Same origin strategy and cross domain

One 、 The same-origin policy

1、 What is homology strategy ？
If two pages of the protocol 、 The domain name and port are the same , The two pages have the same source , The default port number is 80（ Omission ）

The same-origin policy （ English full name same origin policy） It is the eye security function provided by the browser .

**MDN Official concept ：** The same origin policy restricts how documents or scripts loaded from the same source interact with resources from another source , This is an important security mechanism for isolating potentially malicious files .

** Easy to understand ：** Browser rules ,A Website JavaScript, Not allowed with non homologous sites C Between , Resource interaction .
for example ：
1. Can't read non homologous pages cookie,localstorage and indexedDB
2. No access to non homologous web pages DOM
3. Can't send... To a non homologous address Ajax request

Two 、 Cross domain

1、 What is cross-domain ？
Homology refers to two URL The agreement 、 domain name 、 Port consistency , On the contrary, it is cross domain .

2、 The root cause of cross domain ：
The browser's homology policy does not allow non homology URL Interact with resources .
3、 Browser interception of cross domain requests
The browser allows cross domain requests , however , Data returned from cross domain requests , Will be blocked by the browser , Unable to get... By the page .
4、 How to implement cross domain requests
Today, , The two main solutions to realize , Namely JSONP and CORS

JSONP : It's early , Compatibility is good. （ Compatible with lower versions IE） It's the front-end programmer to solve cross domain problems , Forced to come up with a temporary solution ;
** shortcoming ：** Only support GET request I won't support it POST request

**CORS:** Late appearance , yes W3C standard , Cross domain Ajax The underlying solution to the request , Support GET and POST request
shortcoming ： Incompatible with some lower versions of browsers

3、 ... and 、JSONP

1、JSONP Implementation principle of
because Browser homology strategy The limitation of , In the web page Unable to get Ajax Request non homologous interface data , however