Headers

Baseline Widely available *

This feature is well established and works across many devices and browser versions. It’s been available across browsers since March 2017.

* Some parts of this feature may have varying levels of support.

Note: This feature is available in Web Workers.

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 headers from the list of the request's headers.

You can retrieve a Headers object via the Request.headers and Response.headers properties, and create a new Headers object using the Headers() constructor. Compared to using plain objects, using Headers objects to send requests provides some additional input sanitization. For example, it normalizes header names to lowercase, strips leading and trailing whitespace from header values, and prevents certain headers from being set.

Note: You can find out more about the available headers by reading our HTTP headers reference.

Description

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.

An object implementing Headers can directly be used in a for...of structure, instead of entries(): for (const p of myHeaders) is equivalent to for (const p of myHeaders.entries()).

Modification restrictions

Some Headers objects have restrictions on whether the set(), delete(), and append() methods can mutate the header. The modification restrictions are set depending on how the Headers object is created.

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 is immutable. In any other failure case they fail silently.

Constructor

Headers()

Creates a new Headers object.

Instance 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.forEach()

Executes a provided function once for each key/value pair in this Headers object.

Headers.get()

Returns a String sequence of all the values of a header within a Headers object with a given name.

Headers.getSetCookie()

Returns an array containing the values of all Set-Cookie headers associated with a response.

Headers.has()

Returns a boolean stating whether a Headers object contains a certain header.

Headers.keys()

Returns an iterator allowing you to go through all keys of 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 you 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: When Header values are iterated over, they are automatically sorted in lexicographical order, and values from duplicate header names are combined.

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():

js
const myHeaders = new Headers();

myHeaders.append("Content-Type", "text/xml");
myHeaders.get("Content-Type"); // should return 'text/xml'

The same can be achieved by passing an array of arrays or an object literal to the constructor:

js
let myHeaders = new Headers({
  "Content-Type": "text/xml",
});

// or, using an array of arrays:
myHeaders = new Headers([["Content-Type", "text/xml"]]);

myHeaders.get("Content-Type"); // should return 'text/xml'

Specifications

Specification
Fetch
# headers-class

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
Headers
[Symbol.iterator]
Headers() constructor
append
delete
entries
forEach
get
getSetCookie
has
Iteration combines duplicate header names
Iteration is lexicographically sorted
keys
set
values

Legend

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Full support
Full support
No support
No support
See implementation notes.

See also