What is the difference between unit, API and end-to-end tests?

A unit test checks one function directly, an API test sends HTTP requests to the server with no browser, and an end-to-end test drives a real browser through the app the way a user does. They trade speed for coverage: each level runs slower than the one below it but proves more of the system works together.

This answer is one piece of Chapter 29: What to Automate.

What a Test Is Actually For
The Cost of a Test Over Its Lifetime
Where the Check Belongs: Unit, API or UI
Tests That Never Earn Their Keep
Answering "Automate Everything"

The difference is how much of the system each test touches. A unit test calls a function and checks what it returns, so a failure points at that function. An API test checks that the server accepts and answers requests correctly. An end-to-end test proves the screens, the server and the browser are joined up, but a failure could come from any of them.

A useful rule from the course: put a check at the lowest level that can still see the behaviour, and use the level above to prove the pieces are connected. Fourteen password rules belong in unit tests; one browser test proves the form shows the message.

Playwright can run all three in one project. The example has one of each. The unit test calls formatInr() with no browser and no network. The API test uses the request fixture to call /api/products and checks the total is 24. The end-to-end test opens the catalogue, adds the grinder and checks the cart total. The timings in the run show the gap: 70ms, 255ms and 1.2s.

levels.spec.js(https://practice.learnplayforge.com)

import { test, expect } from '@playwright/test';

// A tiny function to unit test: no browser, no network.
const formatInr = (n) => '₹' + n.toLocaleString('en-IN');

test('unit: formatInr formats a price', () => {
  expect(formatInr(11900)).toBe('₹11,900');
});

test('API: the products endpoint returns 24 items', async ({ request }) => {
  const response = await request.get('/api/products');
  await expect(response).toBeOK();
  expect((await response.json()).total).toBe(24);
});

test('end to end: the grinder reaches the cart', async ({ page }) => {
  await page.goto('/catalog.html?mock=static&seed=42&autologin=1');
  await page.locator('article[data-sku="MRD-1003"]')
    .getByRole('button', { name: 'Add to cart' }).click();
  await expect(page.getByTestId('cart-total')).toHaveText('₹11,900');
});

Output

  ok 1 › unit: formatInr formats a price (70ms)
  ok 2 › API: the products endpoint returns 24 items (255ms)
  ok 3 › end to end: the grinder reaches the cart (1.2s)

  3 passed (2.9s)

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A pricing rule has fourteen edge cases. Where do most of those checks belong?

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What is the difference between unit, API and end-to-end tests?

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