2018 latest front-end interview questions seven

This time we bring you the latest front-end interview questions in 2018. We know that interviews are an essential part of front-end work. This time, the common front-end interview questions are sorted and summarized to help you get through the front-end interview. Big trouble. Let’s take a look.

[Related recommendations: Front-end interview questions (2020)]

1. What is cross-domain resource sharing (CORS)? What problem is it used to solve?

By default, to prevent certain behaviors , the browser's XHR object can only access resources originating from the same domain. However, in our daily actual development, we often encounter the need for cross-domain requests, so a cross-domain request solution has emerged: CORS (Cross-Origin Resource Sharing) cross-domain resource sharing. The principle behind CORS is to use custom HTTP headers to communicate with the server, so that the server determines whether the response is successful.
How to use CORS?
Using CORS requires the cooperation of both the client and the server.
1. How does the client initiate a CORS cross-domain request?
At present, CORS is natively supported in most browsers (CORS is supported in each browser). When writing the code, it is similar to requests in the same domain. You only need to pass in the absolute URL when xhr.open() That’s it. For example:

var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function () {    if(xhr.readyState == 4){        if(xhr.status >= 200 && xhr.status < 300 || xhr.status == 304){            console.log(xhr.responseText)
        }else {            console.log(&#39;err&#39; + xhr.status);
        }
    }
};
xhr.open(&#39;get&#39;,&#39;http://www.xxx.com/api/something/&#39;,true);
xhr.send(null);

This way you can send a cross-domain request, but if you just send it like the sample code above, an error will be reported because the server is not set to allow us to make this request, so CORS also needs the server to cooperate. .
2. How does the server allow customers’ CORS cross-domain requests?
The server only needs to set Access-Control-Allow-Origin in the response header to allow the client to access.
Assuming that the domain name of the client is http://www.xxx.com, then as long as the server contains http://www.xxx.com in the Access-Control-Allow-Origin setting, then this CORS request can be made success. If Access-Control- Allow-Origin is set to *, then any domain name can access this server, but for security reasons, this is generally not recommended.

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to the php Chinese website Other related articles!

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