After acquiring knowledge, how can you determine whether it is correct or not? In the field of programming, this is actually a relatively simple question. You just need to check how the specification is written (if there is one).
 ===  is false, why
'b' + 'a' + + 'a' + 'a' is baNaNa, all of these are explained in detail in the specification, using what rules to perform the conversion.
In addition to JS, almost all HTML or other related specifications in the Web field can be found on w3.org or whatwg.org, and the resources are quite rich.
Although the implementation of browsers may be different from what is written in the specification (such as this article), the spec is the most complete and authoritative place, so it is correct to come here to find information.
If you search for the CORS spec, you may find RFC6454 - The Web Origin Concept and W3C’s Cross-Origin Resource Sharing, but these two have been replaced by a document called Fetch.
At first, I was puzzled and thought I had read it wrong. What is the relationship between fetch and CORS? Later, I learned that the fetch here is different from the fetch in the Web API. This specification defines everything related to “fetching data”, as written in its outline:
The Fetch standard defines requests, responses, and the process that binds them: fetching.
In this article, let’s take a look at the CORS-related specifications together, proving that what I said in the previous articles is not nonsense, but based on facts. Since the specification is quite long, I will only pick some key points that I think are important. If you want to understand all the content of the specification, you still need to read it yourself.