Chapter 2: Basics of JavaScript

Syntax and Basic Constructs

JavaScript uses a syntax similar to other C-style languages (like C++ and Java). A JavaScript program is a sequence of statements and comments. Statements end with a semicolon (;), although they can be omitted due to Automatic Semicolon Insertion (ASI).

Here is a simple JavaScript statement:

console.log('Hello, World!');

Comments can be used to make your code more understandable to others (or to yourself). JavaScript supports both single-line comments, started with //, and multi-line comments, enclosed between /* and */.

// This is a single-line comment

/* 
This is a 
multi-line comment 
*/

Variables and Data Types

In JavaScript, we declare variables using the var, let, or const keywords. var is function-scoped, while let and const are block-scoped. const is used for variables that cannot be reassigned.

var name = "John Doe";
let age = 21;
const country = "USA";

JavaScript has dynamic types, meaning a variable can hold different types of values because it’s not tied to any particular type. There are seven basic data types in JavaScript:

1. Number: This includes both integers and floating-point numbers.
   

   let count = 10; // integer
   let price = 9.99; // floating-point number
   

2. String: A sequence of characters used to represent text.
   

   let message = "Hello, World!";
   

3. Boolean: Represents a logical entity and can have two values: true or false.
   

   let isComplete = true;
   

4. Undefined: A variable that has been declared but not yet assigned a value.
   

   let x;
   console.log(x); // undefined
   

5. Null: A special keyword representing a null or "empty" value.
   

   let y = null;
   

6. Symbol: A unique and immutable data type that is often used as an identifier for object properties.

7. Object: Allows you to store collections of data.
   

   let person = {
     name: "John Doe",
     age: 21
   };
   

Operators and Expressions

Operators in JavaScript are used to perform operations on variables and values. The most common operators are:

• Arithmetic operators: +, -, *, /, %, ++, --
  

  let x = 10;
  let y = 20;
  
  console.log(x + y); // 30
  

• Assignment operators: =, +=, -=, *=, /=, %=
  

  let x = 10;
  x += 5;
  
  console.log(x); // 15
  

• Comparison operators: ==, ===, !=, !==, <, >, <=, >=
  

  let x = 10;
  
  console.log(x == "10"); // true
  console.log(x === "10"); // false
  

• Logical operators: &&, ||, !
  

  let x = 10;
  let y = 20;
  
  console.log(x > 5 && y > 5); // true
  

Control Flow (If, Else, Switch, Loops)

Control flow statements are used to conditionally execute blocks of code.

• If-else statements
  

  let age = 21;
  
  if (age >= 18) {
    console.log("You are an adult.");
  } else {
    console.log("You are a minor.");
  }
  

• Switch statement
  

  let grade = 'A';
  
  switch (grade) {
    case 'A':
      console.log("Excellent");
      break;
    case 'B':
      console.log("Good");
      break;
    case 'C':
      console.log("Fair");
      break;
    case 'D':
      console.log("Poor");
      break;
    default:
      console.log("Invalid grade");
  }
  

• For loop
  

  for (let i = 0; i < 5; i++) {
    console.log(i); // 0, 1, 2, 3, 4
  }
  

• While loop
  

  let i = 0;
  
  while (i < 5) {
    console.log(i); // 0, 1, 2, 3, 4
    i++;
  }
  

By understanding these fundamental concepts, you're now ready to explore more complex aspects of the JavaScript language.