Promise Chaining & Error Handling
Promise Chaining Fundamentals
Promise chaining allows you to execute asynchronous operations in sequence, where each operation starts when the previous one completes.
Key Rule: A
.then() handler can return:- A value (which becomes the resolved value)
- Another promise (waits for it to settle)
- A thrown error (which becomes a rejected promise)
Example 1: Basic Promise Chaining
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✅ What You Can Return in .then()
- Primitive values:
return 42; - Objects:
return {data: 'value'}; - Another Promise:
return fetch(url); - Nothing:
return;(returns undefined) - Error:
throw new Error('msg');
⚠️ Common Mistakes
- Forgetting to return from .then()
- Not handling errors properly
- Creating promise pyramids (like callback hell)
- Mixing async/await with .then() unnecessarily
- Not cleaning up resources in finally()
Example 2: Different Return Types
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🔥 Error Handling Patterns
1. Global Catch
promise .then(step1) .then(step2) .then(step3) .catch(handleAllErrors)
2. Recovery After Error
promise
.then(step1)
.catch(error => {
// Recover
return defaultValue;
})
.then(continue)3. Rethrow Error
promise
.then(step1)
.catch(error => {
// Log and rethrow
console.error(error);
throw error;
})
.then(step2)Example 3: Error Handling in Chains
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Example 4: Parallel vs Sequential Execution
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📊 Promise.all() vs Promise.allSettled() vs Promise.race()
| Method | Behavior | Use Case | Result |
|---|---|---|---|
Promise.all() | All must succeed | When all promises are needed | Rejects if ANY fails |
Promise.allSettled() | Wait for all to complete | When you need all results regardless | Never rejects |
Promise.race() | First to settle wins | Timeout patterns, fastest response | First settled (resolved/rejected) |
Promise.any() | First to succeed wins | Multiple fallback sources | First resolved, rejects if ALL fail |
Example 5: Advanced Chain Patterns
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Exercise: Implement Promise Chain
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💡 Best Practices for Promise Chaining
- Always return promises from async functions
- Use
async/awaitfor complex chains - Handle errors at appropriate levels
- Use
Promise.all()for parallel independent operations - Avoid nesting promises (anti-pattern)
- Clean up resources in
finally()
