Values, types and variables

What is a program?

A program is a list of instructions the computer follows, one after another, from top to bottom. JavaScript instructions are called statements, and each usually ends with a semicolon.

console.log('Namaste!');
console.log(2 + 3); // 5

Types of values

Arrays (lists) and objects (groups of named values) are also values; you will meet them soon. The typeof operator tells you the type: typeof 42 is "number", typeof "hi" is "string".

Variables: names for values

const shopName = 'Sharma Kirana';
let itemsInCart = 0;
itemsInCart = itemsInCart + 1; // let can change
// shopName = 'Other';  // Error: const cannot be reassigned
console.log(shopName, itemsInCart); // Sharma Kirana 1

Use const by default. Switch to let only when you really need to change the value. Do not use var: it is the old way and has confusing rules about where the variable can be seen.

Naming rules

• Names can contain letters, digits, _ and $, but cannot start with a digit.
• JavaScript is case-sensitive: total and Total are different variables.
• Use camelCase: first word lowercase, each next word starts with a capital (totalPrice, isPaid).
• Choose names that explain: priceInRupees is better than p.

Strings and numbers mix carefully

console.log(10 + 5);      // 15
console.log('10' + 5);    // '105'  (text joined, not added!)
console.log(Number('10') + 5); // 15
console.log(typeof '10'); // 'string'