Welcome back! I am Mihir, and in this lesson we will write your first proper TypeScript program.

Instead of only printing "Hello World", we will build a small beginner-friendly example that uses variables, functions, object types, and compiler checking. This will show you why TypeScript is useful in real code.

The Goal

We will create a simple course enrollment program.

It will:

• Store student details
• Store course details
• Create a readable enrollment message
• Use TypeScript types to avoid mistakes

Create a File

Create a file named:

index.ts

Add this first line:

console.log("Welcome to TypeScript with Mihir!");

Compile it:

npx tsc index.ts

Run it:

node index.js

Output:

Welcome to TypeScript with Mihir!

This is the most basic TypeScript program. Now let us make it useful.

Add Typed Variables

const studentName: string = "Aarav";
const courseName: string = "TypeScript";
const durationInDays: number = 30;
const isActive: boolean = true;

console.log(studentName);
console.log(courseName);
console.log(durationInDays);
console.log(isActive);

Here we are using four basic types:

TypeScript now knows what kind of value each variable should store.

If we write:

const durationInDays: number = "thirty";

TypeScript warns us because "thirty" is a string, not a number.

Add a Function

Now let us create a function that builds a message.

function createEnrollmentMessage(
  studentName: string,
  courseName: string,
  durationInDays: number
): string {
  return `${studentName} enrolled in ${courseName}. Duration: ${durationInDays} days.`;
}

const message = createEnrollmentMessage("Aarav", "TypeScript", 30);
console.log(message);

Let us understand the function:

studentName: string

This means the first parameter must be a string.

durationInDays: number

This means the third parameter must be a number.

): string

This means the function must return a string.

If we return a number:

function createMessage(): string {
  return 100;
}

TypeScript warns us because the return type is wrong.

Add an Object Type

Passing many separate arguments can become confusing.

Instead, we can group related data in an object.

type Student = {
  id: number;
  name: string;
  email: string;
  isActive: boolean;
};

This creates a custom type named Student.

Now create a student:

const student: Student = {
  id: 1,
  name: "Aarav",
  email: "aarav@example.com",
  isActive: true,
};

If you miss a required property:

const student: Student = {
  id: 1,
  name: "Aarav",
  email: "aarav@example.com",
};

TypeScript warns that isActive is missing.

Add a Course Type

type Course = {
  id: number;
  title: string;
  durationInDays: number;
  level: "beginner" | "intermediate" | "advanced";
};

The level field uses a union of string literals.

This means level can only be:

• "beginner"
• "intermediate"
• "advanced"

Create a course:

const course: Course = {
  id: 101,
  title: "TypeScript",
  durationInDays: 30,
  level: "beginner",
};

This would be invalid:

const course: Course = {
  id: 101,
  title: "TypeScript",
  durationInDays: 30,
  level: "easy",
};

TypeScript warns because "easy" is not allowed.

Complete First Program

Here is the complete program:

type Student = {
  id: number;
  name: string;
  email: string;
  isActive: boolean;
};

type Course = {
  id: number;
  title: string;
  durationInDays: number;
  level: "beginner" | "intermediate" | "advanced";
};

function enrollStudent(student: Student, course: Course): string {
  if (!student.isActive) {
    return `${student.name} is not active and cannot enroll.`;
  }

  return `${student.name} enrolled in ${course.title}, a ${course.level} course for ${course.durationInDays} days.`;
}

const student: Student = {
  id: 1,
  name: "Aarav",
  email: "aarav@example.com",
  isActive: true,
};

const course: Course = {
  id: 101,
  title: "TypeScript",
  durationInDays: 30,
  level: "beginner",
};

const result = enrollStudent(student, course);

console.log(result);

Output:

Aarav enrolled in TypeScript, a beginner course for 30 days.

What TypeScript Checks Here

TypeScript checks:

• student.id is a number
• student.name is a string
• student.email is a string
• student.isActive is a boolean
• course.level is one of the allowed values
• enrollStudent receives a Student and a Course
• enrollStudent returns a string

This gives us safety before the program runs.

Compile and Run

Compile:

npx tsc index.ts

Run:

node index.js

If there are no type errors, TypeScript creates JavaScript output.

If there are type errors, TypeScript shows warnings in the terminal and editor.

Important Beginner Tip

TypeScript errors are not your enemy.

They are feedback.

When TypeScript says something is wrong, it is usually telling you:

• A value has the wrong type
• A property is missing
• A function is being used incorrectly
• A value might be undefined
• Your data model is unclear

Learn to read TypeScript errors slowly. That skill will make you much stronger.

What You've Learned

You now know how to:

• Write a basic TypeScript file
• Use typed variables
• Create typed functions
• Define object types
• Use string literal unions
• Compile .ts to .js
• Run the compiled file with Node.js
• Understand how TypeScript catches mistakes

What's Next?

In the next lesson, we will learn tsconfig.json Basics.

This file controls how the TypeScript compiler behaves in your project.

Need Help?

• Have questions, confusion, or want to know more? Contact me

You have now written your first meaningful TypeScript program. From here, the language will start feeling more natural with practice.