CORS
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CORS (Cross-Origin Resource Sharing) is a system, consisting of transmitting {{Glossary("HTTP_header", "HTTP headers")}} , that determines whether browsers block frontend JavaScript code from accessing responses for cross-origin requests.
The same-origin security policy forbids cross-origin access to resources. But CORS gives web servers the ability to say they want to opt into allowing cross-origin access to their resources.
CORS headers
{{HTTPHeader("Access-Control-Allow-Origin")}}- : Indicates whether the response can be shared.
{{HTTPHeader("Access-Control-Allow-Credentials")}}- : Indicates whether or not the response to the request can be exposed when the credentials flag is true.
{{HTTPHeader("Access-Control-Allow-Headers")}}- : Used in response to a preflight request to indicate which HTTP headers can be used when making the actual request.
{{HTTPHeader("Access-Control-Allow-Methods")}}- : Specifies the method or methods allowed when accessing the resource in response to a preflight request.
{{HTTPHeader("Access-Control-Expose-Headers")}}- : Indicates which headers can be exposed as part of the response by listing their names.
{{HTTPHeader("Access-Control-Max-Age")}}- : Indicates how long the results of a preflight request can be cached.
{{HTTPHeader("Access-Control-Request-Headers")}}- : Used when issuing a preflight request to let the server know which HTTP headers will be used when the actual request is made.
{{HTTPHeader("Access-Control-Request-Method")}}- : Used when issuing a preflight request to let the server know which HTTP method will be used when the actual request is made.
{{HTTPHeader("Origin")}}- : Indicates where a fetch originates from.
{{HTTPHeader("Timing-Allow-Origin")}}- : Specifies origins that are allowed to see values of attributes retrieved via features of the Resource Timing API, which would otherwise be reported as zero due to cross-origin restrictions.