This error occurs when a callback function passed to process.nextTick() throws an uncaught exception. Since nextTick callbacks execute before the event loop continues, unhandled errors in these callbacks will crash the Node.js process.
This error indicates that an exception was thrown inside a callback function passed to `process.nextTick()`, and that exception was not caught or handled properly. The `process.nextTick()` function is used to defer the execution of a callback until after the current operation completes, but before the event loop continues to the next phase. When an error occurs within a nextTick callback, Node.js attempts to propagate it through the normal error handling mechanisms. However, if no error handler catches it (such as an 'uncaughtException' listener or domain error handler), the process will terminate with this error message. This is particularly problematic because nextTick callbacks execute outside the normal event loop phases, making error handling more challenging. The error represents a critical failure in asynchronous code that was scheduled to run immediately after the current operation.
Look at the complete error stack trace to identify which callback is throwing the error:
Error: Callback in process.nextTick threw
at processTicksAndRejections (internal/process/task_queues.js:83:21)
at myFunction (/app/index.js:25:15)The stack trace will show you the exact file and line number where the error originated. Focus on your application code, not the internal Node.js frames.
Add error handling around the code inside your nextTick callbacks:
// Before (unsafe)
process.nextTick(() => {
riskyOperation(); // May throw
});
// After (safe)
process.nextTick(() => {
try {
riskyOperation();
} catch (error) {
console.error('Error in nextTick callback:', error);
// Handle error appropriately
}
});This prevents the error from crashing your process and allows you to handle it gracefully.
Implement a global uncaughtException handler to catch errors that escape local handling:
process.on('uncaughtException', (error, origin) => {
console.error('Uncaught Exception:', error);
console.error('Origin:', origin);
// Log to your error tracking service
// logger.error(error);
// Gracefully shutdown
process.exit(1);
});Note: This should be a last resort. It's better to handle errors properly in your callbacks rather than relying on this global handler.
When using process.nextTick() to defer error handling, follow the error-first callback pattern:
function asyncOperation(callback) {
// Validate inputs
if (!callback || typeof callback !== 'function') {
throw new TypeError('Callback must be a function');
}
// Use nextTick for error propagation
if (someErrorCondition) {
return process.nextTick(callback, new Error('Something went wrong'));
}
// Success case
process.nextTick(callback, null, result);
}
// Usage
asyncOperation((error, result) => {
if (error) {
console.error('Operation failed:', error);
return;
}
console.log('Success:', result);
});This ensures errors are properly propagated through your callback chain.
Modern Node.js code should prefer Promises over callback-based patterns with nextTick:
// Instead of this:
process.nextTick(() => {
try {
const result = riskyOperation();
callback(null, result);
} catch (error) {
callback(error);
}
});
// Use this:
async function safeOperation() {
try {
const result = await riskyOperation();
return result;
} catch (error) {
console.error('Operation failed:', error);
throw error; // Let caller handle it
}
}
// Or with setImmediate for similar deferred execution:
setImmediate(async () => {
try {
await safeOperation();
} catch (error) {
console.error('Deferred operation failed:', error);
}
});Promises and async/await provide better error handling and stack traces.
### Understanding process.nextTick() Execution Timing
The process.nextTick() queue is processed completely before the event loop continues to any I/O operations, timers, or other asynchronous tasks. This means nextTick callbacks can starve the event loop if used recursively or excessively.
### nextTick vs setImmediate
While both defer execution, setImmediate() is often safer for recursive operations because it yields to the event loop, allowing I/O operations to proceed:
// Can starve I/O
function recursiveNextTick() {
process.nextTick(recursiveNextTick);
}
// Allows I/O to proceed
function recursiveImmediate() {
setImmediate(recursiveImmediate);
}### Error Handling in Different Node.js Contexts
- Callbacks: Use error-first pattern (err, result)
- Promises: Use .catch() or try/catch with async/await
- Event Emitters: Listen for 'error' events
- Streams: Handle 'error' events on both readable and writable streams
### Production Considerations
In production environments, always log errors to a monitoring service before exiting:
process.on('uncaughtException', async (error) => {
await logger.fatal('Uncaught Exception', { error });
await cleanup(); // Close DB connections, etc.
process.exit(1);
});Never continue running the process after an uncaughtException - the application state may be inconsistent. Always gracefully shutdown and restart via a process manager like PM2 or systemd.
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