async-wait-until

A lightweight, zero-dependency library for waiting asynchronously until a specific condition is met. Works in any JavaScript environment that supports Promises, including older Node.js versions and browsers (with polyfills if necessary).

✨ Features

• 🚀 Zero dependencies - Lightweight and fast
• 🔧 TypeScript support - Full TypeScript definitions included
• 🌐 Universal compatibility - Works in Node.js and browsers
• ⚡ Flexible configuration - Customizable timeouts and intervals
• 🎯 Promise-based - Clean async/await syntax
• 📦 Multiple formats - UMD, ESM, and additional format bundles
• 🛡️ Error handling - Built-in timeout error handling

📚 Table of Contents

• Installation
• How to Use
• API Reference
• TypeScript Usage
• Recipes
• Browser Compatibility
• Troubleshooting
• Development and Testing
• Links

📖 Detailed Documentation

For detailed documentation, visit https://devlato.github.io/async-wait-until/

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🚀 Installation

Install using npm:

npm install async-wait-until

The library includes UMD and ESM bundles (plus additional formats), so you can use it in any environment.

import { waitUntil } from 'async-wait-until';

// Example: Wait for an element to appear
await waitUntil(() => document.querySelector('#target') !== null);

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🛠️ How to Use

Basic Example: Wait for a DOM Element

import { waitUntil } from 'async-wait-until';

const waitForElement = async () => {
  // Wait for an element with the ID "target" to appear
  const element = await waitUntil(() => document.querySelector('#target'), { timeout: 5000 });
  console.log('Element found:', element);
};

waitForElement();

Handling Timeouts

If the condition is not met within the timeout, a TimeoutError is thrown.

import { waitUntil, TimeoutError } from 'async-wait-until';

const waitForElement = async () => {
  try {
    const element = await waitUntil(() => document.querySelector('#target'), { timeout: 5000 });
    console.log('Element found:', element);
  } catch (error) {
    if (error instanceof TimeoutError) {
      console.error('Timeout: Element not found');
    } else {
      console.error('Unexpected error:', error);
    }
  }
};

waitForElement();

---

📚 API Reference

waitUntil(predicate, options)

Waits for the predicate function to return a truthy value and resolves with that value.

Parameters:

Name	Type	Required	Default	Description
predicate	Function	✅ Yes	-	A function that returns a truthy value (or a Promise for one).
options.timeout	number	🚫 No	5000 ms	Maximum wait time before throwing TimeoutError. Use WAIT_FOREVER for no timeout.
options.intervalBetweenAttempts	number	🚫 No	50 ms	Interval between predicate evaluations.

Exported Constants

Name	Value	Description
WAIT_FOREVER	∞	Use for infinite timeout (no time limit).
DEFAULT_TIMEOUT_IN_MS	5000	Default timeout duration in milliseconds.
DEFAULT_INTERVAL_BETWEEN_ATTEMPTS_IN_MS	50	Default interval between attempts in milliseconds.

Exported Classes

• TimeoutError - Error thrown when timeout is reached before condition is met.

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🔧 TypeScript Usage

This library is written in TypeScript and includes full type definitions. Here are some TypeScript-specific examples:

Basic TypeScript Usage

import { waitUntil, TimeoutError, WAIT_FOREVER } from 'async-wait-until';

// The return type is automatically inferred
const element = await waitUntil(() => document.querySelector('#target'));
// element is typed as Element | null

// With custom timeout and interval
const result = await waitUntil(
  () => someAsyncCondition(),
  { 
    timeout: 10000, 
    intervalBetweenAttempts: 100 
  }
);

Using with Async Predicates

// Async predicate example
const checkApiStatus = async (): Promise<boolean> => {
  const response = await fetch('/api/health');
  return response.ok;
};

try {
  await waitUntil(checkApiStatus, { timeout: 30000 });
  console.log('API is ready!');
} catch (error) {
  if (error instanceof TimeoutError) {
    console.error('API failed to become ready within 30 seconds');
  }
}

Type-Safe Options

import { Options } from 'async-wait-until';

const customOptions: Options = {
  timeout: 15000,
  intervalBetweenAttempts: 200
};

await waitUntil(() => someCondition(), customOptions);

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💡 Recipes

Wait Indefinitely

Use WAIT_FOREVER to wait without a timeout:

import { waitUntil, WAIT_FOREVER } from 'async-wait-until';

await waitUntil(() => someCondition, { timeout: WAIT_FOREVER });

Adjust Retry Interval

Change how often the predicate is evaluated:

await waitUntil(() => someCondition, { intervalBetweenAttempts: 1000 }); // Check every 1 second

Wait for API Response

const waitForApi = async () => {
  const response = await waitUntil(async () => {
    try {
      const res = await fetch('/api/status');
      return res.ok ? res : null;
    } catch {
      return null; // Keep trying on network errors
    }
  }, { timeout: 30000, intervalBetweenAttempts: 1000 });
  
  return response.json();
};

Wait for File System Changes (Node.js)

import fs from 'fs';
import { waitUntil } from 'async-wait-until';

// Wait for a file to be created
const filePath = './important-file.txt';
await waitUntil(() => fs.existsSync(filePath), { timeout: 10000 });

// Wait for file to have content
await waitUntil(() => {
  if (fs.existsSync(filePath)) {
    return fs.readFileSync(filePath, 'utf8').trim().length > 0;
  }
  return false;
});

Wait for Database Connection

const waitForDatabase = async (db) => {
  await waitUntil(async () => {
    try {
      await db.ping();
      return true;
    } catch {
      return false;
    }
  }, { timeout: 60000, intervalBetweenAttempts: 2000 });
  
  console.log('Database is ready!');
};

Wait with Custom Conditions

// Wait for multiple conditions
const waitForComplexCondition = async () => {
  return waitUntil(() => {
    const user = getCurrentUser();
    const permissions = getPermissions();
    const apiReady = isApiReady();
    
    // All conditions must be true
    return user && permissions.length > 0 && apiReady;
  });
};

// Wait for specific value ranges
const waitForTemperature = async () => {
  return waitUntil(async () => {
    const temp = await getSensorTemperature();
    return temp >= 20 && temp <= 25 ? temp : null;
  });
};

---

🌐 Browser Compatibility

This library works in any JavaScript environment that supports Promises:

Node.js: ✅ Version 0.14.0 and above
Modern Browsers: ✅ Chrome 32+, Firefox 29+, Safari 8+, Edge 12+
Legacy Browsers: ✅ With Promise polyfill (e.g., es6-promise)

CDN Usage

<!-- UMD bundle via CDN -->
<script src="https://unpkg.com/async-wait-until@latest/dist/index.js"></script>
<script>
  // Available as global variable
  asyncWaitUntil.waitUntil(() => document.querySelector('#target'))
    .then(element => console.log('Found:', element));
</script>

ES Modules in Browser

<script type="module">
  import { waitUntil } from 'https://unpkg.com/async-wait-until@latest/dist/index.esm.js';
  
  const element = await waitUntil(() => document.querySelector('#target'));
  console.log('Found:', element);
</script>

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🔍 Troubleshooting

Common Issues

Q: My predicate never resolves, what's wrong?
A: Make sure your predicate function returns a truthy value when the condition is met. Common mistakes:

• Forgetting to return a value: () => { someCheck(); } ❌
• Correct: () => { return someCheck(); } ✅ or () => someCheck() ✅

Q: I'm getting unexpected timeout errors
A: Check that:

• Your timeout is long enough for the condition to be met
• Your predicate function doesn't throw unhandled errors
• Network requests in predicates have proper error handling

Q: The function seems to run forever
A: This happens when:

• Using WAIT_FOREVER without proper condition logic
• Predicate always returns falsy values
• Add logging to debug: () => { const result = myCheck(); console.log(result); return result; }

Q: TypeScript compilation errors
A: Ensure you're importing types correctly:

import { waitUntil, Options, TimeoutError } from 'async-wait-until';

Performance Tips

• Use reasonable intervals (50-1000ms) to balance responsiveness and CPU usage
• For expensive operations, increase the interval: { intervalBetweenAttempts: 1000 }
• Implement proper error handling in async predicates to avoid unnecessary retries
• Consider using WAIT_FOREVER with external cancellation for long-running waits

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🧪 Development and Testing

Contributions are welcome! To contribute:

1. Fork and clone the repository.
2. Install dependencies: npm install.
3. Use the following commands:

• Run Tests: npm test
• Lint Code: npm run lint
• Format Code: npm run format
• Build Library: npm run build
• Generate Docs: npm run docs

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📝 Links

• License
• Detailed Documentation
• Changelog - Track version updates and changes
• Contributing Guidelines - How to contribute to the project
• Code of Conduct - Community standards and expectations