Introduction to TypeScript

What is TypeScript

TypeScript is an open-source language maintained and developed by Microsoft.

Basically, TypeScript adds additional syntax to JavaScript to support a tighter integration with your editor. Catch errors early in your editor or in your CI/CD pipeline, and write more maintainable code.

We can talk about other TypeScript benefits later, let's see some examples now!

First TypeScript code

Take a look at this code snippet and then we can unpack it together:

type User = {
  name: string;
  age: number;
};

function isAdult(user: User): boolean {
  return user.age >= 18;
}

const justine = {
  name: 'Justine',
  age: 23,
} satisfies User;

const isJustineAnAdult = isAdult(justine);

The first part (with the type keyword) is responsible for declaring our custom object type representing users. Later we utilize this newly created type to create function isAdult that accepts one argument of type User and returns boolean. After this, we create justine, our example data that can be used for calling the previously defined function. Finally, we create a new variable with information on whether justine is an adult.

There are additional things about this example that you should know. Firstly, if we do not comply with the declared types, TypeScript will inform us that something is wrong and prevent misuse. Secondly, not everything must be typed explicitly—TypeScript infers types for us. For example, the variable isJustineAnAdult is of type boolean even if we didn't type it explicitly, and justine would be a valid argument for our function even though we didn't declare this variable as of User type.

What does TypeScript consist of?

TypeScript consists of two main components: the code itself and type definitions.

TypeScript Code

The code part is regular JavaScript with additional TypeScript-specific syntax for type annotations. When TypeScript code is compiled, all the TypeScript-specific parts are removed, resulting in clean JavaScript that can run in any environment. For example:

function greet(name: string) {
  console.log(`Hello, ${name}!`);
}

Type Definitions

Type definitions describe the shape of existing JavaScript code. They are usually stored in .d.ts files and don't contain any actual implementation—they only describe the types. These definitions are essential for interoperability with JavaScript: code is not usually distributed as TypeScript, but instead transpiled to JavaScript that includes sidecar type definition files.

For example, when you use Node.js with TypeScript, you'll need type definitions for Node.js APIs. This is available via @types/node. Install it using:

npm add --save-dev @types/node

These type definitions allow TypeScript to understand Node.js APIs and provide proper type checking and autocompletion when you use functions like fs.readFile or http.createServer. For example:

import * as fs from 'fs';

fs.readFile('example.txt', 'foo', (err, data) => {
//                          ^^^ Argument of type '"foo"' is not assignable to parameter of type …
  if (err) throw err;
  console.log(data);
});

Many popular JavaScript libraries have their type definitions available under the @types namespace, maintained by the DefinitelyTyped community. This enables seamless integration of existing JavaScript libraries with TypeScript projects.

Transform Capabilities

TypeScript also includes powerful transformation capabilities, particularly for JSX (used in React and similar frameworks). The TypeScript compiler can transform JSX syntax into regular JavaScript, similar to how Babel works. While we won't cover these transformation features in these articles, it's worth noting that TypeScript isn't only a tool for type checking—it's also a build tool for transforming modern JavaScript syntax into compatible versions for different environments.

How to run TypeScript code

Okay, so we have some TypeScript code. Now how do we run it? There are few possible ways to run TypeScript code, we will cover all of them in the next articles.