Your First TypeScript Program

First, create a folder for your TypeScript program. For example:

typescript-first-program

You can create this folder anywhere you normally keep your programming projects.

Open the terminal and move into your project folder.

cd typescript-first-program

The cd command changes the current directory.

Create a file named:

app.ts

The .ts extension indicates that the file contains TypeScript code.

Write the following code inside app.ts:

let message: string = "Hello TypeScript";

console.log(message);

This program creates a string variable and prints its value.

The let keyword is used to declare a variable.

let message: string = "Hello TypeScript";

Here, message is the name of the variable.

The string type tells TypeScript that the variable should contain text.

let message: string = "Hello TypeScript";

The value "Hello TypeScript" is a string.

The console.log() method displays information in the console.

console.log(message);

When the JavaScript program runs, it displays:

Hello TypeScript

If TypeScript is not already installed in the project, install it using npm:

npm install --save-dev typescript

This installs TypeScript as a development dependency.

After writing the program, compile it using:

npx tsc app.ts

The TypeScript compiler processes app.ts and generates JavaScript output.

After compilation, you can get a JavaScript file named:

app.js

The TypeScript type annotation does not need to remain in the generated JavaScript.

let message = "Hello TypeScript";

console.log(message);

If the generated JavaScript program is intended to run with Node.js, use:

node app.js

The output will be:

Hello TypeScript

The complete TypeScript source code is:

let message: string = "Hello TypeScript";

console.log(message);

This is a simple TypeScript program using a variable, a type annotation, and console.log().

You can also create a number variable.

let age: number = 20;

console.log(age);

The number type indicates that age should contain a number.

TypeScript also supports the boolean type.

let isStudent: boolean = true;

console.log(isStudent);

A boolean value can be true or false.

You can create multiple typed variables in the same program.

let name: string = "Rahul";
let age: number = 21;
let student: boolean = true;

console.log(name);
console.log(age);
console.log(student);

TypeScript supports JavaScript arithmetic operations.

let a: number = 10;
let b: number = 20;

let total: number = a + b;

console.log(total);

The output is:

30

You can create functions in TypeScript just as you do in JavaScript, while also adding type information.

function greet(name: string): string {
    return "Hello " + name;
}

console.log(greet("Rahul"));

The parameter name is declared as a string.

function greet(name: string) {
    return "Hello " + name;
}

This tells TypeScript that the function expects a string argument.

You can also specify what type a function should return.

function add(
    a: number,
    b: number
): number {
    return a + b;
}

The return type is number.

TypeScript can detect incorrect type assignments.

let age: number = 20;

age = "Twenty";

The second line causes a type error because a string is assigned to a number variable.

TypeScript can often determine the type automatically from the initial value.

let name = "Rahul";
let age = 20;

TypeScript can infer that name is a string and age is a number.

let course = "TypeScript";
let duration = 6;

console.log(course);
console.log(duration);

Here, TypeScript can infer the types from the assigned values.

The const keyword can also be used in TypeScript.

const language: string = "TypeScript";

console.log(language);

A value declared with const cannot be reassigned.

Let's create a small student example.

let studentName: string = "Amit";
let marks: number = 85;

console.log("Student:", studentName);
console.log("Marks:", marks);

This program stores and displays basic student information.

let price: number = 500;
let quantity: number = 3;

let total: number = price * quantity;

console.log("Total:", total);

The program calculates the total price by multiplying price and quantity.

let student: {
    name: string;
    age: number;
} = {
    name: "Rahul",
    age: 20
};

console.log(student.name);
console.log(student.age);

This example describes the expected structure of the object.

A simple TypeScript project can be organized like this:

typescript-first-program/
│
├── node_modules/
├── app.ts
├── app.js
├── package.json
├── package-lock.json
└── tsconfig.json

The exact structure can change depending on the project and build tools.

Write app.ts
     ↓
TypeScript Code
     ↓
Run tsc
     ↓
Type Checking
     ↓
Generate JavaScript
     ↓
Run JavaScript

This is the basic development flow you should remember.

let studentName: string = "Rahul";
let age: number = 21;
let marks: number = 88;

function showStudent(
    name: string,
    age: number,
    marks: number
): void {

    console.log("Name:", name);
    console.log("Age:", age);
    console.log("Marks:", marks);
}

showStudent(
    studentName,
    age,
    marks
);

This example combines variables, type annotations, a function, parameters, and console output.

The complete beginner workflow is:

1. Create a project folder.
2. Create an .ts file.
3. Write TypeScript code.
4. Install TypeScript if required.
5. Compile the TypeScript file.
6. Check the generated JavaScript.
7. Run the JavaScript program.

app.ts
   ↓
npx tsc app.ts
   ↓
app.js
   ↓
node app.js