aik label wala dabba hai — bahar naam, andar value; const wala dabba lock ho jata hai.)

Key points

  • Declaring a variable: let price = 250; makes a box labelled price and puts 250 inside. The = here means store this value in the box.
  • Naming rules: a name can use letters, numbers, _ and $ — but it cannot start with a number and cannot contain spaces. chaiPrice is fine; 2price and chai price are errors.(Roman Urdu: Naam hamesha letter se shuru hoga, number se nahi, aur beech mein space nahi aayegi.)
  • Use camelCase and meaningful names: write totalBill, not totalbill or tb. A good name tells the reader what is inside the box without opening it.
  • let is for values that change: declare with let when the value must move — like a price that rises, or a counter that grows.
  • const is a locked box: declare with const when the value must never change. Trying to store a new value in a const box throws a TypeError — JavaScript stops and tells you honestly.(Roman Urdu: const ka matlab hai ye value ab change nahi ho sakti — koshish karoge to error milega.)
  • var is the old way — avoid it: var leaks out of blocks and allows silent re-declaring, which creates surprising bugs. Modern code does not use it.
  • The decision rule: const first. Use let only when you know the value must change. Use var never.
  • Name is not the value: the label price is not the number 250. The box can hold a new value later (with let), but the label stays the same.

Code examples

Example 1 — let: a value that changes

let price = 250;
console.log(price);

price = 300;
console.log(price);

let cups = 0;
cups = cups + 1;
cups = cups + 1;
cups = cups + 1;
console.log(cups);

What it does: The box price starts with 250, then we store 300 in the same box — the old value is gone. The box cups starts at 0 and grows by one, three times, like a counter counting cups of chai.

What you will see: three lines in the Console: 250, then 300, then 3.

Example 2 — const: the locked box and its honest error

const shopName = "ChaiPoint";
console.log(shopName);

shopName = "Tea House";
console.log(shopName);

What it does: shopName is declared with const, so its value is locked at "ChaiPoint". The next line tries to store a new value — and JavaScript refuses, loudly and honestly, instead of silently breaking your program later.

What you will see: the Console prints ChaiPoint, then the script stops with a red error: TypeError: Assignment to constant variable. The last line never runs. That error is const protecting you.

Example 3 — Naming rules in real code

let chaiPrice = 250;
let numberOfCups = 3;
let totalBill = chaiPrice * numberOfCups;

console.log(totalBill);
console.log("Total: " + totalBill + " rupees");

What it does: Three variables with clear camelCase names. totalBill is calculated from the other two boxes — 250 * 3 — and printed two ways: as a pure number, and inside a sentence built with +.

What you will see: two lines in the Console: 750 and then Total: 750 rupees.

Quick recap

  • A variable is a labelled box: name outside, value inside.
  • const first, let when the value must change, var never.
  • Names use camelCase, start with a letter, and carry no spaces.

Common mistakes

  1. Changing a const value — storing a new value in a const box throws TypeError: Assignment to constant variable. Fix: if the value must change, declare it with let from the start.
  2. Bad names like 2price or chai price — names cannot start with a number or contain spaces, and the script fails. Fix: start with a letter and use camelCase, like chaiPrice.
  3. Using var out of habit — var leaks and re-declares silently, creating bugs that are hard to find. Fix: follow the rule — const first, then let, never var.

Practice

  1. Declare a const with your shop or city name, a let for a price, and a let counter starting at 0. Increase the counter two times, change the price once, and print all three values.
  2. Deliberately try to change your const value and read the error in the Console. Write one line in your notebook: what does the error say, and why is it helpful?