Typescript fundamental

1. Hello World!

TypeScript is a superset of JavaScript that adds static types to the language. Let's start with a simple "Hello World" program.

Example:

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

To run this:

1. Save it as hello.ts.

2. Compile it to JavaScript using the TypeScript compiler: tsc hello.ts.

3. Run the generated hello.js file with Node.js: node hello.js.

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2. Variables, Constants, and Data Types

TypeScript supports variables and constants with explicit type annotations. Common data types include number, string, boolean, any, and more.

Variables

Use let or const for variables. TypeScript infers types when not explicitly declared.

Example:

let name: string = "Alice";
let age: number = 25;
let isStudent: boolean = true;
let dynamic: any = "Can be anything";

Constants

Use const for values that won't change.

Example:

const PI: number = 3.14;

Other Data Types

• null and undefined

• object, array, tuple, enum, etc. (covered later).

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3. Concatenation and Interpolation

You can combine strings using concatenation (+) or template literals for interpolation.

Concatenation:

let firstName: string = "John";
let lastName: string = "Doe";
let fullName: string = firstName + " " + lastName;
console.log(fullName); // Output: John Doe

Interpolation (using template literals):

let greeting: string = `Hello, ${firstName} ${lastName}!`;
console.log(greeting); // Output: Hello, John Doe!

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4. Objects and Arrays

Objects

Objects in TypeScript can have defined types for their properties.

Example:

let person: { name: string; age: number } = {
    name: "Alice",
    age: 30
};
console.log(person.name); // Output: Alice

Arrays

Arrays hold multiple values of the same type.

Example:

let numbers: number[] = [1, 2, 3, 4];
let names: Array<string> = ["Alice", "Bob", "Charlie"];
console.log(numbers[0]); // Output: 1
console.log(names.length); // Output: 3

Tuples

Fixed-length arrays with specific types.

Example:

let tuple: [string, number] = ["Alice", 25];
console.log(tuple[0]); // Output: Alice

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5. Relational and Equality Operators

elational operators compare values, and equality operators check for equality.

Relational Operators

• <, >, <=, >=

Example:

let a: number = 10;
let b: number = 20;
console.log(a < b); // Output: true
console.log(a >= b); // Output: false

Equality Operators

• == (loose equality, type coercion)

• === (strict equality, no type coercion)

• !=, !==

Example:

let x: number = 5;
let y: string = "5";
console.log(x == y); // Output: true
console.log(x === y); // Output: false

6. Logical Operators

Logical operators combine conditions: && (AND), || (OR), ! (NOT).

Example:

let isAdult: boolean = true;
let hasLicense: boolean = false;

console.log(isAdult && hasLicense); // Output: false
console.log(isAdult || hasLicense); // Output: true
console.log(!isAdult); // Output: false

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7. Conditional Statements

Conditional statements control the flow of execution based on conditions.

If-Else

let age: number = 18;
if (age >= 18) {
    console.log("Adult");
} else {
    console.log("Minor");
}
// Output: Adult

Switch

let day: number = 2;
switch (day) {
    case 1:
        console.log("Monday");
        break;
    case 2:
        console.log("Tuesday");
        break;
    default:
        console.log("Other day");
}
// Output: Tuesday

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8. Loops

Loops allow repetitive execution of code.

For Loop

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

While Loop

let count: number = 0;
while (count < 3) {
    console.log(count);
    count++;
}
// Output: 0, 1, 2

For...of Loop (for arrays)

let fruits: string[] = ["Apple", "Banana", "Orange"];
for (let fruit of fruits) {
    console.log(fruit);
}
// Output: Apple, Banana, Orange

9. Functions

Functions in TypeScript can have typed parameters and return types.

Example:

function add(a: number, b: number): number {
    return a + b;
}
console.log(add(5, 3)); // Output: 8

Optional Parameters

function greet(name: string, greeting?: string): string {
    return `${greeting || "Hello"}, ${name}!`;
}
console.log(greet("Alice")); // Output: Hello, Alice!
console.log(greet("Bob", "Hi")); // Output: Hi, Bob!

Arrow Functions

const multiply = (x: number, y: number): number => x * y;
console.log(multiply(4, 5)); // Output: 20

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10. Classes and Methods

Classes define blueprints for objects with properties and methods.

Example:

class Person {
    name: string;
    age: number;

    constructor(name: string, age: number) {
        this.name = name;
        this.age = age;
    }

    introduce(): string {
        return `Hi, I'm ${this.name} and I'm ${this.age} years old.`;
    }
}

let person: Person = new Person("Alice", 25);
console.log(person.introduce()); // Output: Hi, I'm Alice and I'm 25 years old.

Inheritance

class Student extends Person {
    grade: number;

    constructor(name: string, age: number, grade: number) {
        super(name, age);
        this.grade = grade;
    }

    study(): string {
        return `${this.name} is studying.`;
    }
}

let student: Student = new Student("Bob", 20, 10);
console.log(student.introduce()); // Output: Hi, I'm Bob and I'm 20 years old.
console.log(student.study()); // Output: Bob is studying.

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11. Practice Exercises

Try these exercises to test your understanding of TypeScript fundamentals.

1. Hello World Modification Create a program that prints "Welcome to TypeScript!" to the console.

2. Variable Swap Declare two variables a and b with numbers, swap their values, and print the result.

3. String Interpolation Create a function that takes a name and age and returns a string like "Hello, [name]! You are [age] years old."

4. Array Sum Write a function that takes an array of numbers and returns their sum.

5. Object Creation Define an object type for a Car with properties brand (string) and year (number). Create an instance and print its properties.

6. Equality Check Write a function that checks if two numbers are equal using both == and === and explains the difference in output.

7. Logical Operator Task Create a function that takes age and hasLicense and returns true only if the person is over 18 and has a license.

8. Grade Calculator Write a program that takes a score (0-100) and prints a letter grade (A, B, C, D, F) using if-else or switch.

9. Loop Exercise Write a loop that prints all even numbers from 1 to 20.

10. Class Creation Create a Rectangle class with width and height properties and a method getArea() that returns the area. Test it with an instance.