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.

name

The name of the HTTP header you want to set to a new value. If the given name is not the name of an HTTP header, this method throws a TypeError.

value

The new value you want to set.

None (undefined).

Creating an empty Headers object is simple:

const myHeaders = new Headers(); // Currently empty

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"); // Returns 'gzip'

You'd need Headers.append to append the new value onto the values, not overwrite it.

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