How to Handle Uncaught (in promise) TypeError

You’re debugging your frontend application when the console displays: Uncaught (in promise) TypeError: Cannot read property 'type' of undefined. This error message contains two distinct problems that developers often conflate. Understanding both is essential for proper JavaScript Promise error handling.

This article explains what this error actually means, why it appears, and how to handle unhandled Promise rejections correctly in browser environments.

What ‘Uncaught (in promise) TypeError’ Actually Means

This error message communicates two separate issues:

1. TypeError: A runtime error occurred—typically from accessing a property on undefined or null.
2. Uncaught (in promise): The Promise that produced this error had no rejection handler attached.

The browser distinguishes these because Promise rejections behave differently from synchronous errors. When a synchronous TypeError occurs, execution stops immediately. When the same error occurs inside a Promise, the rejection propagates through the Promise chain. If no .catch() or try/catch handles it, the browser logs it as an unhandled Promise rejection.

For a deeper reference on Promise behavior and error propagation, see the MDN documentation on Promises.

Common Root Causes in Frontend Code

Errors Thrown Inside Async Functions

Any exception inside an async function automatically rejects the returned Promise:

async function fetchUserData(userId) {
  const response = await fetch(`/api/users/${userId}`)
  const data = await response.json()
  return data.profile.name // TypeError if profile is undefined
}

fetchUserData(123) // Floating Promise—no handler attached

Missing Await Keywords

Forgetting await creates a floating Promise that executes independently:

async function processData() {
  fetchUserData(123) // Missing await—rejection goes unhandled
  console.log('Done')
}

Forgotten .catch() Handlers

Promise chains without a terminal .catch() block leave rejections unhandled:

fetch('/api/data')
  .then(res => res.json())
  .then(data => data.items[0].name) // No .catch()—TypeError propagates

Late Handler Attachment and Microtask Timing

Browsers determine whether a rejection is handled after the current microtask checkpoint. If a rejection does not have a handler by that point, the browser may emit an unhandledrejection event and log a console warning. Attaching a handler shortly afterward can still trigger the warning in development builds:

const promise = Promise.reject(new Error('Failed'))

promise.catch(err => console.log('Handled'))

This timing nuance explains why some properly handled rejections can still appear as unhandled during development.

Proper Async/Await Error Handling Patterns

Local try/catch Around Await

The most reliable pattern wraps await expressions in try/catch:

async function fetchPokemon(id) {
  try {
    const response = await fetch(`https://pokeapi.co/api/v2/pokemon/${id}`)
    const data = await response.json()
    return {
      name: data.name,
      type: data.types[0]?.type.name,
      type2: data.types[1]?.type.name ?? null
    }
  } catch (error) {
    console.error('Fetch failed:', error.message)
    return null
  }
}

Returning Promises with Attached Handlers

When calling async functions, always handle the returned Promise:

// Option 1: await with try/catch
await fetchPokemon(25)

// Option 2: .catch() on the call
fetchPokemon(25).catch(handleError)

Chaining .catch() Appropriately

Place .catch() at the end of Promise chains to capture any rejection in the chain:

Promise.all(urls.map(url => fetch(url).then(r => r.json())))
  .then(results => processResults(results))
  .catch(error => showErrorUI(error))

Using the unhandledrejection Event for Monitoring

Browsers provide the unhandledrejection event for global error monitoring—not as a primary error handling mechanism:

window.addEventListener('unhandledrejection', event => {
  // Log to error monitoring service
  errorTracker.capture(event.reason)

  // Optionally prevent the default console error
  event.preventDefault()
})

The companion rejectionhandled event fires when a previously unhandled rejection later receives a handler. These events are useful for telemetry and for catching errors that slip through your local handling, but they should not replace proper try/catch and .catch() patterns.

Conclusion

The “Uncaught (in promise) TypeError” message signals both a runtime error and missing error handling. Address both sides of the problem: use optional chaining (?.) and nullish coalescing (??) to prevent TypeErrors from undefined properties, and wrap async operations in try/catch or attach .catch() handlers to every Promise chain. Reserve the unhandledrejection event for monitoring, not control flow. These patterns will keep your frontend code resilient and your console clean.

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