Essential Concepts
Variables
In JavaScript, variables are used to store data values. You can declare variables using var, let, or const.
// Using var (function-scoped)
var name = "John";
// Using let (block-scoped)
let age = 25;
// Using const (block-scoped, immutable reference)
const PI = 3.14;
var name = "John";
// Using let (block-scoped)
let age = 25;
// Using const (block-scoped, immutable reference)
const PI = 3.14;
Functions
Functions are blocks of code designed to perform specific tasks. They can take parameters and return values.
// Function declaration
function greet(name) {
return "Hello, " + name + "!";
}
// Arrow function (ES6+)
const add = (a, b) => a + b;
console.log(greet("Alice")); // Output: Hello, Alice!
console.log(add(5, 3)); // Output: 8
function greet(name) {
return "Hello, " + name + "!";
}
// Arrow function (ES6+)
const add = (a, b) => a + b;
console.log(greet("Alice")); // Output: Hello, Alice!
console.log(add(5, 3)); // Output: 8
Arrays
Arrays are used to store multiple values in a single variable. They can contain any data type.
// Creating an array
const fruits = ["apple", "banana", "orange"];
// Adding elements
fruits.push("grape");
// Iterating through array
fruits.forEach(fruit => console.log(fruit));
const fruits = ["apple", "banana", "orange"];
// Adding elements
fruits.push("grape");
// Iterating through array
fruits.forEach(fruit => console.log(fruit));
Object Example
Objects are collections of key-value pairs that represent entities with properties and methods.
// Creating an object
const person = {
name: "John",
age: 30,
greet: function() {
return "Hello, I'm " + this.name;
}
};
console.log(person.name); // Output: John
console.log(person.greet()); // Output: Hello, I'm John
const person = {
name: "John",
age: 30,
greet: function() {
return "Hello, I'm " + this.name;
}
};
console.log(person.name); // Output: John
console.log(person.greet()); // Output: Hello, I'm John
Interactive Example
Try out this simple calculator function:
function calculate(operation, num1, num2) {
switch(operation) {
case 'add':
return num1 + num2;
case 'subtract':
return num1 - num2;
case 'multiply':
return num1 * num2;
case 'divide':
if(num2 !== 0) return num1 / num2;
else return "Cannot divide by zero";
default:
return "Invalid operation";
}
}
// Examples
console.log(calculate('add', 10, 5)); // Output: 15
console.log(calculate('multiply', 4, 6)); // Output: 24
switch(operation) {
case 'add':
return num1 + num2;
case 'subtract':
return num1 - num2;
case 'multiply':
return num1 * num2;
case 'divide':
if(num2 !== 0) return num1 / num2;
else return "Cannot divide by zero";
default:
return "Invalid operation";
}
}
// Examples
console.log(calculate('add', 10, 5)); // Output: 15
console.log(calculate('multiply', 4, 6)); // Output: 24