The get() method of the Headers interface returns a byte string of all the values of a header within a Headers object with a given name. If the requested header doesn't exist in the Headers object, it returns null.

For security reasons, some headers can only be controlled by the user agent. These headers include the forbidden header names and forbidden response header names.

name

The name of the HTTP header whose values you want to retrieve from the Headers object. If the given name is not the name of an HTTP header, this method throws a TypeError. The name is case-insensitive.

A String sequence representing the values of the retrieved header or null if this header is not set.

Creating an empty Headers object is simple:

const myHeaders = new Headers(); // Currently empty
myHeaders.get("Not-Set"); // Returns null

You could add a header to this using Headers.append, then retrieve it using get():

myHeaders.append("Content-Type", "image/jpeg");
myHeaders.get("Content-Type"); // Returns "image/jpeg"

If the header has multiple values associated with it, the byte string will contain all the values, in the order they were added to the Headers object:

myHeaders.append("Accept-Encoding", "deflate");
myHeaders.append("Accept-Encoding", "gzip");
myHeaders.get("Accept-Encoding"); // Returns "deflate, gzip"
myHeaders
  .get("Accept-Encoding")
  .split(",")
  .map((v) => v.trimStart()); // Returns [ "deflate", "gzip" ]

• ServiceWorker API
• HTTP access control (CORS)
• HTTP