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 Headers.set and append() is that if the specified header already exists and accepts multiple values, Headers.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
Fetch
The definition of 'append()' in that specification.
Living Standard  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support

42

412

Yes

39

343

No

29

284

10.1
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support

42

411

42

412

Yes No

29

284

No4.0

1. From version 41: this feature is behind the Experimental Web Platform Features preference.

2. From version 41: this feature is behind the Experimental Web Platform Features preference. To change preferences in Chrome, visit chrome://flags.

3. From version 34: this feature is behind the dom.fetch.enabled preference. To change preferences in Firefox, visit about:config.

4. From version 28: this feature is behind the Experimental Web Platform Features preference.

See also

Document Tags and Contributors

 Contributors to this page: fscholz, Rob W, ibnesayeed, chrisdavidmills, erikadoyle, jpmedley, kscarfone
 Last updated by: fscholz,