Introduction to TypeScript

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

TypeScript is a superset of JavaScript that adds a static type system. It helps catch type-related errors during development, provides better editor support through features such as autocompletion and safe refactoring, and makes codebases easier to maintain as they grow.

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

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);

Let's understand what happens here.

First, we define a custom type named User. This type describes the structure that every user object should follow. In this example, a user must have a name of type string and an age of type number.

Next, we create the isAdult function. It accepts a parameter of type User and returns a boolean, indicating whether the user's age is at least 18.

We then create a justine object. The satisfies operator verifies that the object conforms to the User type while preserving the object's most specific inferred type. This allows TypeScript to validate the object's structure without changing its inferred properties.

Finally, we call isAdult(justine) and store the result in isJustineAnAdult.

One of TypeScript's strengths is its ability to infer types automatically. For example, even though we never explicitly declared the type of isJustineAnAdult, TypeScript correctly infers it as boolean.

If the code doesn't match the declared types---for example, if age were a string instead of a number---TypeScript reports an error during development, helping you catch mistakes before your code runs.

Note

TypeScript's type system exists only during development. During compilation, all type annotations are removed, so they do not affect your application's runtime behavior.

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

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 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:

// @errors: 2345
import { resolve } from 'node:path';

resolve(123, 456);

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.

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.

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.