3 · Connect, test and build · 18 MIN

Write repeatable tests for the logic

A test should check behavior, including boundaries and failures.

Manual checks are useful but easy to forget. Node’s built-in test runner executes named tests and reports failures. Extract pure logic from HTTP handlers so it can be tested with simple values. deepEqual compares the structure of arrays and objects; equal suits primitive values. This example tests filtering and a zero limit. It also checks that the input array is unchanged. Add separate tests for each meaningful contract instead of copying the function’s implementation into the test.

Run node --test lesson.mjs in the terminal. Read each test name before its assertion. Add one edge case per test. Break the function, observe a red result, then restore it.

Before you start

Install Node.js 22.12+ (or a supported newer LTS). Create a folder named codingneed-node. Open a terminal in that folder. Save ONE lesson example as lesson.mjs and run node lesson.mjs. No npm dependencies are required. HTTP lessons keep running until you press Ctrl+C; stop the previous server before starting another.

File for this example: lesson.mjs

New words, explained

testRunner: A program that executes tests and reports their outcomes.
pureFunction: A calculation depending on its inputs without external side effects.
regression: A previously working behavior broken by a change.

Follow the example step by step

Run node --test lesson.mjs in the terminal.
Read each test name before its assertion.
Add one edge case per test.
Break the function, observe a red result, then restore it.

You are ready to move on when: All five behavior tests pass after restoring the implementation. An intentional filter bug causes a failure. Explain why helper and HTTP contracts can have different allowed inputs.

Read the example

import test from 'node:test';
import assert from 'node:assert/strict';
function findTopics(topics, query, limit) {
  return topics.filter(topic => topic.toLowerCase().includes(query.trim().toLowerCase())).slice(0, limit);
}
test('matches case and ignores surrounding spaces', () => {
  assert.deepEqual(findTopics(['React', 'SQL'], ' sql ', 2), ['SQL']);
});
test('zero limit returns nothing without mutating the list', () => {
  const topics = ['React'];
  assert.deepEqual(findTopics(topics, '', 0), []);
  assert.deepEqual(topics, ['React']);
});

Check the expected output

Run node --test lesson.mjs. Both named tests pass. This helper accepts a zero limit, unlike the previous HTTP endpoint’s stricter contract.

Your challenge

Add tests for an empty list, no matching topic and applying a limit after filtering. Deliberately introduce a bug and confirm one of your tests fails.

Solution cost: Discuss the operations in this small example; rendering and I/O costs depend on the host. time · Proportional to the example’s retained data. space

Common trap

Passing tests demonstrate the cases checked; they do not prove every possible input is correct.

Study the project implementation

import test from 'node:test';
import assert from 'node:assert/strict';
function findTopics(topics, query, limit) {
  return topics.filter(topic => topic.toLowerCase().includes(query.trim().toLowerCase())).slice(0, limit);
}
test('matches case and ignores surrounding spaces', () => {
  assert.deepEqual(findTopics(['React', 'SQL'], ' sql ', 2), ['SQL']);
});
test('zero limit returns nothing without mutating the list', () => {
  const topics = ['React'];
  assert.deepEqual(findTopics(topics, '', 0), []);
  assert.deepEqual(topics, ['React']);
});

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