#javascript

Master async programming. Patterns: 1. async/await for readability. 2. Promise.all for parallel execution. 3. Promise.allSettled for all results. 4. Promise.race for timeout handling. 5. Try/catch for error handling. 6. Avoid callback hell. 7. Handle unhandled rejections. 8. Sequential vs parallel trade-offs. Use for I/O operations. Don't block event loop. Implement retry logic for resilience.

Write a set of unit tests for the following JavaScript function, which takes an array of numbers and returns the sum. Use a testing framework like Jest. Cover edge cases like an empty array, an array with non-numeric values, and a very large array.

I have an old JavaScript codebase that uses ES5 syntax (var, function declarations). Help me modernize it to ES6+ syntax (let/const, arrow functions, classes). Take the following ES5 code and rewrite it using modern JavaScript features.

I'm getting the following error message in my browser console: "Uncaught TypeError: Cannot read properties of undefined (reading 'map')". Explain what this error means in simple terms, what are the common causes for it, and how I can go about debugging it in my JavaScript code.

I need to add charts and graphs to my web application. What are some popular JavaScript libraries for data visualization? Compare libraries like D3.js, Chart.js, and Highcharts based on ease of use, customization options, and performance.

Explain how `async` and `await` work in JavaScript for handling asynchronous operations. How do they differ from using `.then()` with Promises? Provide a code example that fetches data from an API, first using Promises with `.then()` and then refactored to use `async/await`.

Rewrite the following imperative JavaScript code, which uses a for-loop to double every number in an array, into a functional style. Use higher-order functions like `map`. Explain the benefits of the functional approach, such as immutability and readability.

Show me how to create a simple, self-contained Web Component for a custom button. The component should use a Shadow DOM to encapsulate its styles and markup. It should also have a `disabled` attribute that can be toggled. Use vanilla JavaScript (ES6 classes).

I have a computationally expensive task in my web app that is blocking the main UI thread. Show me how to offload this task to a Web Worker. Provide the code for the main script that creates the worker and the code for the worker script itself (`worker.js`) that performs the calculation and sends the result back.