6 min read

uvu + Sinon: Fast, Lightweight Testing That Actually Feels Good

# testing# javascript# uvu# sinon# jest# performance

I want to talk about testing frameworks. Jest dominated for years. Then Vitest came along and made things faster. But they’re both still heavy, complex tools with lots of magic.

I tried a lightweight approach on my newsletter digester project and testing feels like magic. 205 tests run in 190ms. 🤯 The entire test suite including mocking external APIs completes faster than Jest can even start up.

Let me show you why uvu + Sinon is worth considering.

The Testing Framework Fatigue

Jest changed JavaScript testing. Before Jest, testing was fragmented and painful. Jest unified everything: test runner, assertions, mocking, coverage. One tool, everything works.

But Jest is slow. Really slow. Even with the --maxWorkers flag and all the optimizations, a medium-sized test suite takes 5-10 seconds. Large suites take minutes.

Vitest improved this significantly. It’s faster, uses Vite’s infrastructure, and has a better developer experience. But it’s still 10MB+ of dependencies with lots of moving parts.

For my newsletter digester, I wanted something simpler. Something that runs tests fast without needing configuration or magic.

uvu: The 5KB Test Runner

uvu is a test runner that’s 5KB. Five kilobytes. It has one job: run tests fast.

No built-in assertions (use Node’s assert or any library). No built-in mocking (use Sinon). No magic. Just a blazingly fast test runner.

Here’s what 205 tests look like with uvu (using a custom reporter I built to see the whole picture - more on that below):

config-api

    ✓ getAll should return config as object
    ✓ getAll should return all config keys
    ✓ update should update config values
    ✓ update should update multiple config values
    ✓ update should handle schedule updates and trigger cron reschedule
    ✓ update should handle empty body
    ✓ testAI should return 400 when API key missing
    ✓ testAI should return 400 when base URL missing
    ✓ testAI should return 400 when both key and URL missing

cron-api

    ✓ runNow should trigger background check and return success
    ✓ runNow should not wait for check to complete
    ✓ runNow should start check even with no active sites
    ✓ runNow should handle multiple concurrent calls

...

Tests:  205 passed, 205 total
Time:   0.19s

190 milliseconds for 205 tests. That includes database operations, API mocking, and full integration tests.

Jest takes longer just to initialize.

The Setup

Install uvu and sinon:

npm install -D uvu sinon c8

That’s 3 packages. Total size: ~1MB. Compare that to Jest’s 30MB or Vitest’s 10MB.

Here’s a test file:

import { suite } from "uvu";
import * as assert from "uvu/assert";
import sinon from "sinon";

const SitesDB = suite("Sites DB");

SitesDB.before.each(context => {
  // Setup runs before each test
  context.db = initTestDatabase();
  context.stubs = [];
});

SitesDB.after.each(async context => {
  // Cleanup runs after each test
  context.stubs.forEach(stub => stub.restore());
  await context.db.close();
});

SitesDB("should create a new site", async ({ db }) => {
  const site = await db.createSite({
    url: "https://example.com/rss",
    title: "Test Blog",
    type: "rss",
  });

  assert.ok(site.id);
  assert.is(site.title, "Test Blog");
  assert.is(site.type, "rss");
});

SitesDB("should fetch all sites", async ({ db }) => {
  await db.createSite({
    url: "https://example.com/rss",
    title: "Blog 1",
    type: "rss",
  });
  await db.createSite({
    url: "https://example.com/feed",
    title: "Blog 2",
    type: "rss",
  });

  const sites = await db.getAllSites();

  assert.is(sites.length, 2);
});

SitesDB.run();

Clean, simple, fast. No magic, no hidden configuration.

Mocking with Sinon

uvu doesn’t include mocking. That’s by design. Use Sinon, which is the industry standard for mocking.

Here’s how I mock OpenAI API calls:

import { suite } from "uvu";
import * as assert from "uvu/assert";
import sinon from "sinon";
import axios from "axios";

const Extractors = suite("Extractors");

Extractors("should extract posts using LLM", async () => {
  // Mock axios to return HTML
  const axiosStub = sinon.stub(axios, "get").resolves({
    data: "<html><article><h2>Post Title</h2></article></html>",
  });

  // Mock OpenAI response
  const openaiStub = sinon.stub(global, "fetch").resolves({
    ok: true,
    json: async () => ({
      choices: [
        {
          message: {
            content: JSON.stringify([
              { title: "Post Title", url: "https://example.com/post1" },
            ]),
          },
        },
      ],
    }),
  });

  const posts = await extractWithLLM("https://example.com");

  assert.is(posts.length, 1);
  assert.is(posts[0].title, "Post Title");

  // Cleanup
  axiosStub.restore();
  openaiStub.restore();
});

Extractors.run();

Sinon stubs are powerful. You can stub any function, spy on calls, fake timers, create test doubles. Everything you need for comprehensive testing.

Custom Test Reporter

uvu’s default output is minimal. I wanted something prettier with test names and clear visual hierarchy.

I built a custom test runner that parses uvu output and reformats it:

#!/usr/bin/env node
import { exec } from "child_process";
import { promisify } from "util";

const execAsync = promisify(exec);

const colors = {
  reset: "\x1b[0m",
  green: "\x1b[32m",
  red: "\x1b[31m",
  gray: "\x1b[90m",
  bold: "\x1b[1m",
};

async function runTests() {
  const startTime = Date.now();

  const { stdout } = await execAsync(
    "NODE_OPTIONS=--experimental-vm-modules node node_modules/uvu/bin.js src/server/__tests__"
  );

  // Parse and reformat output
  const lines = stdout.split("\n");
  let passed = 0;

  for (const line of lines) {
    if (line.includes(".test.js")) {
      const fileName = line.replace(/.*\//, "").replace(".test.js", "");
      console.log(`${colors.bold}${fileName}${colors.reset}`);
    } else if (line.includes("✓")) {
      const testName = extractTestName(line);
      console.log(
        `    ${colors.green}✓${colors.reset} ${colors.gray}${testName}${colors.reset}`
      );
      passed++;
    }
  }

  const duration = ((Date.now() - startTime) / 1000).toFixed(2);

  console.log("");
  console.log(
    `${colors.bold}Tests:${colors.reset}  ${colors.green}${passed} passed${colors.reset}, ${passed} total`
  );
  console.log(`${colors.bold}Time:${colors.reset}   ${duration}s`);
}

runTests();

This gives me nice, readable output with test names, file grouping, and timing:

config-api
    ✓ getAll should return config as object
    ✓ update should update config values
    ✓ testAI should validate API keys

cron
    ✓ runCheck() should process active sites
    ✓ updateSchedule() should validate cron expressions

Tests:  205 passed, 205 total
Time:   0.19s

Clean, fast, informative.

Coverage with c8

uvu doesn’t include coverage. Use c8, which is a native V8 coverage tool:

{
  "scripts": {
    "test": "uvu src/server/__tests__",
    "test:coverage": "c8 uvu src/server/__tests__"
  }
}

Run npm run test:coverage and you get full coverage reports:

File                | % Stmts | % Branch | % Funcs | % Lines
--------------------|---------|----------|---------|--------
All files           |   94.23 |    89.47 |   92.85 |   94.23
 api/config.js      |     100 |      100 |     100 |     100
 api/cron.js        |     100 |      100 |     100 |     100
 api/sites.js       |   96.15 |    88.23 |     100 |   96.15
 cron.js            |   91.66 |    85.71 |      90 |   91.66
 db.js              |   97.56 |    92.85 |     100 |   97.56

c8 is fast and accurate because it uses Node’s built-in coverage.

Try It Yourself

The newsletter digester project uses uvu for all its tests:

github.com/mfyz/newsletter-blog-digester

Clone it, run npm test. You’ll see 205 tests complete in under 200ms.

Look at the test files in src/server/__tests__/. They’re simple, explicit, and fast.

The custom test runner is in run-tests.js. Feel free to copy it for your own projects.

Testing Should Be Fast

We’ve normalized slow test suites. “Tests take 30 seconds” is considered acceptable. Some projects have test suites that take minutes.

It doesn’t have to be this way.

uvu proves that testing can be fast without sacrificing capability. 205 comprehensive tests in 190ms. That’s not magic, that’s just good engineering.

Fast tests change how you work. They become part of your development flow instead of something you run occasionally. That makes you more productive and your code more reliable.

Try uvu. You might not go back.

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