Unit Tests

Overview

A unit test checks one small piece of code, usually a single function, in isolation. Because they touch no database, network, or file system, unit tests run in milliseconds and pin down exactly where a failure happens. They form the wide base of most testing strategies: fast, numerous, and cheap to write.

Syntax / Usage

Group related tests with describe and write one test per behavior. Keep each test focused on a single expectation so a failure points at one cause.

// cart.js
function totalPrice(items) {
  return items.reduce((sum, item) => sum + item.price * item.quantity, 0);
}
module.exports = { totalPrice };

// cart.test.js
const { totalPrice } = require("./cart");

describe("totalPrice", () => {
  test("returns 0 for an empty cart", () => {
    expect(totalPrice([])).toBe(0);
  });

  test("multiplies price by quantity", () => {
    const items = [{ price: 5, quantity: 2 }];
    expect(totalPrice(items)).toBe(10);
  });
});

Examples

Testing several inputs for the same rule:

function isEven(n) {
  return n % 2 === 0;
}

test.each([
  [2, true],
  [3, false],
  [0, true],
])("isEven(%i) returns %s", (input, expected) => {
  expect(isEven(input)).toBe(expected);
});

Testing edge cases that are easy to get wrong:

function initials(name) {
  return name
    .trim()
    .split(/\s+/)
    .map((part) => part[0].toUpperCase())
    .join("");
}

test("handles extra whitespace", () => {
  expect(initials("  ada  lovelace ")).toBe("AL");
});

Common Mistakes

• Testing many behaviors in one giant test, so a failure is hard to diagnose
• Reaching into a database or API, which makes it an integration test, not a unit test
• Only testing the happy path and ignoring empty, null, or boundary inputs
• Coupling tests to private internals instead of the function's public output
• Duplicating logic from the code under test inside the assertion

See Also

testing-fundamentals testing-integration-tests testing-mocking