Your setImmediate() callback threw an unhandled exception or failed during execution. This error occurs because exceptions in setImmediate callbacks cannot be caught with try-catch and propagate unchecked through the Node.js event loop, crashing your process.
When you use setImmediate() to schedule a callback for execution after the current event loop phase completes, any error thrown within that callback executes asynchronously—after your synchronous code has already exited. Try-catch blocks only work for synchronous code in the same call stack, so they cannot intercept these asynchronous errors. When an error escapes a setImmediate callback, it propagates to the Node.js internals as an uncaught exception. If no global error handler is registered, Node.js terminates the entire process with an uncaught exception error.
Add a try-catch block inside your setImmediate callback to handle any synchronous errors that might occur during execution:
setImmediate(() => {
try {
// Synchronous operations
const data = JSON.parse(jsonString);
processData(data);
} catch (error) {
console.error('Error in setImmediate callback:', error);
// Handle error appropriately
}
});This catches synchronous errors like JSON parsing failures or TypeError from undefined properties, but will not catch asynchronous errors (like rejected promises).
Add a global uncaught exception handler at the top level of your application to catch errors that escape setImmediate callbacks:
process.on('uncaughtException', (error) => {
console.error('Uncaught exception:', error);
// Log the error, send to monitoring service, or gracefully shutdown
// Important: After this handler, Node.js behavior is unpredictable
// Consider restarting the process
process.exit(1);
});
setImmediate(() => {
throw new Error('This error will be caught by uncaughtException handler');
});Note: After an 'uncaughtException' handler runs, the application is in an undefined state. Best practice is to log the error and restart the process.
If you use promises within setImmediate, register a handler for unhandled promise rejections:
process.on('unhandledRejection', (reason, promise) => {
console.error('Unhandled rejection at:', promise, 'reason:', reason);
// Handle the rejected promise
});
setImmediate(async () => {
// This rejection would be caught by unhandledRejection handler
return Promise.reject(new Error('Async operation failed'));
});This is especially important if you're using async/await or returning promises from setImmediate callbacks.
Instead of throwing errors, use error-first callback patterns or promises to propagate errors from setImmediate:
// Good: Using error-first callback pattern
function asyncOperation(callback) {
setImmediate(() => {
try {
const result = performSomeOperation();
callback(null, result);
} catch (error) {
callback(error);
}
});
}
asyncOperation((error, result) => {
if (error) {
console.error('Operation failed:', error);
return;
}
console.log('Result:', result);
});
// Alternative: Using promises
function asyncOperationPromise() {
return new Promise((resolve, reject) => {
setImmediate(() => {
try {
const result = performSomeOperation();
resolve(result);
} catch (error) {
reject(error);
}
});
});
}
asyncOperationPromise()
.then(result => console.log('Result:', result))
.catch(error => console.error('Operation failed:', error));This approach gives you control over error handling without relying on global handlers.
For cleaner code, use async/await inside setImmediate, which works well with try-catch:
async function main() {
setImmediate(async () => {
try {
const result = await someAsyncOperation();
console.log('Success:', result);
} catch (error) {
console.error('Error in async setImmediate:', error);
// Handle error without crashing the process
}
});
}
main().catch(error => {
console.error('Fatal error:', error);
process.exit(1);
});Async functions return promises, which can be properly awaited and error-handled with try-catch.
Add defensive checks to prevent runtime errors from undefined or invalid data:
setImmediate(() => {
try {
// Defensive checks
if (!data || typeof data !== 'object') {
console.error('Invalid data received');
return;
}
if (!data.id || typeof data.id !== 'number') {
console.error('Invalid data.id');
return;
}
// Safe to proceed
processData(data);
} catch (error) {
console.error('Error processing data:', error);
}
});This prevents TypeError exceptions from accessing properties on null/undefined values.
setImmediate schedules callbacks for execution during the Check phase of the Node.js event loop, after the I/O phase completes. Importantly, errors thrown in setImmediate callbacks cannot be caught by try-catch blocks because the callback executes asynchronously in a different call stack. Starting with Node.js 13+, error handling in setImmediate became more deterministic—if an error occurs during Immediate processing, the remaining queue is scheduled to finish after all error handling code runs. For mission-critical operations, consider using async/await with promises instead of raw setImmediate, as they provide more predictable error handling semantics. In Express.js applications, errors from setImmediate should never be thrown directly; instead, pass them to the next() middleware to be caught by error handling middleware. If you're using worker threads or child processes, remember that errors in the main process and worker thread need separate handlers.
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