Скачать или смотреть How to Use Promises with Recursion and setTimeout for DOM Element Checks

Learn how to create a JavaScript function that uses promises and recursion to check for the presence of a DOM element every 2 seconds. This guide breaks down the solution step-by-step.
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This video is based on the question https://stackoverflow.com/q/77423094/ asked by the user 'fciri' ( https://stackoverflow.com/u/1267608/ ) and on the answer https://stackoverflow.com/a/77423101/ provided by the user 'Seonghun' ( https://stackoverflow.com/u/16596710/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Checking for a DOM Element with Promises and Recursion

In today's world of web development, ensuring that certain elements are present in the DOM before executing further actions is a common requirement. You might want to wait for a specific element to appear on a page, and periodically check for its presence. This is where the combination of Promises, recursion, and setTimeout in JavaScript can help you create efficient and clean solutions.

The Problem

Imagine you are building a dynamic web application where a specific DOM element loads asynchronously. You want to execute some code as soon as that element is available. The traditional approach might involve using setInterval, but using a promise provides a much cleaner, more manageable way of handling this situation.

In our example, we want a function that checks every 2 seconds if an element with a specific ID exists in the DOM. Once the element is present, we want to resolve a promise so that we can execute further actions using .then().

Here’s what the initial code looks like:

[[See Video to Reveal this Text or Code Snippet]]

The issue here is that the then() method is never executed, meaning the promise resolves only when the element is found, but this isn't currently happening correctly due to the way the recursive call is structured.

The Solution

To solve this issue, we can modify the code to return the result of the recursive call within the setTimeout. This way, when the element is not found, it creates a new promise that will resolve after a 2-second delay, continuing to check for the element until it is found.

Here’s a step-by-step breakdown of the correct implementation:

Step 1: Setup the Function

Define the function checkElement, which will be responsible for checking if the element is present:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Check for the Element

Using jQuery, check if the element exists in the DOM:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Handle the Retry Logic

If the element is not found, we need to create a new promise that resolves after 2 seconds. Inside this promise, we set a setTimeout to call checkElement again:

[[See Video to Reveal this Text or Code Snippet]]

Full Implementation

Here’s the complete function:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By following these steps, you can build a robust solution for checking the presence of a DOM element using promises, recursion, and setTimeout. This method not only simplifies code management but also enhances asynchronous behavior by utilizing promises effectively. Next time you need to wait for a DOM element, remember this efficient approach!