Break Up Long Tasks

Web features: Scheduler API

Heavy computations or long loops can block the main thread, causing the page to become unresponsive. To prevent this, you should yield control back to the browser periodically. The scheduler.yield() API allows you to pause a long task and let the browser handle user input or rendering before continuing.

Breaking up long tasks

Use scheduler.yield() inside async functions to break up work.

async function processLargeArray(items) {
  // DO: Set a time-based deadline 50 milliseconds into the future. 50
  // milliseconds is the boundary for when a task becomes a long task.
  let deadline = performance.now() + 50; // 50ms budget

  for (const item of items) {
    // Process the item
    processItem(item);
    
    // MANDATORY: Yield to the main thread periodically to keep the UI
    // responsive. This can be done by checking if the deadline set earlier
    // has been exceeded. When it has been, yield, then reset the deadline
    // another 50 milliseconds into the future.
    if (performance.now() >= deadline) {
      await scheduler.yield();
      deadline = performance.now() + 50;
    }
  }
}

Fallback strategies

Some browsers may not support the scheduler API. You MUST implement a fallback using setTimeout to ensure code executes without breaking.

Fallback for scheduler.yield()

async function processLargeArrayWithFallback(items) {
  // DO: Set a time-based deadline 50 milliseconds into the future.
  let deadline = performance.now() + 50;

  for (const item of items) {
    processItem(item);
    
    // MANDATORY: Yield to the main thread periodically to keep the UI responsive.
    if (performance.now() >= deadline) {
      // DO: Feature detect scheduler.yield
      if ('scheduler' in window && 'yield' in window.scheduler) {
        await scheduler.yield();
      } else {
        // DO: Fallback to setTimeout for older browsers
        await new Promise(resolve => setTimeout(resolve, 0));
      }
      deadline = performance.now() + 50;
    }
  }
}