Скачать или смотреть Why does my alert trigger when readyState isn't 4 in an AJAX request?

Encountering an issue where your alert triggers unexpectedly during an AJAX request because `readyState` isn't `4`? Understand why this happens and learn how to fix it for smoother AJAX interactions.
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Why does my alert trigger when readyState isn't 4 in an AJAX request?

When working with AJAX requests in JavaScript, it’s common to monitor the readyState property to understand the status of an HTTP request. However, an alert may unexpectedly trigger when the readyState isn't 4, causing confusion. Let's delve into why this might happen and how you can handle it more effectively.

Understanding readyState
The readyState property of an XMLHttpRequest object can have one of five values:

0 (UNSENT): The request has not been initialized.

1 (OPENED): The request has been opened.

2 (HEADERS_RECEIVED): The request has been sent, and headers and status are available.

3 (LOADING): The response is being downloaded; responseText holds partial data.

4 (DONE): The operation is complete.

In most cases, developers check for readyState === 4 to ensure the request has been completed before trying to process the data returned by the server.

Common Pitfalls
If the alert you’ve set up to monitor the AJAX request state triggers when readyState isn't 4, several issues might be at play:

Improper Condition Checking: Ensure that your condition explicitly checks for readyState === 4 and not generally for other states. A common mistake is to check for != 4 or similar, which inadvertently matches states other than 4.

Timing Issues: AJAX requests can progress through states quickly. If you’re setting an alert inside the readystatechange event handler without proper checks, you might see alerts for states 1, 2, and 3 as well.

Overly General Alerts: Make sure that the logic inside your readystatechange event handler captures specific cases related to readyState values. Avoid placing alert statements in the general scope of the handler.

Example Code

Here's an example to ensure alerts trigger only when readyState is 4:

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

In this example, the alert only triggers when the request is truly complete (readyState === 4).

Conclusion
Monitoring the readyState property in an AJAX request is crucial for understanding the request lifecycle. Ensuring that alerts and other actions only trigger when readyState === 4 can prevent unexpected behaviors. Perform precise condition checks and handle states appropriately to achieve smoother AJAX interactions in your applications.