How Promises work in backend code — Basic

Node / Express Async and await in Express

A Promise represents async work that will finish later (database, file, network). Knex queries return promises; Express handlers use async/await to read results cleanly.

Learn: Before async/await, Promises were the standard way to handle async results in Node. Knex still returns a Promise from every query builder chain. Understanding then/catch helps you read older examples; writing new code with async/await is usually clearer in controllers and services.

Supplementary examples

Promise.all for parallel queries

const [users, pastes] = await Promise.all([
  knex("users").select("*"),
  knex("pastes").select("*"),
]);

Course example

// Knex returns a Promise
const promise = knex("users").select("*");

promise
  .then((rows) => console.log(rows))
  .catch((err) => console.error(err));

// Same query with async/await (preferred in controllers/services)
async function listUsers() {
  const rows = await knex("users").select("*");
  return rows;
}

Additional references & examples

MDN: Using Promises
Promise states, then/catch, and chaining.
Node.js documentation
Official Node APIs: modules, events, and runtime behavior.