The set()
method of the Headers
interface sets a new value for an existing header inside a Headers
object, or adds the header if it does not already exist.
The difference between set()
and Headers.append
is that if the specified header already exists and accepts multiple values, set()
overwrites the existing value with the new one, whereas Headers.append
appends the new value to the end of the set of values.
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.
Creating an empty Headers
object is simple:
const myHeaders = new Headers();
You could add a header to this using Headers.append
, then set a new value for this header using set()
:
myHeaders.append("Content-Type", "image/jpeg");
myHeaders.set("Content-Type", "text/html");
If the specified header does not already exist, set()
will create it and set its value to the specified value. If the specified header does already exist and does accept multiple values, set()
will overwrite the existing value with the new one:
myHeaders.set("Accept-Encoding", "deflate");
myHeaders.set("Accept-Encoding", "gzip");
myHeaders.get("Accept-Encoding");
You'd need Headers.append
to append the new value onto the values, not overwrite it.