Race conditions in JavaScript occur when multiple asynchronous operations compete, leading to unpredictable outcomes; to handle them: 1. Cancel previous requests using AbortController to ensure only the latest request affects the UI; 2. Track request order with sequence numbers to process only the most recent response when cancellation isn’t possible; 3. Use async/await with proper error handling and update shared state only when the response is the latest; 4. Avoid shared state in concurrent operations by using immutable updates, state management libraries, or queuing mechanisms to process tasks sequentially; 5. Use Promise.race() only when the fastest result is intended, otherwise prefer Promise.all() or Promise.allSettled() to handle all results reliably—always plan for unpredictable completion order by controlling how results are applied.
Race conditions in JavaScript happen when multiple asynchronous operations compete to complete first, and the final outcome depends on the order they finish — which you can't reliably predict. This is common in web apps when making multiple API calls, handling user interactions, or managing state updates. Here’s how to handle them effectively.
1. Cancel Previous Requests Using AbortController
One of the most common race conditions occurs in search inputs or auto-suggestions, where fast user typing leads to multiple API calls, and slower ones may return after faster ones, showing outdated results.
Solution: Cancel outdated requests using AbortController.
let controller;
async function search(query) {
// Cancel any previous request
if (controller) {
controller.abort();
}
controller = new AbortController();
try {
const response = await fetch(`/api/search?q=${query}`, {
signal: controller.signal,
});
const results = await response.json();
updateUI(results); // Only the latest query updates UI
} catch (error) {
if (error.name !== 'AbortError') {
console.error('Fetch error:', error);
}
}
}This ensures only the most recent request affects the UI — older ones are canceled before they resolve.
2. Track Request Order with Sequence Numbers
Sometimes you can't cancel a request (e.g., older browsers, non-fetch APIs), but you still want to ignore outdated responses.
Solution: Assign a sequence number to each request and only process the latest one.
let requestCount = 0;
async function fetchData(query) {
const currentRequestId = requestCount;
const response = await fetch(`/api/data?q=${query}`);
const data = await response.json();
// Only handle result if this is the latest request
if (currentRequestId === requestCount) {
updateUI(data);
}
// Ignore if a newer request has already been made
}This pattern is useful when cancellation isn’t possible or when working with polling mechanisms.
3. Use Async/Await with Proper Error Handling
Race conditions can also occur when multiple async operations modify shared state. For example, two fetch calls updating the same variable might overwrite each other.
Solution: Ensure state updates are based on the latest data, or use locking patterns if needed.
let appState = { data: null };
async function refreshData() {
const response = await fetch('/api/latest');
const newData = await response.json();
// Only update if this response is intended to be the latest
// (Combine with sequence or timestamp checks if needed)
appState.data = newData;
}If multiple async functions can mutate appState, consider using a state management approach (like a reducer or mutex-style guard) to serialize updates.
4. Avoid Shared State in Concurrent Operations
When multiple async functions write to the same variable without coordination, race conditions are inevitable.
Best Practices:
- Use immutable updates.
- Avoid side effects in async functions.
- Use state management libraries (e.g., Redux, Zustand) that handle ordering and updates predictably.
Alternatively, queue operations if order matters:
const queue = [];
let isProcessing = false;
async function enqueue(task) {
queue.push(task);
if (!isProcessing) {
processQueue();
}
}
async function processQueue() {
isProcessing = true;
while (queue.length > 0) {
const task = queue.shift();
await task(); // Process one at a time
}
isProcessing = false;
}This ensures tasks run sequentially and don’t interfere.
5. Use Promises Carefully with Promise.race()
Promise.race() intentionally returns the first settled promise — which is designed for race conditions. Use it only when you truly want the fastest result.
But if you need all results or the last result, use:
-
Promise.all()– waits for all. -
Promise.allSettled()– waits for all, even if some fail. - Avoid
Promise.race()unless the race is intentional.
Handling race conditions comes down to:
- Canceling or ignoring outdated operations
- Tracking request order
- Avoiding uncoordinated state mutations
- Using proper async patterns
It’s not always about preventing concurrency — it’s about controlling how results are applied.
Basically: always assume async operations finish in unpredictable order — plan accordingly.
The above is the detailed content of How to Handle Race Conditions in JavaScript. For more information, please follow other related articles on the PHP Chinese website!
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