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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 controller by the user agent. These headers include the forbidden header names  and forbidden response header names.




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.
The new value you want to set.




Creating an empty Headers object is simple:

var 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.getAll('Accept-Encoding'); // Returns [ "gzip" ]

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


Specification Status Comment
The definition of 'set()' in that specification.
Living Standard  

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 42
41 behind pref
(Yes) 39 (39)
34 behind pref
No support

28 behind pref

No support
Feature Android Edge Firefox Mobile (Gecko) Firefox OS (Gecko) IE Phone Opera Mobile Safari Mobile Chrome for Android
Basic support No support (Yes) No support No support No support No support No support No support

See also

Document Tags and Contributors

 Contributors to this page: chrisdavidmills, erikadoyle, jpmedley, kscarfone
 Last updated by: chrisdavidmills,