Web/API/Headers/append

The append() method of the Headers interface appends a new value onto 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() will overwrite the existing value with the new one, whereas append() will append the new value onto 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.

Syntax

myHeaders.append(name, value);

Parameters

name: The name of the HTTP header you want to add to the Headers object.
value: The value of the HTTP header you want to add.

Returns

Void.

Example

Creating an empty Headers object is simple:

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

You could add a header to this using append():

myHeaders.append('Content-Type', 'image/jpeg');
myHeaders.get('Content-Type'); // Returns 'image/jpeg'

If the specified header already exists, append() will change its value to the specified value. If the specified header already exists and accepts multiple values, append() will append the new value to the end of the value set:

myHeaders.append('Accept-Encoding', 'deflate');
myHeaders.append('Accept-Encoding', 'gzip');
myHeaders.get('Accept-Encoding'); // Returns 'deflate, gzip'

To overwrite the old value with a new one, use Headers.set.

Specifications

Specification	Status	Comment
FetchThe definition of 'append()' 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
`append` 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 Full support 10.1	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.