async-wait-until
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Waits until the given predicate function returns a truthy value, then resolves
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# 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](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise), including older Node.js versions and browsers (with polyfills if necessary).
[](https://npmjs.org/package/async-wait-until)
[](https://qlty.sh/gh/devlato/projects/async-wait-until)
## ✨ 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](#-installation)
- [How to Use](#️-how-to-use)
- [API Reference](#-api)
- [TypeScript Usage](#-typescript-usage)
- [Recipes](#-recipes)
- [Browser Compatibility](#-browser-compatibility)
- [Troubleshooting](#-troubleshooting)
- [Development and Testing](#-development-and-testing)
- [Links](#-links)
## 📖 Detailed Documentation
For detailed documentation, visit [https://devlato.github.io/async-wait-until/](https://devlato.github.io/async-wait-until/)
## 🚀 Installation
Install using npm:
```sh
npm install async-wait-until
```
The library includes UMD and ESM bundles (plus additional formats), so you can use it in any environment.
```javascript
import { waitUntil } from 'async-wait-until';
// Example: Wait for an element to appear
await waitUntil(() => document.querySelector('#target') !== null);
```
## 🛠️ How to Use
### Basic Example: Wait for a DOM Element
```javascript
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.
```javascript
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.
## 🔧 TypeScript Usage
This library is written in TypeScript and includes full type definitions. Here are some TypeScript-specific examples:
### Basic TypeScript Usage
```typescript
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
```typescript
// 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
```typescript
import { Options } from 'async-wait-until';
const customOptions: Options = {
timeout: 15000,
intervalBetweenAttempts: 200
};
await waitUntil(() => someCondition(), customOptions);
```
## 💡 Recipes
### Wait Indefinitely
Use `WAIT_FOREVER` to wait without a timeout:
```javascript
import { waitUntil, WAIT_FOREVER } from 'async-wait-until';
await waitUntil(() => someCondition, { timeout: WAIT_FOREVER });
```
### Adjust Retry Interval
Change how often the predicate is evaluated:
```javascript
await waitUntil(() => someCondition, { intervalBetweenAttempts: 1000 }); // Check every 1 second
```
### Wait for API Response
```javascript
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)
```javascript
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
```javascript
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
```javascript
// 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
```html
<!-- 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
```html
<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>
```
## 🔍 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:
```typescript
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
## 🧪 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`
## 📝 Links
- [License](./LICENSE)
- [Detailed Documentation](https://devlato.github.io/async-wait-until/)
- [Changelog](./CHANGELOG.md) - Track version updates and changes
- [Contributing Guidelines](./CONTRIBUTING.md) - How to contribute to the project
- [Code of Conduct](./CODE_OF_CONDUCT.md) - Community standards and expectations