JavaScript is a versatile programming language commonly used for web development. Below are some fundamental JavaScript commands, concepts, and examples that you might use when building a website:

1. Variables:

• Variables are used to store data values. They can be declared using var, let, or const.

var x = 10; // Declaring a variable named x with a value of 10

2. Data Types:

• JavaScript has various data types, including strings, numbers, booleans, objects, and arrays.

var name = "John";
var age = 30;
var isStudent = true;

3. Functions:

• Functions are reusable blocks of code. They can be defined and called.

function greet(name) {
    console.log("Hello, " + name + "!");
}
greet("John"); // Output: Hello, John!

4. Conditional Statements:

• Use if, else if, and else for conditional logic.

var hour = 15;
if (hour < 12) {
    console.log("Good morning!");
} else if (hour < 18) {
    console.log("Good afternoon!");
} else {
    console.log("Good evening!");
}

5. Loops:

• Loops, like for and while, help execute code repeatedly.

for (var i = 0; i < 5; i++) {
    console.log("Count: " + i);
}

6. Arrays:

• Arrays store multiple values and can be accessed by index.

var colors = ["red", "green", "blue"];
console.log(colors[0]); // Output: red

7. Objects:

• Objects store data as key-value pairs.

var person = {
    firstName: "John",
    lastName: "Doe"
};
console.log(person.firstName); // Output: John

8. DOM Manipulation:

• JavaScript can interact with the Document Object Model (DOM) to change HTML content or styles.
  

var element = document.getElementById("myElement");
element.innerHTML = "New content";

9. Event Handling:

• You can attach JavaScript functions to HTML events.

var button = document.getElementById("myButton");
button.addEventListener("click", function() {
    console.log("Button clicked!");
});

‍10. AJAX and Fetch:

• Use AJAX or the Fetch API to make asynchronous requests to a server.

fetch('https://api.example.com/data')
    .then(response => response.json())
    .then(data => console.log(data))
    .catch(error => console.error(error));

‍

These are some fundamental JavaScript concepts and commands used when building websites. JavaScript offers a wide range of capabilities for web development, from simple interactivity to complex web applications.