Web Workers is a multi-threaded mechanism provided by the browser to solve the problem of page lag caused by single thread in JavaScript. 1. It runs in an independent context and cannot access the DOM. It is suitable for processing time-consuming tasks such as data encryption, image processing, and large number of JSON parsing; 2. Creation is divided into three steps: writing worker scripts, creating Worker instances, and communicating through postMessage and onmessage; 3. When using it, you need to pay attention to the data transfer as copy rather than reference. Frequent communication and big data transmission may affect performance, and the debugging method is also different; 4. The standards for determining whether to use Web Worker include the function execution time exceeding 50ms, the page lag caused by the task or the asynchronous completion does not affect the main process. The rational use of Web Workers can significantly improve application responsiveness and user experience.

JavaScript is single-threaded, which means it can only do one thing at the same time. If you perform a time-consuming operation in the main thread, such as parsing a large amount of data or performing complex calculations, the page will be stuck and the user interface will not respond. This experience is awful, especially in the context of increasingly complex modern web pages.

The solution is to hand over heavy tasks to Web Workers, so that they can run in the background without interfering with the main thread.

What are Web Workers?

Web Workers is a multi-threaded mechanism provided by the browser that allows you to run scripts outside the main JavaScript thread. They run in a separate context and cannot directly access the DOM, but can handle compute-intensive tasks such as:

• Data encryption
• Data preparation before image processing
• A large number of JSON parsing
• AI computing in the game

You can think of it as a "behind the scenes employee", you are only responsible for assigning tasks and receiving results, and it handles the intermediate process by itself.

How to create and use a Web Worker?

Creating a Web Worker is very simple, and it is mainly divided into three steps:

1. Write a worker script file
   For example, worker.js writes the task logic you want to execute.

2. Create Worker instance in the main thread

    const myWorker = new Worker('worker.js');

3. Send messages through postMessage and receive results onmessage

    myWorker.postMessage('start'); // Send task parameters myWorker.onmessage = function(e) {
     console.log('Result received:', e.data);
   };

In worker.js:

 onmessage = function(e) {
  // Execute complex tasks const result = heavyComputation(e.data);
  postMessage(result); // Return result};

Note: The data passed to the worker will be copied (not referenced), so try to pass objects of basic type or simple structure.

Notes on using Web Workers

Although Web Workers are useful, they are not easy to improve performance. There are a few points to note:

• DOM cannot be accessed : because the worker does not have a window object and cannot operate on page elements.
• Communication overhead exists : frequent transmission of big data between the main thread and the worker will affect performance.
• Compatibility is basically OK : mainstream browsers support it, but if you are still considering IE support, it may not be appropriate.
• Different debugging methods : the worker's console output will be displayed on another tab of DevTools, which is sometimes easily overlooked.

The recommended scenarios are: the task is indeed time-consuming and does not require frequent interactions, such as one-time data processing, timed background computing, etc.

Tips: How to determine whether to use Web Worker?

Here are a few small judgment criteria to help you decide whether to introduce Web Worker:

• If a function executes for more than 50ms, it is worth considering putting it in the worker.
• If you find that the page becomes "stuttered" or "unresponsive" when performing a task, this is the obvious signal.
• If the task can be completed asynchronously and does not affect the current process, it is more suitable to throw it to the worker.

Of course, you can also use the performance tool to measure the specific time-consuming, and don’t draw conclusions based on your feelings.

Basically that's it. Web Workers are not difficult to use, but they are easily overlooked. Reasonable use of it can effectively improve the application's response speed and user experience, especially when processing large chunks of data or complex logic.

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