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This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.
The Headers
interface of the Fetch API allows you to perform various actions on HTTP request and response headers. These actions include retrieving, setting, adding to, and removing. A Headers
object has an associated header list, which is initially empty and consists of zero or more name and value pairs. You can add to this using methods like append()
(see Examples.) In all methods of this interface, header names are matched by case-insensitive byte sequence.
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.
A Headers object also has an associated guard, which takes a value of immutable
, request
, request-no-cors
, response
, or none
. This affects whether the set()
, delete()
, and append()
methods will mutate the header. For more information see Guard.
You can retrieve a Headers
object via the Request.headers
and Response.headers
properties, and create a new Headers
object using the Headers.Headers()
constructor.
An object implementing Headers
can directly be used in a for...of
structure, instead of entries()
: for (var p of myHeaders)
is equivalent to for (var p of myHeaders.entries())
.
Note: you can find more out about the available headers by reading our HTTP headers reference.
Constructor
Headers.Headers()
- Creates a new
Headers
object.
Methods
Headers.append()
- Appends a new value onto an existing header inside a
Headers
object, or adds the header if it does not already exist. Headers.delete()
- Deletes a header from a
Headers
object. Headers.entries()
- Returns an
iterator
allowing to go through all key/value pairs contained in this object. Headers.get()
- Returns the first value of a given header from within a
Headers
object. Headers.getAll()
- Returns an array of all the values of a header within a
Headers
object with a given name. Headers.has()
- Returns a boolean stating whether a
Headers
object contains a certain header. Headers.keys()
- Returns an
iterator
allowing to go through all keys f the key/value pairs contained in this object. Headers.set()
- Sets a new value for an existing header inside a
Headers
object, or adds the header if it does not already exist. Headers.values()
- Returns an
iterator
allowing to go through all values of the key/value pairs contained in this object.
Note: To be clear, the difference between Headers.set()
and Headers.append()
is that if the specified header does already exist and does accept multiple values, Headers.set()
will overwrite the existing value with the new one, whereas Headers.append()
will append the new value onto the end of the set of values. See their dedicated pages for example code.
Note: All of the Headers methods will throw a TypeError
if you try to pass in a reference to a name that isn't a valid HTTP Header name. The mutation operations will throw a TypeError
if the header has an immutable Guard. In any other failure case they fail silently.
Examples
In the following snippet, we create a new header using the Headers()
constructor, add a new header to it using append()
, then return that header value using get()
:
var myHeaders = new Headers(); myHeaders.append('Content-Type', 'text/xml'); myHeaders.get('Content-Type') // should return 'text/xml'
Specifications
Specification | Status | Comment |
---|---|---|
Fetch The definition of 'Headers' in that specification. |
Living Standard |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari (WebKit) |
---|---|---|---|---|---|
Basic support | 42 41 behind pref |
39 (39) 34 behind pref |
No support |
29 |
No support |
entries() , keys() , values() , and support of for...of |
? | 44 (44) | ? | ? | ? |
Feature | Android | Firefox Mobile (Gecko) | Firefox OS (Gecko) | IE Phone | Opera Mobile | Safari Mobile | Chrome for Android |
---|---|---|---|---|---|---|---|
Basic support | No support | No support | No support | No support | No support | No support | No support |
entries() , keys() , values() , and support of for...of |
? | 44.0 (44) | 2.5 | ? | ? | ? | ? |
See also