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 |
---|---|---|
Fetch The definition of 'append()' in that specification. |
Living Standard |
Browser compatibility
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
append | Chrome
Full support
42
| Edge Full support 14 | Firefox
Full support
39
| IE No support No | Opera
Full support
29
| Safari Full support 10.1 | WebView Android Full support 42 | Chrome Android
Full support
42
| Firefox Android No support No | Opera Android
Full support
29
| 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.