Wait until scrollTo is complete before running a command

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07-10-2024

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How to Wait for scrollTo Completion Before Executing Your Next Command

The Problem:

Let's say you want to smoothly scroll to a specific element on your webpage using JavaScript's scrollTo method. But then, you need to execute another command only after the scrolling animation has finished. How do you ensure the command is not triggered prematurely, potentially causing unexpected behavior?

Scenario:

Imagine you have a button that, when clicked, should:

1. Scroll the user to a specific element (#target-element)
2. Open a modal (#my-modal)

Here's the naive approach:

const scrollButton = document.querySelector('.scroll-button');
const targetElement = document.querySelector('#target-element');
const modal = document.querySelector('#my-modal');

scrollButton.addEventListener('click', () => {
  window.scrollTo({
    top: targetElement.offsetTop,
    behavior: 'smooth'
  });

  // This will likely open the modal before scrolling finishes
  modal.style.display = 'block';
});

The issue is that modal.style.display = 'block' runs immediately after window.scrollTo. The modal will open before the scrolling animation completes, resulting in a poor user experience.

The Solution:

We need a way to "pause" the execution of our code until the scrollTo animation finishes. Here's how we can achieve this using Promises:

const scrollButton = document.querySelector('.scroll-button');
const targetElement = document.querySelector('#target-element');
const modal = document.querySelector('#my-modal');

scrollButton.addEventListener('click', () => {
  const scrollPromise = new Promise(resolve => {
    window.scrollTo({
      top: targetElement.offsetTop,
      behavior: 'smooth',
      // Resolve the promise when scrolling is complete
      complete: resolve 
    });
  });

  // Execute modal opening after scrolling is complete
  scrollPromise.then(() => {
    modal.style.display = 'block';
  });
});

Explanation:

1. We create a Promise (scrollPromise) that will resolve when the scrollTo animation completes.
2. Inside the window.scrollTo function, we set the complete callback property to resolve the Promise when the scrolling finishes.
3. Using the then method on the Promise, we chain our desired action (opening the modal) to execute only after the Promise resolves.

Key Points:

• Browser Support: This approach relies on the complete property in window.scrollTo, which is relatively new and might not be supported in older browsers.
• Alternative: If you need to support older browsers, you can use techniques like requestAnimationFrame or setTimeout with a slight delay to achieve a similar effect. However, these methods will not be as precise as using the complete callback.
• Performance: While this approach is effective, keep in mind that using Promises and callbacks can introduce a slight overhead compared to a synchronous execution. For simple scenarios, the performance difference might be negligible, but for complex animations, consider optimization techniques.

Additional Considerations:

• Asynchronous Nature: Always be aware of the asynchronous nature of JavaScript. Be careful when executing actions that depend on the results of previous asynchronous operations.
• Timing: Even with the complete callback, the animation might not be entirely finished by the time the Promise resolves. If you need absolute precision, consider additional techniques like using a callback function within the animation itself.

Conclusion:

By using Promises and the complete callback in window.scrollTo, you can reliably wait for the scrolling animation to finish before executing other commands. This ensures a smooth and predictable user experience. Remember to consider browser support and performance implications when choosing your approach.