wap-notes/Frontend-dev/Session7.md at main · wapinfotech/wap-notes

Inline Script: You can write JavaScript code directly inside the HTML file using the <script> tag.

<html>
  <body>
    <h1>My Web Page</h1>
    <script>
      console.log("Hello, World!");
    </script>
  </body>
</html>

External JavaScript File: You can also write JavaScript in a separate file and link it to your HTML using the src attribute in the <script> tag.
```
<html>
  <body>
    <h1>My Web Page</h1>
    <script src="script.js"></script>
  </body>
</html>
```
Note: Ensure the script.js file is in the same directory as your HTML file, or specify the correct path.
In the Head Section: You can place the <script> tag inside the <head> section of your HTML file.
```
<html>
  <head>
    <script>
      console.log("Hello from the head section!");
    </script>
  </head>
  <body>
    <h1>My Web Page</h1>
  </body>
</html>
```
Note: It's generally a good practice to place JavaScript at the end of the body for better performance unless it needs to run early.

console.log: This function is used to print or log messages to the browser's console (useful for debugging).
```
console.log("This is a log message!");
```

prompt: This function displays a dialog box that prompts the user for input.

let userInput = prompt("Enter your name:");
console.log("User's name is " + userInput);

alert: This function displays a simple message in a dialog box.
```
alert("This is an alert message!");
```

var: Older way of declaring variables. It has function scope and can be redeclared.
```
var name = "John";
console.log(name);
```
let: Modern way of declaring variables. It has block scope and cannot be redeclared in the same block.
```
let age = 25;
console.log(age);
```
const: Declares a constant variable that cannot be reassigned after its initial assignment. It also has block scope.
```
const pi = 3.14;
console.log(pi);
```
Note: Use let and const for variable declarations in modern JavaScript, as they provide better scoping and prevent errors.

JavaScript has different data types, which can be categorized into two main types: Primitive and Non-Primitive (Reference).

Primitive Data Types:
1. String: A sequence of characters used to represent text.
```
let greeting = "Hello, World!";
```
2. Number: Represents both integer and floating-point numbers.
```
let age = 30;
let price = 9.99;
```
3. Boolean: Represents logical values: true or false.
```
let isAvailable = true;
```
4. Undefined: A variable that has been declared but not assigned a value.
```
let name;
console.log(name); // Output: undefined
```
5. Null: Represents an intentionally empty value.
```
let value = null;
```
6. Symbol: Represents a unique identifier.
```
let sym = Symbol("id");
```
7. BigInt: Represents large integers beyond the safe integer limit.
```
let bigNumber = BigInt(9007199254740991);
```
Non-Primitive Data Types:
1. Object: Used to store collections of data or more complex entities.
```
let person = {
  name: "John",
  age: 30
};
```
2. Array: A special type of object used to store ordered lists of items.
```
let fruits = ["Apple", "Banana", "Mango"];
```
3. Function: A block of code designed to perform a particular task.
```
function greet() {
  console.log("Hello, World!");
}
```

Creating an Object: An object is a collection of properties, where each property has a key and a value.
```
let person = {
  firstName: "John",
  lastName: "Doe",
  age: 30
};
```

Accessing Object Properties: You can access properties using dot notation or bracket notation.

console.log(person.firstName); // Dot notation
console.log(person["lastName"]); // Bracket notation

Adding or Modifying Properties: You can add or modify properties of an object.

person.job = "Developer"; // Adding a new property
person.age = 31; // Modifying an existing property
console.log(person);

Deleting Properties: You can delete properties from an object using the delete keyword.
```
delete person.age;
console.log(person);
```

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