Testing Fundamentals

Overview

Automated tests are code that checks whether your other code behaves as expected. They catch bugs before users do, document how a system is meant to work, and let you change code with confidence. Most test suites follow the same shape: set up some inputs, run the code, and assert the result matches what you expected.

Syntax / Usage

A test file describes behavior with a name and verifies it with an assertion. The common pattern is Arrange–Act–Assert: prepare inputs, call the code, then check the outcome.

// sum.js
function sum(a, b) {
  return a + b;
}
module.exports = { sum };

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

describe("sum", () => {
  test("adds two positive numbers", () => {
    // Arrange
    const a = 2;
    const b = 3;
    // Act
    const result = sum(a, b);
    // Assert
    expect(result).toBe(5);
  });
});

Run tests with npx jest. A green run means every assertion passed.

Examples

A test that checks a specific expected value:

test("adds negative numbers", () => {
  expect(sum(-4, -6)).toBe(-10);
});

A test that verifies an error is thrown for bad input:

function divide(a, b) {
  if (b === 0) throw new Error("Cannot divide by zero");
  return a / b;
}

test("throws when dividing by zero", () => {
  expect(() => divide(10, 0)).toThrow("Cannot divide by zero");
});

Common Mistakes

• Writing tests that assert nothing, so they always pass no matter what
• Testing implementation details instead of observable behavior, making tests brittle
• Depending on test order, where one test only passes because another ran first
• Using vague test names like "works" that don't describe the expected behavior
• Skipping the "arrange" step and reusing shared mutable state between tests

See Also

testing-unit-tests testing-integration-tests testing-code-coverage