The key to JavaScript application performance optimization is to reduce DOM operations, rational use of anti-shake throttling, lazy resource loading, cache and multiplexing calculation results. 1. Reduce unnecessary DOM operations, merge updates, use document fragments or strings to splice HTML content to reduce re-arrangement and redrawing; 2. Use anti-shake (such as search box input) and throttling (such as scrolling monitoring) to control execution frequency; 3. Lazy load resources, control script loading, Intersection Observer to achieve lazy image loading, and routing-level code segmentation to reduce the loading pressure on the first screen; 4. Use cache to avoid repeated calculations, including variable multiplexing, API request local cache, function return value memoization and other strategies to improve execution efficiency.

JavaScript application performance optimization is actually not that mysterious, the key is to grasp several core points. Many developers only focus on function implementation at the beginning, and only after the project grows bigger will they find that problems such as lag and slow loading affect the user experience. If you want to make JS applications run faster and smoother, the following directions are the most worthy of effort.

Reduce unnecessary DOM operations

Frequent operation of DOM is a common reason for slowing down JavaScript performance. Browser rendering pages requires time and resources. Reflow and repaint can be triggered every time the DOM is modified. If it is modified multiple times in a row, the efficiency will be very low.

suggestion:

• Merge multiple DOM operations into one update
• Use Document Fragment to batch insert nodes
• Avoid direct operation of DOM in loops

For example, if you want to add 10 elements to a list, don’t append one by one in the loop, but first splice the HTML content with a string and insert it at once.

Use anti-shake and throttling properly

During user interaction, events such as window adjustment, scrolling, and input box search suggestions will be triggered at high frequency. If there is no control, it is easy to cause too many functions to be executed, resulting in page stuttering.

At this time, you need to use:

• Debounce : suitable for scenarios where "it is triggered only after the last operation", such as automatically completing the search box when inputting
• Throttle : suitable for limiting the trigger frequency, such as scroll monitoring and window adjustment

For example, if you do a search suggestion function, the user sends a request every time he types a letter, which not only wastes resources, but also is easily restricted by the backend. Adding 300ms anti-shake can effectively reduce the number of requests.

Lazy loading and asynchronous loading of resources

Modern web applications often contain a large number of resources such as JS files, pictures, fonts, etc., and loading all the content from the beginning will cause the first screen to load longer.

Optimizations that can be made include:

• Use <script defer></script> or <script async></script> to control script loading timing
• Lazy image loading, use Intersection Observer API
• Code splitting at route level

If you use a framework like React or Vue, you usually have an off-the-shelf dynamic import mechanism, which is particularly convenient to use with lazy loading of routing.

Utilize cache and avoid duplicate calculations

Some data or calculation results can be cached after the first acquisition and reused directly afterwards, rather than recalculating them every time. For example:

• Don't create variables inside the function repeatedly every time, you can propose them to cache them.
• For API requests, you can set a local cache policy, such as no request will be initiated within 5 minutes with the same parameters.
• Use Memoization trick to cache functions to return values, especially for recursive or complex calculations

For example, write a simple memoize function, save the parameters and results, and return the result directly the next time you encounter the same input, without calculating it again.

Basically that's it. Performance optimization is not something that can be achieved overnight, but as long as you start from common bottlenecks and gradually improve it, you can significantly improve the response speed and user experience of the application.

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