Your First TypeScript Program

Now that your environment is set up, let us write, compile, and execute your first TypeScript program. We will also introduce a development tool called ts-node that streamlines this process.

1. Writing Your Code

First, make sure you have the directory structure set up as configured in your tsconfig.json.

Create a new folder named src in your project root, and then create a file named index.ts inside it:

mkdir src
touch src/index.ts

Open src/index.ts in your text editor and add the following code:

const username: string = "TypeScript Developer";
const birthYear: number = 2012;

function getAge(year: number): number {
  const currentYear = new Date().getFullYear();
  return currentYear - year;
}

console.log(`Hello, ${username}!`);
console.log(`TypeScript has been active for ${getAge(birthYear)} years.`);

2. Compiling to JavaScript

To run this code, we first need to compile it to JavaScript.

In your terminal, run the compiler:

npx tsc

This command reads your tsconfig.json file, locates your files inside the src folder, performs type checks, and writes the output files to your compiled build folder.

If you list the files in your project, you will see a new directory called dist containing the compiled JavaScript:

# Verify the output
cat dist/index.js

The compiled dist/index.js file will look like this:

"use strict";
const username = "TypeScript Developer";
const birthYear = 2012;
function getAge(year) {
    const currentYear = new Date().getFullYear();
    return currentYear - year;
}
console.log(`Hello, ${username}!`);
console.log(`TypeScript has been active for ${getAge(birthYear)} years.`);

Notice how the type annotations (: string, : number) have been removed by the compiler.

3. Running the Program

Now, execute the output JavaScript file using Node.js:

node dist/index.js

You should see the output text printed in your terminal.

4. Faster Development with ts-node

Compiling code manually with tsc and running it with node every time you make a change can be slow.

To speed up development, we can use ts-node. This tool compiles TypeScript on-the-fly in memory and runs it directly, without writing JavaScript files to your hard drive.

Installing ts-node

Install it locally in your project:

npm install -g ts-node

Running Your TypeScript Code Directly

Now, run your TypeScript file in one command:

ts-node src/index.ts

This compiles and runs the program instantly, providing a much faster feedback loop during development.

5. Summary

• Write TypeScript code in files with the .ts extension.
• Use the command npx tsc to compile TypeScript into JavaScript.
• Run the resulting JavaScript files using Node.js.
• Use ts-node during development to run TypeScript files directly without manual compilation steps.