<!--
title: Concurrent operations completion management
domain: app-frameworks
topic: Concurrency
language: JavaScript
source: fastify/fastify
updated: 
url: https://awesomereviewers.com/reviewers/fastify-concurrent-operations-completion-management/
-->

When running concurrent operations in tests, ensure all operations complete before concluding the test. Race conditions occur when your test calls `done()` or resolves before all async operations finish, leading to flaky tests or missed assertions.

**For multiple concurrent HTTP/API calls:**
```javascript
// AVOID: Race condition - test might complete before all operations finish
test('API calls', (t, testDone) => {
  sget(url1, (err, res) => {
    t.assert.ifError(err)
    t.assert.strictEqual(res.statusCode, 200)
    testDone() // BAD: Other calls might still be running!
  })
  sget(url2, (err, res) => { /* assertions */ })
})

// BETTER: Use Promise.all for truly concurrent operations
test('API calls', async (t) => {
  const results = await Promise.all([
    fetch(url1),
    fetch(url2),
    fetch(url3)
  ])
  
  // All operations are complete before assertions run
  t.assert.strictEqual(results[0].status, 200)
  t.assert.strictEqual(results[1].status, 200)
})

// ALTERNATIVE: Use completion counter for callback style
test('API calls', (t, testDone) => {
  let pending = 3
  function completed() {
    if (--pending === 0) {
      testDone()
    }
  }
  
  sget(url1, (err, res) => {
    t.assert.ifError(err)
    t.assert.strictEqual(res.statusCode, 200)
    completed()
  })
  sget(url2, (err, res) => {
    t.assert.ifError(err)
    completed()
  })
  sget(url3, (err, res) => {
    t.assert.ifError(err)
    completed()
  })
})

// UTILITY APPROACH: Use waitForCb or similar utilities
test('API calls', (t, testDone) => {
  const completion = waitForCb({ steps: 3 })
  
  sget(url1, (err, res) => {
    t.assert.ifError(err)
    t.assert.strictEqual(res.statusCode, 200)
    completion.stepIn()
  })
  sget(url2, (err, res) => {
    // assertions
    completion.stepIn()
  })
  sget(url3, (err, res) => {
    // assertions
    completion.stepIn()
  })
  
  completion.patience.then(testDone)
})
```