JavaScript Language Reference

Core Concepts

Variables
Declare and manage data.
Data Types
Understanding primitive and object types.
Operators
Perform operations on values.
Control Flow
Conditional statements and loops.
Functions
Reusable blocks of code.
Objects
Key-value pairs and properties.
Arrays
Ordered collections of data.

Advanced Topics

Prototypes and Inheritance
Object-oriented programming concepts.
Closures
Accessing outer function scope from inner functions.
Asynchronous JavaScript
Callbacks, Promises, and Async/Await.
Modules
Organizing code into reusable units.
Error Handling
Managing exceptions and errors.

Built-in Objects

String Object
Manipulating text.
Number Object
Mathematical operations.
Math Object
Constants and mathematical functions.
Date Object
Working with dates and times.
RegExp Object
Regular expressions for pattern matching.
JSON Object
Parsing and stringifying JSON data.

Key Features

Variables

Variables are containers for storing data values. In modern JavaScript, `let` and `const` are preferred over `var`.


// Declaring variables
let message = "Hello, World!";
const PI = 3.14159;

// Reassigning a 'let' variable
message = "Greetings!";

// Attempting to reassign a 'const' variable will cause an error
// PI = 3.14; // TypeError

Data Types

JavaScript has primitive types (String, Number, Boolean, Null, Undefined, Symbol, BigInt) and the Object type.


let name = "Alice";          // String
let age = 30;                // Number
let isStudent = false;       // Boolean
let job = null;              // Null (intentional absence of a value)
let apartment;               // Undefined (variable declared but not assigned)
const id = Symbol('unique'); // Symbol
const bigNum = 9007199254740991n; // BigInt

let person = { name: "Bob", age: 25 }; // Object
let numbers = [1, 2, 3];            // Array (a type of object)

Operators

Operators perform operations on operands. Common operators include arithmetic, assignment, comparison, and logical.


// Arithmetic
let sum = 10 + 5;      // 15
let diff = 10 - 5;     // 5
let product = 10 * 5;  // 50
let quotient = 10 / 5; // 2
let remainder = 10 % 3; // 1

// Comparison
let isEqual = (10 == '10');     // true (type coercion)
let isStrictEqual = (10 === '10'); // false (no type coercion)
let isGreater = (10 > 5);       // true

// Logical
let isTrue = true;
let isFalse = false;
let logicalAnd = isTrue && isFalse; // false
let logicalOr = isTrue || isFalse;  // true
let logicalNot = !isTrue;           // false

Control Flow

Control flow statements dictate the order in which code is executed.


// if-else statement
let score = 75;
if (score >= 90) {
    console.log("Excellent!");
} else if (score >= 70) {
    console.log("Good job!");
} else {
    console.log("Keep practicing.");
}

// for loop
for (let i = 0; i < 5; i++) {
    console.log(`Iteration ${i}`);
}

// while loop
let count = 0;
while (count < 3) {
    console.log(`While count: ${count}`);
    count++;
}

Functions

Functions are blocks of reusable code that perform a specific task.


// Function declaration
function greet(name) {
    return `Hello, ${name}!`;
}

// Function expression
const add = function(a, b) {
    return a + b;
};

// Arrow function (concise syntax)
const multiply = (x, y) => x * y;

console.log(greet("Alice"));       // Output: Hello, Alice!
console.log(add(5, 3));            // Output: 8
console.log(multiply(4, 6));       // Output: 24

Objects

Objects are collections of properties, where each property is a key-value pair.


let car = {
    make: "Toyota",
    model: "Camry",
    year: 2022,
    isRunning: false,
    start: function() {
        this.isRunning = true;
        console.log("Engine started.");
    }
};

console.log(car.make);       // Output: Toyota
console.log(car['model']);   // Output: Camry
car.start();                 // Output: Engine started.
console.log(car.isRunning);  // Output: true

Arrays

Arrays are ordered lists of values, accessed by index.


let fruits = ["Apple", "Banana", "Cherry"];

console.log(fruits[0]); // Output: Apple
console.log(fruits.length); // Output: 3

fruits.push("Date"); // Add to end
console.log(fruits); // ["Apple", "Banana", "Cherry", "Date"]

fruits.pop(); // Remove from end
console.log(fruits); // ["Apple", "Banana", "Cherry"]

fruits.forEach(fruit => {
    console.log(`I like ${fruit}`);
});