Variables

In the last lesson, the canteen bill typed the samosa price in two places. If the price changed, you would have to find and fix every copy. Programmers solve this with variables: names that hold values, so you write a value once and use its name everywhere.

In this lesson you will create variables in Python and JavaScript, change their values while the program runs, learn the rules for naming them, and see the mistakes that trip up almost every beginner.

What is a variable?

A variable is a name that refers to a value stored in the computer's memory. Think of the contacts on your phone: you do not remember ten-digit numbers, you tap "Papa" or "Meera" and the phone finds the number. A variable name works the same way: you use the name, and the computer finds the value.

Creating a variable and giving it a value is called assignment:

samosa_price = 15
chai_price = 12
samosas = 4
chais = 2

total = samosas * samosa_price + chais * chai_price
print("Total to pay:", total)

Total to pay: 84

Read samosa_price = 15 as "store 15 under the name samosa_price". The = sign does not mean "is equal to" like in maths. It is an instruction: work out the value on the right, then store it under the name on the left.

Variables in JavaScript: let and const

JavaScript asks you to announce a new variable with a keyword. Use const for a value that will not change, and let for a value that will:

const samosaPrice = 15;
const chaiPrice = 12;
let samosas = 4;
let chais = 2;

const total = samosas * samosaPrice + chais * chaiPrice;
console.log("Total to pay:", total);

Total to pay: 84

If you later try to change a const, JavaScript stops with TypeError: Assignment to constant variable. That error is helpful: it protects values that should stay fixed, such as a pass mark or a tax rate. Start with const and switch to let only when the value really needs to change.

Python has no const. By custom, Python programmers write fixed values in capital letters, such as PASS_MARK = 33, as a signal to other people: "please do not change this".

Changing a variable

A variable can be given a new value at any time. The old value is replaced. A common pattern is to use the variable's current value to work out its new value:

score = 0
print("Start:", score)
score = score + 4
print("After a four:", score)
score = score + 6
print("After a six:", score)
score = score + 1
print("After a single:", score)

Start: 0
After a four: 4
After a six: 10
After a single: 11

score = score + 4 means: take the current score (0), add 4, and store the answer (4) back in score. In maths, "x = x + 4" would be impossible, which shows again that = in code means "store", not "equals".

The JavaScript version uses let, because score changes:

let score = 0;
console.log("Start:", score);
score = score + 4;
console.log("After a four:", score);
score = score + 6;
console.log("After a six:", score);

Start: 0
After a four: 4
After a six: 10

Notice that let appears only once, when the variable is created. After that you just use the name.

Naming rules and good names

Every language has rules for names. In both Python and JavaScript, a name:

• can contain letters, digits and the underscore _ (JavaScript also allows $);
• cannot start with a digit, so 2nd_place is not allowed but second_place is;
• cannot contain spaces or symbols like - or @;
• cannot be a keyword, a word the language already uses, such as if, for or while;
• is case-sensitive: marks, Marks and MARKS are three different names.

Beyond the rules, choose names that explain the value. The two languages also have different habits for joining words:

Python style is snake_case (lower case words joined by underscores). JavaScript style is camelCase (each new word starts with a capital letter). Code with clear names is easier to read, and easier to fix six months later.

Advanced: swapping two values

Suppose Aarav and Priya swap seats. If you write left = right first, the old value of left is lost. The classic solution uses a third, temporary variable. Python and JavaScript also have a shortcut that swaps in one line:

left = "Aarav"
right = "Priya"

# the classic way, with a temporary variable
temp = left
left = right
right = temp
print("Left:", left, "| Right:", right)

# Python's one-line swap
left, right = right, left
print("Left:", left, "| Right:", right)

Left: Priya | Right: Aarav
Left: Aarav | Right: Priya

In JavaScript the one-line swap is written [left, right] = [right, left];. You will meet the brackets in this style again when you learn about lists and arrays.

Common mistakes

• Using a variable before creating it. print(total) before any total = ... line fails with NameError: name 'total' is not defined. JavaScript says ReferenceError: total is not defined.
• A typing mistake in the name. In Python, totl = total + 5 silently creates a new variable called totl, and total stays the same. Check spellings carefully.
• Writing let again for the same name. let score = 0; then let score = 4; is an error in JavaScript. Use let once, then score = 4;.
• Changing a const. If a value must change, create it with let.

Key points

• A variable is a name for a value stored in memory; assignment with = stores the value.
• = means "store the value on the right under the name on the left", not "equals".
• In JavaScript, use const for values that never change and let for values that do.
• A variable can be updated using its own old value: score = score + 4.
• Names use letters, digits and _, cannot start with a digit, and are case-sensitive.
• Python prefers snake_case names; JavaScript prefers camelCase names.

pocket_money = 500
movie_ticket = 120
snacks = 85

money_left = pocket_money - movie_ticket - snacks
print("Money left:", money_left)

money_left = money_left + 100
print("After Nani's gift:", money_left)

Money left: 295
After Nani's gift: 395

const pocketMoney = 500;
const movieTicket = 120;
const snacks = 85;

let moneyLeft = pocketMoney - movieTicket - snacks;
console.log("Money left:", moneyLeft);

moneyLeft = moneyLeft + 100;
console.log("After Nani's gift:", moneyLeft);