Web/API/Headers/set

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.

Syntax

myHeaders.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.

Returns

Void.

Example

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.get('Accept-Encoding'); // Returns 'gzip'

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

Specifications

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

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`set` Experimental'	Chrome Full support 42 Full support 42 Full support 41 Disabled' Disabled' From version 41: this feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Edge Full support 14	Firefox Full support 39 Full support 39 Full support 34 Disabled' Disabled' From version 34: this feature is behind the `dom.fetch.enabled` preference. To change preferences in Firefox, visit about:config.	IE No support No	Opera Full support 29 Full support 29 Full support 28 Disabled' Disabled' From version 28: this feature is behind the `Experimental Web Platform Features` preference.	Safari No support No	WebView Android Full support 42	Chrome Android Full support 42 Full support 42 Full support 41 Disabled' Disabled' From version 41: this feature is behind the `Experimental Web Platform Features` preference. To change preferences in Chrome, visit chrome://flags.	Firefox Android No support No	Opera Android Full support 29 Full support 29 Full support 28 Disabled' Disabled' From version 28: this feature is behind the `Experimental Web Platform Features` preference.	Safari iOS No support No	Samsung Internet Android Full support 4.0

Legend

Full support: Full support
No support: No support
Experimental. Expect behavior to change in the future.': Experimental. Expect behavior to change in the future.
User must explicitly enable this feature.': User must explicitly enable this feature.