Provisional headers are shown

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When we encounter problems in sending asynchronous requests, we will first open the chrome console to look at HTTP requests. Sometimes, we will see proactive headers are show errors in the request header

Problem Description:

The problem literally means “temporary header displayed”. The browser sent the request for the first time, but the request was blocked and did not receive a response. When the browser is asked to send this request again, the browser will report this warning if the last same request has not received a response

In short, the request is not sent to

Cause analysis

There are many cases of this kind of situation, but the reasons are various, generally as follows:

1. Cross domain request blocked by browser

Now the static resources of the website will exist under the special static domain name, which may be inconsistent with the actual domain name

This situation basically occurs in Ajax requests, which are based on CORS to solve this problem

      url: ajaxUrl,
      type: 'GET',
      success: function (res) {}

Can be replaced by

  $.cors(ajaxUrl, {}, function (json) {
    if (json.code == 0) {}

2. The server did not respond in time (timeout) 2

This will only appear under specific circumstances, which is not related to the server

3. Request blocked by browser plug-in

As a developer, you must have a lot of plug-ins installed on chrome. We can use chrome://net-internals , this address has been abandoned, new address: chrome://net-export/
, to find relevant requests according to keywords, and then to see the relevant status in addition, if you are blocked by a browser, you can try another browser

Processing method: call Chrome’s chrome://net-internals/#events , and then trigger the request again, and view the log of the request that appears the preventive headers are show

Check whether there is a delegate_ blocked_ The key word of by; This is generally due to the browser plug-in or client software to intercept the request; We are intercepted by Websense endpoint

If this is the case, we can basically ignore the problems of the client itself; You can consider unloading the plug-in or software and try again to see if it still appears

4. The data is directly cached and no request is sent

Only the communication obtained from the cache is displayed as “show temporary title” (or “execute”), because the file is obtained from the cache and is not communicated, so the detailed header is not displayed

If the previous resource fails to load, the resource loaded from the cache may fail, that is, the request before the cache resource request cannot fail. Strong cache from disk cache or from memory cache will not be displayed at this time

In many cases mentioned above, the correct communication with the server is not carried out or completed, so only temporary information is displayed


CORS is a W3C standard, whose full name is “cross origin resource sharing”. It allows browsers to send XMLHttpRequest requests to cross source servers, thus overcoming the limitation that Ajax can only use the same source to avoid the browser’s same source policy. It is a modern version of jsonp mode

Unlike jsonp, CORS supports other HTTP requests in addition to the get request method. CORS allows web designers to use general XMLHttpRequest, which is better than jsonp in error handling. On the other hand, jsonp can operate on old browsers that do not support CORS, and modern browsers support CORS


This article is shared in the blog “Zhou Xiaodong” (CSDN).

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