TypeScript

TypeScript adds static type checking to JavaScript. It helps describe data and function contracts, find mistakes before execution, and make refactoring safer. The runtime still executes JavaScript; types do not become automatic checks on every value.

One function, two views

TypeScript source:

function greet(name: string): string {
  return `Hello, ${name}`;
}

greet('Ada');

The relevant emitted JavaScript, with type annotations removed:

function greet(name) {
  return `Hello, ${name}`;
}

greet('Ada');

TypeScript would reject greet(42) when checking this program. A JavaScript caller can still pass 42 at runtime: the annotation did not insert a validation function. This distinction matters especially for JSON, user input and DOM lookups.

Which tool does what?

Operation	Purpose	Typical tool
Type-checking	Check whether the source obeys its declared contracts	TypeScript compiler (tsc)
Transpilation	Produce JavaScript from TypeScript or newer syntax	tsc, or a configured loader/transpiler
Bundling	Follow imports and produce browser assets	Webpack
Runtime validation	Check actual input values	Code you execute, or a validation library

Types are mostly erased; some TypeScript features also generate JavaScript. Browser code must be emitted in a form the target browsers support. A successful type check does not prove the business logic is correct—tests cover behavior.

Learning path

1. Types and contracts: inference, narrowing, interfaces, classes and generics.
2. Compiler and DOM: configuration, modules, null checks and API boundaries.
3. Webpack: connect the typed application to its HTML and bundle.

Try it: change the function call to greet(42), inspect the type error, then compare the emitted JavaScript with the TypeScript source. Do not suppress the error with any; decide which input the function should actually accept.

TypeScript Handbook (opens new window)