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 controller 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.getAll('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
Feature | Chrome | Edge | Firefox (Gecko) | Internet Explorer | Opera | Safari (WebKit) |
---|---|---|---|---|---|---|
Basic support | 42 41 behind pref |
(Yes) | 39 (39) 34 behind pref |
No support |
29 |
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 |