TypeScript Application Architecture

An application architecture determines which parts own business rules, how data crosses boundaries, and which dependencies are replaceable. A small TypeScript service can separate domain calculations from transport handlers and databases. Keeping the central rules independent of Express, React, and a database makes it easier to test and reuse them.

The domain function accepts read-only line items, calculates an amount, and returns a number. An HTTP handler or command-line program can call the same function without putting network code into the calculation.

A directory called services does not create clean architecture if the functions still import every framework and persistence implementation.

This source was compiled using TypeScript 5.8.3 with strict checking, then executed with Node.js 22.16.0. Its stdout below is the actual captured output. The core logic is runnable without a framework or a remote API; integration-specific examples appear in a separate lab when needed.

TypeScript source

type Line = Readonly<{ price: number; quantity: number }>;
function orderTotal(lines: readonly Line[]): number {
  return lines.reduce((sum, line) => sum + line.price * line.quantity, 0);
}
const order: Line[] = [{ price: 15, quantity: 2 }, { price: 7, quantity: 1 }];
console.log(`Order total: ${orderTotal(order)}`);

Verified output

Order total: 37

Compile: npx tsc --strict --target es2022 --module commonjs application-architecture.ts; run: node application-architecture.js. For examples importing node: modules, install @types/node as a development dependency; for browser or React projects use the separate lab setup.

The domain function accepts read-only line items, calculates an amount, and returns a number. An HTTP handler or command-line program can call the same function without putting network code into the calculation.

Before editing the code, predict what the output will be for one changed input. Run it and compare your prediction with the actual result.

A directory called services does not create clean architecture if the functions still import every framework and persistence implementation.

Determine whether the issue belongs to the type system, a trust boundary, or actual runtime behavior before choosing a correction.

Add a discount policy as a pure function. Write one test that calls it without starting a server.

Record expected success and failure outcomes, run the code, and compare the observed behavior. Keep the exercise compatible with strict type checking.

Review the official documentation for the provider, framework or runtime, especially when installing a newer major version.

The domain function accepts read-only line items, calculates an amount, and returns a number. An HTTP handler or command-line program can call the same function without putting network code into the calculation.

Apply the same reasoning to the practice task, then integrate it into a real app only when the runtime and dependency setup are ready.