Headers: set() method

Available in workers

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 append is that if the specified header already exists and accepts multiple values, set() overwrites the existing value with the new one, whereas 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 request headers and forbidden response header names.

Syntax

set(name, value)

Parameters

• 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.

Return value

None (undefined).

Examples

Creating an empty Headers object is simple:

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

You could add a header to this using 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 append to append the new value onto the values, not overwrite it.

Specifications

Browser compatibility

See also

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