Optimizing Interaction to Next Paint (INP) for Frontend Applications

Description
Interaction to Next Paint (INP) is a Core Web Vital metric proposed by Google in 2022, used to measure a page's responsiveness to user interactions (such as clicks, taps, or keyboard inputs). INP records the time from the start of a user interaction until the next frame is painted, focusing on all interaction delays throughout the page's lifecycle. It ultimately takes the worst interaction delay as the page's INP value. Optimizing INP can significantly improve user experience by avoiding interface lag or unresponsiveness.

Problem-Solving Process

1. Understanding INP Calculation Logic
   INP measurement is based on the event handling process:

   • Interaction Start: The user triggers an event (e.g., click).
   • Event Processing: The browser executes the relevant event listeners (including capture, target, and bubbling phases).
   • Next Paint: The browser updates the interface (e.g., re-renders after DOM modifications).
     INP Value = Event Processing Time + Subsequent Rendering Time. If an interaction triggers multiple events (e.g., click may be accompanied by mousedown and mouseup), INP takes the total duration of these events.
   • Key Point: INP focuses on the worst interaction delay (e.g., the 98th percentile delay), not the average.

2. Analyzing Common Causes of High INP

   • Long Tasks: The main thread is blocked by JavaScript tasks, causing event processing delays.
   • Complex Calculations or Frequent DOM Operations: Executing too many computations or DOM updates within a single interaction.
   • Third-Party Scripts: Scripts for ads, analytics tools, etc., may occupy the main thread.
   • Heavy Input Handling (e.g., Scrolling): Unoptimized event listeners (like scroll or resize) firing frequently.

3. Optimization Strategy: Reducing Main Thread Load

   • Break Up Long Tasks: Split large JavaScript tasks into chunks smaller than 50ms, using setTimeout, queueMicrotask, or requestIdleCallback for incremental execution.

     // Original long task
     function processLargeData() {
       for (let i = 0; i < 1e6; i++) {
         heavyCalculation(i); // Blocks main thread
       }
     }
     
     // After splitting
     function processInChunks(data, chunkSize = 1000) {
       let index = 0;
       function nextChunk() {
         const end = Math.min(index + chunkSize, data.length);
         for (; index < end; index++) {
           heavyCalculation(data[index]);
         }
         if (index < data.length) {
           setTimeout(nextChunk, 0); // Yield the main thread
         }
       }
       nextChunk();
     }
     

   • Use Web Workers: Move non-UI related complex calculations (e.g., data processing) to a Worker thread.

     // Main thread
     const worker = new Worker('compute.js');
     worker.postMessage(data);
     worker.onmessage = (e) => updateUI(e.data);
     
     // compute.js
     self.onmessage = (e) => {
       const result = heavyCalculation(e.data);
       self.postMessage(result);
     };
     

4. Optimization Strategy: Optimize Event Handling Logic

   • Debouncing and Throttling: Limit the firing frequency of high-frequency events (like resize, scroll).

     // Throttle scroll event
     const throttle = (fn, delay) => {
       let lastCall = 0;
       return (...args) => {
         const now = Date.now();
         if (now - lastCall >= delay) {
           fn(...args);
           lastCall = now;
         }
       };
     };
     window.addEventListener('scroll', throttle(handleScroll, 100));
     

   • Avoid Synchronous Layout Thrashing: Avoid alternating reads and writes of layout properties (like offsetHeight) in loops, which causes forced reflows.

     // Bad example: Layout thrashing
     function resizeAll() {
       for (let i = 0; i < items.length; i++) {
         items[i].style.height = items[i].offsetHeight + 10 + 'px'; // Write immediately after read, triggers reflow
       }
     }
     
     // Optimized: Read first, then write
     function resizeAll() {
       const heights = items.map(item => item.offsetHeight); // Batch read
       for (let i = 0; i < items.length; i++) {
         items[i].style.height = heights[i] + 10 + 'px'; // Batch write
       }
     }
     

5. Optimization Strategy: Reduce Input Delay

   • Use passive Event Listeners: Add { passive: true } to touch/wheel events that don't block scrolling, preventing the browser from waiting for listener execution before scrolling.

     // Default behavior may cause scroll jank
     element.addEventListener('touchstart', handleTouch, { passive: true });
     

   • Avoid Executing Excessive Tasks in Critical Interactions: For example, when a button is clicked, only perform necessary operations (like sending a request), and defer non-urgent tasks (like logging).

6. Monitoring and Debugging INP

   • Use Chrome DevTools:
     • Performance panel: Record interaction processes to analyze event handling duration and long tasks.
     • Core Web Vitals panel: View real-user INP data.
   • Field Tools: Obtain real-world INP percentiles via Chrome UX Report or PageSpeed Insights.

Summary
The core of optimizing INP is ensuring the main thread can quickly respond to user interactions. By breaking up long tasks, offloading non-critical logic, optimizing event handling, and combining performance monitoring tools for continuous iteration, page responsiveness can be significantly improved.