Variables and Data Types

JavaScript has two everyday keywords for creating variables:

• const for values that will not be reassigned. Use this by default.
• let for values that will change later.

You may see var in older code; it still works but has confusing scoping rules, so modern code avoids it.

const course = "JavaScript Basics";
let lessonsDone = 1;
lessonsDone = lessonsDone + 1;

console.log(course, "-", lessonsDone, "lessons done");

JavaScript Basics - 2 lessons done

Types of values

The typeof operator tells you the type of a value.

console.log(typeof "text");
console.log(typeof 42, typeof 3.5);
console.log(typeof true);
console.log(typeof undefined);
console.log(typeof { name: "Asha" });
console.log(typeof [1, 2, 3]);

string
number number
boolean
undefined
object
object

• string: text, in single quotes, double quotes or backticks.
• number: whole numbers and decimals are the same type.
• boolean: true or false.
• undefined: a variable that has not been given a value yet.
• object: groups of named values, and also arrays.

Template literals

Strings written with backticks can contain ${...} placeholders. Any expression inside is evaluated and inserted.

const item = "notebook";
const price = 45;
const qty = 3;
console.log(`${qty} x ${item} = Rs ${price * qty}`);

3 x notebook = Rs 135

Arrays and objects

An array is an ordered list. An object stores values under names called keys.

const scores = [72, 88, 95];
const student = { name: "Asha", year: 2, scores };

console.log(scores.length, scores[0]);
console.log(student.name);
console.log(student);

3 72
Asha
{ name: 'Asha', year: 2, scores: [ 72, 88, 95 ] }