node/doc/api/http.md

# HTTP

> Stability: 2 - Stable

To use the HTTP server and client one must `require('http')`.

The HTTP interfaces in Node.js are designed to support many features
of the protocol which have been traditionally difficult to use.
In particular, large, possibly chunk-encoded, messages. The interface is
careful to never buffer entire requests or responses--the
user is able to stream data.

HTTP message headers are represented by an object like this:

```js
{ 'content-length': '123',
  'content-type': 'text/plain',
  'connection': 'keep-alive',
  'host': 'mysite.com',
  'accept': '*/*' }
```

Keys are lowercased. Values are not modified.

In order to support the full spectrum of possible HTTP applications, Node.js's
HTTP API is very low-level. It deals with stream handling and message
parsing only. It parses a message into headers and body but it does not
parse the actual headers or the body.

See [`message.headers`][] for details on how duplicate headers are handled.

The raw headers as they were received are retained in the `rawHeaders`
property, which is an array of `[key, value, key2, value2, ...]`.  For
example, the previous message header object might have a `rawHeaders`
list like the following:

```js
[ 'ConTent-Length', '123456',
  'content-LENGTH', '123',
  'content-type', 'text/plain',
  'CONNECTION', 'keep-alive',
  'Host', 'mysite.com',
  'accepT', '*/*' ]
```

## Class: http.Agent
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The HTTP Agent is used for pooling sockets used in HTTP client
requests.

The HTTP Agent also defaults client requests to using
`Connection: keep-alive`. If no pending HTTP requests are waiting on a
socket to become free the socket is closed. This means that Node.js's
pool has the benefit of keep-alive when under load but still does not
require developers to manually close the HTTP clients using
KeepAlive.

If you opt into using HTTP KeepAlive, you can create an Agent object
with that flag set to `true`.  (See the [constructor options][].)
Then, the Agent will keep unused sockets in a pool for later use.  They
will be explicitly marked so as to not keep the Node.js process running.
However, it is still a good idea to explicitly [`destroy()`][] KeepAlive
agents when they are no longer in use, so that the Sockets will be shut
down.

Sockets are removed from the agent's pool when the socket emits either
a `'close'` event or a special `'agentRemove'` event. This means that if
you intend to keep one HTTP request open for a long time and don't
want it to stay in the pool you can do something along the lines of:

```js
http.get(options, (res) => {
  // Do stuff
}).on('socket', (socket) => {
  socket.emit('agentRemove');
});
```

Alternatively, you could just opt out of pooling entirely using
`agent:false`:

```js
http.get({
  hostname: 'localhost',
  port: 80,
  path: '/',
  agent: false  // create a new agent just for this one request
}, (res) => {
  // Do stuff with response
});
```

### new Agent([options])
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* `options` {Object} Set of configurable options to set on the agent.
  Can have the following fields:
  * `keepAlive` {Boolean} Keep sockets around in a pool to be used by
    other requests in the future. Default = `false`
  * `keepAliveMsecs` {Integer} When using HTTP KeepAlive, how often
    to send TCP KeepAlive packets over sockets being kept alive.
    Default = `1000`.  Only relevant if `keepAlive` is set to `true`.
  * `maxSockets` {Number} Maximum number of sockets to allow per
    host.  Default = `Infinity`.
  * `maxFreeSockets` {Number} Maximum number of sockets to leave open
    in a free state.  Only relevant if `keepAlive` is set to `true`.
    Default = `256`.

The default [`http.globalAgent`][] that is used by [`http.request()`][] has all
of these values set to their respective defaults.

To configure any of them, you must create your own [`http.Agent`][] object.

```js
const http = require('http');
var keepAliveAgent = new http.Agent({ keepAlive: true });
options.agent = keepAliveAgent;
http.request(options, onResponseCallback);
```

### agent.createConnection(options[, callback])
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* `options` {Object} Options containing connection details. Check
  [`net.createConnection()`][] for the format of the options
* `callback` {Function} Callback function that receives the created socket
* Returns: {net.Socket}

Produces a socket/stream to be used for HTTP requests.

By default, this function is the same as [`net.createConnection()`][]. However,
custom Agents may override this method in case greater flexibility is desired.

A socket/stream can be supplied in one of two ways: by returning the
socket/stream from this function, or by passing the socket/stream to `callback`.

`callback` has a signature of `(err, stream)`.

### agent.destroy()
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Destroy any sockets that are currently in use by the agent.

It is usually not necessary to do this.  However, if you are using an
agent with KeepAlive enabled, then it is best to explicitly shut down
the agent when you know that it will no longer be used.  Otherwise,
sockets may hang open for quite a long time before the server
terminates them.

### agent.freeSockets
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* {Object}

An object which contains arrays of sockets currently awaiting use by
the Agent when HTTP KeepAlive is used.  Do not modify.

### agent.getName(options)
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* `options` {Object} A set of options providing information for name generation
  * `host` {String} A domain name or IP address of the server to issue the request to
  * `port` {Number} Port of remote server
  * `localAddress` {String} Local interface to bind for network connections
    when issuing the request
* Returns: {String}

Get a unique name for a set of request options, to determine whether a
connection can be reused.  In the http agent, this returns
`host:port:localAddress`.  In the https agent, the name includes the
CA, cert, ciphers, and other HTTPS/TLS-specific options that determine
socket reusability.

### agent.maxFreeSockets
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* {Number}

By default set to 256.  For Agents supporting HTTP KeepAlive, this
sets the maximum number of sockets that will be left open in the free
state.

### agent.maxSockets
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* {Number}

By default set to Infinity. Determines how many concurrent sockets the agent
can have open per origin. Origin is either a 'host:port' or
'host:port:localAddress' combination.

### agent.requests
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* {Object}

An object which contains queues of requests that have not yet been assigned to
sockets. Do not modify.

### agent.sockets
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* {Object}

An object which contains arrays of sockets currently in use by the
Agent.  Do not modify.

## Class: http.ClientRequest
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This object is created internally and returned from [`http.request()`][].  It
represents an _in-progress_ request whose header has already been queued.  The
header is still mutable using the `setHeader(name, value)`, `getHeader(name)`,
`removeHeader(name)` API.  The actual header will be sent along with the first
data chunk or when closing the connection.

To get the response, add a listener for [`'response'`][] to the request object.
[`'response'`][] will be emitted from the request object when the response
headers have been received.  The [`'response'`][] event is executed with one
argument which is an instance of [`http.IncomingMessage`][].

During the [`'response'`][] event, one can add listeners to the
response object; particularly to listen for the `'data'` event.

If no [`'response'`][] handler is added, then the response will be
entirely discarded.  However, if you add a [`'response'`][] event handler,
then you **must** consume the data from the response object, either by
calling `response.read()` whenever there is a `'readable'` event, or
by adding a `'data'` handler, or by calling the `.resume()` method.
Until the data is consumed, the `'end'` event will not fire.  Also, until
the data is read it will consume memory that can eventually lead to a
'process out of memory' error.

Note: Node.js does not check whether Content-Length and the length of the body
which has been transmitted are equal or not.

The request implements the [Writable Stream][] interface. This is an
[`EventEmitter`][] with the following events:

### Event: 'abort'
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Emitted when the request has been aborted by the client. This event is only
emitted on the first call to `abort()`.

### Event: 'aborted'
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Emitted when the request has been aborted by the server and the network
socket has closed.

### Event: 'connect'
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* `response` {http.IncomingMessage}
* `socket` {net.Socket}
* `head` {Buffer}

Emitted each time a server responds to a request with a `CONNECT` method. If this
event isn't being listened for, clients receiving a `CONNECT` method will have
their connections closed.

A client and server pair that shows you how to listen for the `'connect'` event:

```js
const http = require('http');
const net = require('net');
const url = require('url');

// Create an HTTP tunneling proxy
var proxy = http.createServer( (req, res) => {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('okay');
});
proxy.on('connect', (req, cltSocket, head) => {
  // connect to an origin server
  var srvUrl = url.parse(`http://${req.url}`);
  var srvSocket = net.connect(srvUrl.port, srvUrl.hostname, () => {
    cltSocket.write('HTTP/1.1 200 Connection Established\r\n' +
                    'Proxy-agent: Node.js-Proxy\r\n' +
                    '\r\n');
    srvSocket.write(head);
    srvSocket.pipe(cltSocket);
    cltSocket.pipe(srvSocket);
  });
});

// now that proxy is running
proxy.listen(1337, '127.0.0.1', () => {

  // make a request to a tunneling proxy
  var options = {
    port: 1337,
    hostname: '127.0.0.1',
    method: 'CONNECT',
    path: 'www.google.com:80'
  };

  var req = http.request(options);
  req.end();

  req.on('connect', (res, socket, head) => {
    console.log('got connected!');

    // make a request over an HTTP tunnel
    socket.write('GET / HTTP/1.1\r\n' +
                 'Host: www.google.com:80\r\n' +
                 'Connection: close\r\n' +
                 '\r\n');
    socket.on('data', (chunk) => {
      console.log(chunk.toString());
    });
    socket.on('end', () => {
      proxy.close();
    });
  });
});
```

### Event: 'continue'
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Emitted when the server sends a '100 Continue' HTTP response, usually because
the request contained 'Expect: 100-continue'. This is an instruction that
the client should send the request body.

### Event: 'response'
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* `response` {http.IncomingMessage}

Emitted when a response is received to this request. This event is emitted only
once.

### Event: 'socket'
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* `socket` {net.Socket}

Emitted after a socket is assigned to this request.

### Event: 'upgrade'
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* `response` {http.IncomingMessage}
* `socket` {net.Socket}
* `head` {Buffer}

Emitted each time a server responds to a request with an upgrade. If this
event isn't being listened for, clients receiving an upgrade header will have
their connections closed.

A client server pair that show you how to listen for the `'upgrade'` event.

```js
const http = require('http');

// Create an HTTP server
var srv = http.createServer( (req, res) => {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('okay');
});
srv.on('upgrade', (req, socket, head) => {
  socket.write('HTTP/1.1 101 Web Socket Protocol Handshake\r\n' +
               'Upgrade: WebSocket\r\n' +
               'Connection: Upgrade\r\n' +
               '\r\n');

  socket.pipe(socket); // echo back
});

// now that server is running
srv.listen(1337, '127.0.0.1', () => {

  // make a request
  var options = {
    port: 1337,
    hostname: '127.0.0.1',
    headers: {
      'Connection': 'Upgrade',
      'Upgrade': 'websocket'
    }
  };

  var req = http.request(options);
  req.end();

  req.on('upgrade', (res, socket, upgradeHead) => {
    console.log('got upgraded!');
    socket.end();
    process.exit(0);
  });
});
```

### request.abort()
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Marks the request as aborting. Calling this will cause remaining data
in the response to be dropped and the socket to be destroyed.

### request.end([data][, encoding][, callback])
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* `data` {String | Buffer}
* `encoding` {String}
* `callback` {Function}

Finishes sending the request. If any parts of the body are
unsent, it will flush them to the stream. If the request is
chunked, this will send the terminating `'0\r\n\r\n'`.

If `data` is specified, it is equivalent to calling
[`response.write(data, encoding)`][] followed by `request.end(callback)`.

If `callback` is specified, it will be called when the request stream
is finished.

### request.flushHeaders()
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Flush the request headers.

For efficiency reasons, Node.js normally buffers the request headers until you
call `request.end()` or write the first chunk of request data.  It then tries
hard to pack the request headers and data into a single TCP packet.

That's usually what you want (it saves a TCP round-trip) but not when the first
data isn't sent until possibly much later.  `request.flushHeaders()` lets you bypass
the optimization and kickstart the request.

### request.setNoDelay([noDelay])
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* `noDelay` {Boolean}

Once a socket is assigned to this request and is connected
[`socket.setNoDelay()`][] will be called.

### request.setSocketKeepAlive([enable][, initialDelay])
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* `enable` {Boolean}
* `initialDelay` {Number}

Once a socket is assigned to this request and is connected
[`socket.setKeepAlive()`][] will be called.

### request.setTimeout(timeout[, callback])
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* `timeout` {Number} Milliseconds before a request is considered to be timed out.
* `callback` {Function} Optional function to be called when a timeout occurs. Same as binding to the `timeout` event.

Once a socket is assigned to this request and is connected
[`socket.setTimeout()`][] will be called.

Returns `request`.

### request.write(chunk[, encoding][, callback])
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* `chunk` {String | Buffer}
* `encoding` {String}
* `callback` {Function}

Sends a chunk of the body.  By calling this method
many times, the user can stream a request body to a
server--in that case it is suggested to use the
`['Transfer-Encoding', 'chunked']` header line when
creating the request.

The `encoding` argument is optional and only applies when `chunk` is a string.
Defaults to `'utf8'`.

The `callback` argument is optional and will be called when this chunk of data
is flushed.

Returns `request`.

## Class: http.Server
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This class inherits from [`net.Server`][] and has the following additional events:

### Event: 'checkContinue'
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* `request` {http.IncomingMessage}
* `response` {http.ServerResponse}

Emitted each time a request with an HTTP `Expect: 100-continue` is received.
If this event isn't listened for, the server will automatically respond
with a `100 Continue` as appropriate.

Handling this event involves calling [`response.writeContinue()`][] if the client
should continue to send the request body, or generating an appropriate HTTP
response (e.g. 400 Bad Request) if the client should not continue to send the
request body.

Note that when this event is emitted and handled, the [`'request'`][] event will
not be emitted.

### Event: 'checkExpectation'
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* `request` {http.ClientRequest}
* `response` {http.ServerResponse}

Emitted each time a request with an HTTP `Expect` header is received, where the
value is not `100-continue`. If this event isn't listened for, the server will
automatically respond with a `417 Expectation Failed` as appropriate.

Note that when this event is emitted and handled, the [`'request'`][] event will
not be emitted.

### Event: 'clientError'
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* `exception` {Error}
* `socket` {net.Socket}

If a client connection emits an `'error'` event, it will be forwarded here.
Listener of this event is responsible for closing/destroying the underlying
socket. For example, one may wish to more gracefully close the socket with an
HTTP '400 Bad Request' response instead of abruptly severing the connection.

Default behavior is to destroy the socket immediately on malformed request.

`socket` is the [`net.Socket`][] object that the error originated from.

```js
const http = require('http');

const server = http.createServer((req, res) => {
  res.end();
});
server.on('clientError', (err, socket) => {
  socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});
server.listen(8000);
```

When the `'clientError'` event occurs, there is no `request` or `response`
object, so any HTTP response sent, including response headers and payload,
*must* be written directly to the `socket` object. Care must be taken to
ensure the response is a properly formatted HTTP response message.

### Event: 'close'
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Emitted when the server closes.

### Event: 'connect'
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* `request` {http.IncomingMessage} Arguments for the HTTP request, as it is in
  the [`'request'`][] event
* `socket` {net.Socket} Network socket between the server and client
* `head` {Buffer} The first packet of the tunneling stream (may be empty)

Emitted each time a client requests an HTTP `CONNECT` method. If this event isn't
listened for, then clients requesting a `CONNECT` method will have their
connections closed.

After this event is emitted, the request's socket will not have a `'data'`
event listener, meaning you will need to bind to it in order to handle data
sent to the server on that socket.

### Event: 'connection'
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* `socket` {net.Socket}

When a new TCP stream is established. `socket` is an object of type
[`net.Socket`][]. Usually users will not want to access this event. In
particular, the socket will not emit `'readable'` events because of how
the protocol parser attaches to the socket. The `socket` can also be
accessed at `request.connection`.

### Event: 'request'
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* `request` {http.IncomingMessage}
* `response` {http.ServerResponse}

Emitted each time there is a request. Note that there may be multiple requests
per connection (in the case of keep-alive connections).

### Event: 'upgrade'
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* `request` {http.IncomingMessage} Arguments for the HTTP request, as it is in
  the [`'request'`][] event
* `socket` {net.Socket} Network socket between the server and client
* `head` {Buffer} The first packet of the upgraded stream (may be empty)

Emitted each time a client requests an HTTP upgrade. If this event isn't
listened for, then clients requesting an upgrade will have their connections
closed.

After this event is emitted, the request's socket will not have a `'data'`
event listener, meaning you will need to bind to it in order to handle data
sent to the server on that socket.

### server.close([callback])
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* `callback` {Function}

Stops the server from accepting new connections.  See [`net.Server.close()`][].

### server.listen(handle[, callback])
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* `handle` {Object}
* `callback` {Function}

The `handle` object can be set to either a server or socket (anything
with an underlying `_handle` member), or a `{fd: <n>}` object.

This will cause the server to accept connections on the specified
handle, but it is presumed that the file descriptor or handle has
already been bound to a port or domain socket.

Listening on a file descriptor is not supported on Windows.

This function is asynchronous. `callback` will be added as a listener for the
[`'listening'`][] event. See also [`net.Server.listen()`][].

Returns `server`.

*Note*: The `server.listen()` method may be called multiple times. Each
subsequent call will *re-open* the server using the provided options.

### server.listen(path[, callback])
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* `path` {String}
* `callback` {Function}

Start a UNIX socket server listening for connections on the given `path`.

This function is asynchronous. `callback` will be added as a listener for the
[`'listening'`][] event.  See also [`net.Server.listen(path)`][].

*Note*: The `server.listen()` method may be called multiple times. Each
subsequent call will *re-open* the server using the provided options.

### server.listen([port][, hostname][, backlog][, callback])
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* `port` {Number}
* `hostname` {String}
* `backlog` {Number}
* `callback` {Function}

Begin accepting connections on the specified `port` and `hostname`. If the
`hostname` is omitted, the server will accept connections on any IPv6 address
(`::`) when IPv6 is available, or any IPv4 address (`0.0.0.0`) otherwise.
Omit the port argument, or use a port value of `0`, to have the operating system
assign a random port, which can be retrieved by using `server.address().port`
after the `'listening'` event has been emitted.

To listen to a unix socket, supply a filename instead of port and hostname.

`backlog` is the maximum length of the queue of pending connections.
The actual length will be determined by your OS through sysctl settings such as
`tcp_max_syn_backlog` and `somaxconn` on linux. The default value of this
parameter is 511 (not 512).

This function is asynchronous. `callback` will be added as a listener for the
[`'listening'`][] event.  See also [`net.Server.listen(port)`][].

*Note*: The `server.listen()` method may be called multiple times. Each
subsequent call will *re-open* the server using the provided options.

### server.listening
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* {Boolean}

A Boolean indicating whether or not the server is listening for
connections.

### server.maxHeadersCount
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* {Number}

Limits maximum incoming headers count, equal to 1000 by default. If set to 0 -
no limit will be applied.

### server.setTimeout(msecs, callback)
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* `msecs` {Number}
* `callback` {Function}

Sets the timeout value for sockets, and emits a `'timeout'` event on
the Server object, passing the socket as an argument, if a timeout
occurs.

If there is a `'timeout'` event listener on the Server object, then it
will be called with the timed-out socket as an argument.

By default, the Server's timeout value is 2 minutes, and sockets are
destroyed automatically if they time out.  However, if you assign a
callback to the Server's `'timeout'` event, then you are responsible
for handling socket timeouts.

Returns `server`.

### server.timeout
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* {Number} Default = 120000 (2 minutes)

The number of milliseconds of inactivity before a socket is presumed
to have timed out.

Note that the socket timeout logic is set up on connection, so
changing this value only affects *new* connections to the server, not
any existing connections.

Set to 0 to disable any kind of automatic timeout behavior on incoming
connections.

## Class: http.ServerResponse
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This object is created internally by an HTTP server--not by the user. It is
passed as the second parameter to the [`'request'`][] event.

The response implements, but does not inherit from, the [Writable Stream][]
interface. This is an [`EventEmitter`][] with the following events:

### Event: 'close'
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Indicates that the underlying connection was terminated before
[`response.end()`][] was called or able to flush.

### Event: 'finish'
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Emitted when the response has been sent. More specifically, this event is
emitted when the last segment of the response headers and body have been
handed off to the operating system for transmission over the network. It
does not imply that the client has received anything yet.

After this event, no more events will be emitted on the response object.

### response.addTrailers(headers)
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* `headers` {Object}

This method adds HTTP trailing headers (a header but at the end of the
message) to the response.

Trailers will **only** be emitted if chunked encoding is used for the
response; if it is not (e.g. if the request was HTTP/1.0), they will
be silently discarded.

Note that HTTP requires the `Trailer` header to be sent if you intend to
emit trailers, with a list of the header fields in its value. E.g.,

```js
response.writeHead(200, { 'Content-Type': 'text/plain',
                          'Trailer': 'Content-MD5' });
response.write(fileData);
response.addTrailers({'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667'});
response.end();
```

Attempting to set a header field name or value that contains invalid characters
will result in a [`TypeError`][] being thrown.

### response.end([data][, encoding][, callback])
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* `data` {String | Buffer}
* `encoding` {String}
* `callback` {Function}

This method signals to the server that all of the response headers and body
have been sent; that server should consider this message complete.
The method, `response.end()`, MUST be called on each response.

If `data` is specified, it is equivalent to calling
[`response.write(data, encoding)`][] followed by `response.end(callback)`.

If `callback` is specified, it will be called when the response stream
is finished.

### response.finished
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* {Boolean}

Boolean value that indicates whether the response has completed. Starts
as `false`. After [`response.end()`][] executes, the value will be `true`.

### response.getHeader(name)
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* `name` {String}
* Returns: {String}

Reads out a header that's already been queued but not sent to the client.  Note
that the name is case insensitive.  This can only be called before headers get
implicitly flushed.

Example:

```js
var contentType = response.getHeader('content-type');
```

### response.headersSent
<!-- YAML
added: v0.9.3
-->

* {Boolean}

Boolean (read-only). True if headers were sent, false otherwise.

### response.removeHeader(name)
<!-- YAML
added: v0.4.0
-->

* `name` {String}

Removes a header that's queued for implicit sending.

Example:

```js
response.removeHeader('Content-Encoding');
```

### response.sendDate
<!-- YAML
added: v0.7.5
-->

* {Boolean}

When true, the Date header will be automatically generated and sent in
the response if it is not already present in the headers. Defaults to true.

This should only be disabled for testing; HTTP requires the Date header
in responses.

### response.setHeader(name, value)
<!-- YAML
added: v0.4.0
-->

* `name` {String}
* `value` {String}

Sets a single header value for implicit headers.  If this header already exists
in the to-be-sent headers, its value will be replaced.  Use an array of strings
here if you need to send multiple headers with the same name.

Example:

```js
response.setHeader('Content-Type', 'text/html');
```

or

```js
response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);
```

Attempting to set a header field name or value that contains invalid characters
will result in a [`TypeError`][] being thrown.

When headers have been set with [`response.setHeader()`][], they will be merged with
any headers passed to [`response.writeHead()`][], with the headers passed to
[`response.writeHead()`][] given precedence.

```js
// returns content-type = text/plain
const server = http.createServer((req,res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('ok');
});
```

### response.setTimeout(msecs, callback)
<!-- YAML
added: v0.9.12
-->

* `msecs` {Number}
* `callback` {Function}

Sets the Socket's timeout value to `msecs`.  If a callback is
provided, then it is added as a listener on the `'timeout'` event on
the response object.

If no `'timeout'` listener is added to the request, the response, or
the server, then sockets are destroyed when they time out.  If you
assign a handler on the request, the response, or the server's
`'timeout'` events, then it is your responsibility to handle timed out
sockets.

Returns `response`.

### response.statusCode
<!-- YAML
added: v0.4.0
-->

* {Number}

When using implicit headers (not calling [`response.writeHead()`][] explicitly),
this property controls the status code that will be sent to the client when
the headers get flushed.

Example:

```js
response.statusCode = 404;
```

After response header was sent to the client, this property indicates the
status code which was sent out.

### response.statusMessage
<!-- YAML
added: v0.11.8
-->

* {String}

When using implicit headers (not calling [`response.writeHead()`][] explicitly), this property
controls the status message that will be sent to the client when the headers get
flushed. If this is left as `undefined` then the standard message for the status
code will be used.

Example:

```js
response.statusMessage = 'Not found';
```

After response header was sent to the client, this property indicates the
status message which was sent out.

### response.write(chunk[, encoding][, callback])
<!-- YAML
added: v0.1.29
-->

* `chunk` {String | Buffer}
* `encoding` {String}
* `callback` {Function}
* Returns: {Boolean}

If this method is called and [`response.writeHead()`][] has not been called,
it will switch to implicit header mode and flush the implicit headers.

This sends a chunk of the response body. This method may
be called multiple times to provide successive parts of the body.

`chunk` can be a string or a buffer. If `chunk` is a string,
the second parameter specifies how to encode it into a byte stream.
By default the `encoding` is `'utf8'`. `callback` will be called when this chunk
of data is flushed.

**Note**: This is the raw HTTP body and has nothing to do with
higher-level multi-part body encodings that may be used.

The first time [`response.write()`][] is called, it will send the buffered
header information and the first body to the client. The second time
[`response.write()`][] is called, Node.js assumes you're going to be streaming
data, and sends that separately. That is, the response is buffered up to the
first chunk of body.

Returns `true` if the entire data was flushed successfully to the kernel
buffer. Returns `false` if all or part of the data was queued in user memory.
`'drain'` will be emitted when the buffer is free again.

### response.writeContinue()
<!-- YAML
added: v0.3.0
-->

Sends a HTTP/1.1 100 Continue message to the client, indicating that
the request body should be sent. See the [`'checkContinue'`][] event on `Server`.

### response.writeHead(statusCode[, statusMessage][, headers])
<!-- YAML
added: v0.1.30
-->

* `statusCode` {Number}
* `statusMessage` {String}
* `headers` {Object}

Sends a response header to the request. The status code is a 3-digit HTTP
status code, like `404`. The last argument, `headers`, are the response headers.
Optionally one can give a human-readable `statusMessage` as the second
argument.

Example:

```js
var body = 'hello world';
response.writeHead(200, {
  'Content-Length': Buffer.byteLength(body),
  'Content-Type': 'text/plain' });
```

This method must only be called once on a message and it must
be called before [`response.end()`][] is called.

If you call [`response.write()`][] or [`response.end()`][] before calling this,
the implicit/mutable headers will be calculated and call this function for you.

When headers have been set with [`response.setHeader()`][], they will be merged with
any headers passed to [`response.writeHead()`][], with the headers passed to
[`response.writeHead()`][] given precedence.

```js
// returns content-type = text/plain
const server = http.createServer((req,res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('ok');
});
```

Note that Content-Length is given in bytes not characters. The above example
works because the string `'hello world'` contains only single byte characters.
If the body contains higher coded characters then `Buffer.byteLength()`
should be used to determine the number of bytes in a given encoding.
And Node.js does not check whether Content-Length and the length of the body
which has been transmitted are equal or not.

Attempting to set a header field name or value that contains invalid characters
will result in a [`TypeError`][] being thrown.

## Class: http.IncomingMessage
<!-- YAML
added: v0.1.17
-->

An `IncomingMessage` object is created by [`http.Server`][] or
[`http.ClientRequest`][] and passed as the first argument to the [`'request'`][]
and [`'response'`][] event respectively. It may be used to access response status,
headers and data.

It implements the [Readable Stream][] interface, as well as the
following additional events, methods, and properties.

### Event: 'aborted'
<!-- YAML
added: v0.3.8
-->

Emitted when the request has been aborted by the client and the network
socket has closed.

### Event: 'close'
<!-- YAML
added: v0.4.2
-->

Indicates that the underlying connection was closed.
Just like `'end'`, this event occurs only once per response.

### message.destroy([error])
<!-- YAML
added: v0.3.0
-->

* `error` {Error}

Calls `destroy()` on the socket that received the `IncomingMessage`. If `error`
is provided, an `'error'` event is emitted and `error` is passed as an argument
to any listeners on the event.

### message.headers
<!-- YAML
added: v0.1.5
-->

* {Object}

The request/response headers object.

Key-value pairs of header names and values. Header names are lower-cased.
Example:

```js
// Prints something like:
//
// { 'user-agent': 'curl/7.22.0',
//   host: '127.0.0.1:8000',
//   accept: '*/*' }
console.log(request.headers);
```

Duplicates in raw headers are handled in the following ways, depending on the
header name:

* Duplicates of `age`, `authorization`, `content-length`, `content-type`,
`etag`, `expires`, `from`, `host`, `if-modified-since`, `if-unmodified-since`,
`last-modified`, `location`, `max-forwards`, `proxy-authorization`, `referer`,
`retry-after`, or `user-agent` are discarded.
* `set-cookie` is always an array. Duplicates are added to the array.
* For all other headers, the values are joined together with ', '.

### message.httpVersion
<!-- YAML
added: v0.1.1
-->

* {String}

In case of server request, the HTTP version sent by the client. In the case of
client response, the HTTP version of the connected-to server.
Probably either `'1.1'` or `'1.0'`.

Also `message.httpVersionMajor` is the first integer and
`message.httpVersionMinor` is the second.

### message.method
<!-- YAML
added: v0.1.1
-->

* {String}

**Only valid for request obtained from [`http.Server`][].**

The request method as a string. Read only. Example:
`'GET'`, `'DELETE'`.

### message.rawHeaders
<!-- YAML
added: v0.11.6
-->

* {Array}

The raw request/response headers list exactly as they were received.

Note that the keys and values are in the same list.  It is *not* a
list of tuples.  So, the even-numbered offsets are key values, and the
odd-numbered offsets are the associated values.

Header names are not lowercased, and duplicates are not merged.

```js
// Prints something like:
//
// [ 'user-agent',
//   'this is invalid because there can be only one',
//   'User-Agent',
//   'curl/7.22.0',
//   'Host',
//   '127.0.0.1:8000',
//   'ACCEPT',
//   '*/*' ]
console.log(request.rawHeaders);
```

### message.rawTrailers
<!-- YAML
added: v0.11.6
-->

* {Array}

The raw request/response trailer keys and values exactly as they were
received.  Only populated at the `'end'` event.

### message.setTimeout(msecs, callback)
<!-- YAML
added: v0.5.9
-->

* `msecs` {Number}
* `callback` {Function}

Calls `message.connection.setTimeout(msecs, callback)`.

Returns `message`.

### message.statusCode
<!-- YAML
added: v0.1.1
-->

* {Number}

**Only valid for response obtained from [`http.ClientRequest`][].**

The 3-digit HTTP response status code. E.G. `404`.

### message.statusMessage
<!-- YAML
added: v0.11.10
-->

* {String}

**Only valid for response obtained from [`http.ClientRequest`][].**

The HTTP response status message (reason phrase). E.G. `OK` or `Internal Server Error`.

### message.socket
<!-- YAML
added: v0.3.0
-->

* {net.Socket}

The [`net.Socket`][] object associated with the connection.

With HTTPS support, use [`request.socket.getPeerCertificate()`][] to obtain the
client's authentication details.

### message.trailers
<!-- YAML
added: v0.3.0
-->

* {Object}

The request/response trailers object. Only populated at the `'end'` event.

### message.url
<!-- YAML
added: v0.1.90
-->

* {String}

**Only valid for request obtained from [`http.Server`][].**

Request URL string. This contains only the URL that is
present in the actual HTTP request. If the request is:

```txt
GET /status?name=ryan HTTP/1.1\r\n
Accept: text/plain\r\n
\r\n
```

Then `request.url` will be:

```js
'/status?name=ryan'
```

If you would like to parse the URL into its parts, you can use
`require('url').parse(request.url)`.  Example:

```txt
$ node
> require('url').parse('/status?name=ryan')
{
  href: '/status?name=ryan',
  search: '?name=ryan',
  query: 'name=ryan',
  pathname: '/status'
}
```

If you would like to extract the parameters from the query string,
you can use the `require('querystring').parse` function, or pass
`true` as the second argument to `require('url').parse`.  Example:

```txt
$ node
> require('url').parse('/status?name=ryan', true)
{
  href: '/status?name=ryan',
  search: '?name=ryan',
  query: {name: 'ryan'},
  pathname: '/status'
}
```

## http.METHODS
<!-- YAML
added: v0.11.8
-->

* {Array}

A list of the HTTP methods that are supported by the parser.

## http.STATUS_CODES
<!-- YAML
added: v0.1.22
-->

* {Object}

A collection of all the standard HTTP response status codes, and the
short description of each.  For example, `http.STATUS_CODES[404] === 'Not
Found'`.

## http.createServer([requestListener])
<!-- YAML
added: v0.1.13
-->

* Returns: {http.Server}

Returns a new instance of [`http.Server`][].

The `requestListener` is a function which is automatically
added to the [`'request'`][] event.

## http.get(options[, callback])
<!-- YAML
added: v0.3.6
-->

* `options` {Object}
* `callback` {Function}
* Returns: {http.ClientRequest}

Since most requests are GET requests without bodies, Node.js provides this
convenience method. The only difference between this method and
[`http.request()`][] is that it sets the method to GET and calls `req.end()`
automatically. Note that response data must be consumed in the callback
for reasons stated in [`http.ClientRequest`][] section.

The `callback` is invoked with a single argument that is an instance of
[`http.IncomingMessage`][]

JSON Fetching Example:

```js
http.get('http://nodejs.org/dist/index.json', (res) => {
  const statusCode = res.statusCode;
  const contentType = res.headers['content-type'];

  let error;
  if (statusCode !== 200) {
    error = new Error(`Request Failed.\n` +
                      `Status Code: ${statusCode}`);
  } else if (!/^application\/json/.test(contentType)) {
    error = new Error(`Invalid content-type.\n` +
                      `Expected application/json but received ${contentType}`);
  }
  if (error) {
    console.log(error.message);
    // consume response data to free up memory
    res.resume();
    return;
  }

  res.setEncoding('utf8');
  let rawData = '';
  res.on('data', (chunk) => rawData += chunk);
  res.on('end', () => {
    try {
      let parsedData = JSON.parse(rawData);
      console.log(parsedData);
    } catch (e) {
      console.log(e.message);
    }
  });
}).on('error', (e) => {
  console.log(`Got error: ${e.message}`);
});
```

## http.globalAgent
<!-- YAML
added: v0.5.9
-->

* {http.Agent}

Global instance of Agent which is used as the default for all HTTP client
requests.

## http.request(options[, callback])
<!-- YAML
added: v0.3.6
-->

* `options` {Object}
  * `protocol` {String} Protocol to use. Defaults to `'http:'`.
  * `host` {String} A domain name or IP address of the server to issue the
    request to. Defaults to `'localhost'`.
  * `hostname` {String} Alias for `host`. To support [`url.parse()`][],
    `hostname` is preferred over `host`.
  * `family` {Number} IP address family to use when resolving `host` and
    `hostname`. Valid values are `4` or `6`. When unspecified, both IP v4 and
    v6 will be used.
  * `port` {Number} Port of remote server. Defaults to 80.
  * `localAddress` {String} Local interface to bind for network connections.
  * `socketPath` {String} Unix Domain Socket (use one of host:port or
    socketPath).
  * `method` {String} A string specifying the HTTP request method. Defaults to
    `'GET'`.
  * `path` {String} Request path. Defaults to `'/'`. Should include query
    string if any. E.G. `'/index.html?page=12'`. An exception is thrown when
    the request path contains illegal characters. Currently, only spaces are
    rejected but that may change in the future.
  * `headers` {Object} An object containing request headers.
  * `auth` {String} Basic authentication i.e. `'user:password'` to compute an
    Authorization header.
  * `agent` {http.Agent|Boolean} Controls [`Agent`][] behavior. When an Agent
    is used request will default to `Connection: keep-alive`. Possible values:
   * `undefined` (default): use [`http.globalAgent`][] for this host and port.
   * `Agent` object: explicitly use the passed in `Agent`.
   * `false`: opts out of connection pooling with an Agent, defaults request to
     `Connection: close`.
  * `createConnection` {Function} A function that produces a socket/stream to
    use for the request when the `agent` option is not used. This can be used to
    avoid creating a custom Agent class just to override the default
    `createConnection` function. See [`agent.createConnection()`][] for more
    details.
  * `timeout` {Integer}: A number specifying the socket timeout in milliseconds.
    This will set the timeout before the socket is connected.
* `callback` {Function}
* Returns: {http.ClientRequest}

Node.js maintains several connections per server to make HTTP requests.
This function allows one to transparently issue requests.

`options` can be an object or a string. If `options` is a string, it is
automatically parsed with [`url.parse()`][].

The optional `callback` parameter will be added as a one time listener for
the [`'response'`][] event.

`http.request()` returns an instance of the [`http.ClientRequest`][]
class. The `ClientRequest` instance is a writable stream. If one needs to
upload a file with a POST request, then write to the `ClientRequest` object.

Example:

```js
var postData = querystring.stringify({
  'msg' : 'Hello World!'
});

var options = {
  hostname: 'www.google.com',
  port: 80,
  path: '/upload',
  method: 'POST',
  headers: {
    'Content-Type': 'application/x-www-form-urlencoded',
    'Content-Length': Buffer.byteLength(postData)
  }
};

var req = http.request(options, (res) => {
  console.log(`STATUS: ${res.statusCode}`);
  console.log(`HEADERS: ${JSON.stringify(res.headers)}`);
  res.setEncoding('utf8');
  res.on('data', (chunk) => {
    console.log(`BODY: ${chunk}`);
  });
  res.on('end', () => {
    console.log('No more data in response.');
  });
});

req.on('error', (e) => {
  console.log(`problem with request: ${e.message}`);
});

// write data to request body
req.write(postData);
req.end();
```

Note that in the example `req.end()` was called. With `http.request()` one
must always call `req.end()` to signify that you're done with the request -
even if there is no data being written to the request body.

If any error is encountered during the request (be that with DNS resolution,
TCP level errors, or actual HTTP parse errors) an `'error'` event is emitted
on the returned request object. As with all `'error'` events, if no listeners
are registered the error will be thrown.

There are a few special headers that should be noted.

* Sending a 'Connection: keep-alive' will notify Node.js that the connection to
  the server should be persisted until the next request.

* Sending a 'Content-length' header will disable the default chunked encoding.

* Sending an 'Expect' header will immediately send the request headers.
  Usually, when sending 'Expect: 100-continue', you should both set a timeout
  and listen for the `'continue'` event. See RFC2616 Section 8.2.3 for more
  information.

* Sending an Authorization header will override using the `auth` option
  to compute basic authentication.

[`'checkContinue'`]: #http_event_checkcontinue
[`'listening'`]: net.html#net_event_listening
[`'request'`]: #http_event_request
[`'response'`]: #http_event_response
[`Agent`]: #http_class_http_agent
[`agent.createConnection()`]: #http_agent_createconnection_options_callback
[`destroy()`]: #http_agent_destroy
[`EventEmitter`]: events.html#events_class_eventemitter
[`http.Agent`]: #http_class_http_agent
[`http.ClientRequest`]: #http_class_http_clientrequest
[`http.globalAgent`]: #http_http_globalagent
[`http.IncomingMessage`]: #http_class_http_incomingmessage
[`http.request()`]: #http_http_request_options_callback
[`http.Server`]: #http_class_http_server
[`message.headers`]: #http_message_headers
[`net.createConnection()`]: net.html#net_net_createconnection_options_connectlistener
[`net.Server`]: net.html#net_class_net_server
[`net.Server.close()`]: net.html#net_server_close_callback
[`net.Server.listen()`]: net.html#net_server_listen_handle_backlog_callback
[`net.Server.listen(path)`]: net.html#net_server_listen_path_backlog_callback
[`net.Server.listen(port)`]: net.html#net_server_listen_port_hostname_backlog_callback
[`net.Socket`]: net.html#net_class_net_socket
[`request.socket.getPeerCertificate()`]: tls.html#tls_tlssocket_getpeercertificate_detailed
[`response.end()`]: #http_response_end_data_encoding_callback
[`response.setHeader()`]: #http_response_setheader_name_value
[`response.write()`]: #http_response_write_chunk_encoding_callback
[`response.write(data, encoding)`]: #http_response_write_chunk_encoding_callback
[`response.writeContinue()`]: #http_response_writecontinue
[`response.writeHead()`]: #http_response_writehead_statuscode_statusmessage_headers
[`socket.setKeepAlive()`]: net.html#net_socket_setkeepalive_enable_initialdelay
[`socket.setNoDelay()`]: net.html#net_socket_setnodelay_nodelay
[`socket.setTimeout()`]: net.html#net_socket_settimeout_timeout_callback
[`TypeError`]: errors.html#errors_class_typeerror
[`url.parse()`]: url.html#url_url_parse_urlstring_parsequerystring_slashesdenotehost
[constructor options]: #http_new_agent_options
[Readable Stream]: stream.html#stream_class_stream_readable
[Writable Stream]: stream.html#stream_class_stream_writable