SIBS2024/node_modules/node-fetch

node-fetch

A light-weight module that brings window.fetch to Node.js

(We are looking for v2 maintainers and collaborators)

• Motivation
• Features
• Difference from client-side fetch
• Installation
• Loading and configuring the module
• Common Usage
  • Plain text or HTML
  • JSON
  • Simple Post
  • Post with JSON
  • Post with form parameters
  • Handling exceptions
  • Handling client and server errors
• Advanced Usage
  • Streams
  • Buffer
  • Accessing Headers and other Meta data
  • Extract Set-Cookie Header
  • Post data using a file stream
  • Post with form-data (detect multipart)
  • Request cancellation with AbortSignal
• API
  • fetch(url[, options])
  • Options
  • Class: Request
  • Class: Response
  • Class: Headers
  • Interface: Body
  • Class: FetchError
• License
• Acknowledgement

Motivation

Instead of implementing XMLHttpRequest in Node.js to run browser-specific Fetch polyfill, why not go from native http to fetch API directly? Hence, node-fetch, minimal code for a window.fetch compatible API on Node.js runtime.

See Matt Andrews’ isomorphic-fetch or Leonardo Quixada’s cross-fetch for isomorphic usage (exports node-fetch for server-side, whatwg-fetch for client-side).

Features

• Stay consistent with window.fetch API.
• Make conscious trade-off when following WHATWG fetch spec and stream spec implementation details, document known differences.
• Use native promise but allow substituting it with [insert your favorite promise library].
• Use native Node streams for body on both request and response.
• Decode content encoding (gzip/deflate) properly and convert string output (such as res.text() and res.json()) to UTF-8 automatically.
• Useful extensions such as timeout, redirect limit, response size limit, explicit errors for troubleshooting.

Difference from client-side fetch

• See Known Differences for details.
• If you happen to use a missing feature that window.fetch offers, feel free to open an issue.
• Pull requests are welcomed too!

Installation

Current stable release (2.x)

$ npm install node-fetch

Loading and configuring the module

We suggest you load the module via require until the stabilization of ES modules in node:

const fetch = require('node-fetch');

If you are using a Promise library other than native, set it through fetch.Promise:

const Bluebird = require('bluebird');

fetch.Promise = Bluebird;

Common Usage

NOTE: The documentation below is up-to-date with 2.x releases; see the 1.x readme, changelog and 2.x upgrade guide for the differences.

Plain text or HTML

fetch('https://github.com/')
    .then(res => res.text())
    .then(body => console.log(body));

JSON

fetch('https://api.github.com/users/github')
    .then(res => res.json())
    .then(json => console.log(json));

Simple Post

fetch('https://httpbin.org/post', { method: 'POST', body: 'a=1' })
    .then(res => res.json()) // expecting a json response
    .then(json => console.log(json));

Post with JSON

const body = { a: 1 };

fetch('https://httpbin.org/post', {
        method: 'post',
        body:    JSON.stringify(body),
        headers: { 'Content-Type': 'application/json' },
    })
    .then(res => res.json())
    .then(json => console.log(json));

Post with form parameters

URLSearchParams is available in Node.js as of v7.5.0. See official documentation for more usage methods.

NOTE: The Content-Type header is only set automatically to x-www-form-urlencoded when an instance of URLSearchParams is given as such:

const { URLSearchParams } = require('url');

const params = new URLSearchParams();
params.append('a', 1);

fetch('https://httpbin.org/post', { method: 'POST', body: params })
    .then(res => res.json())
    .then(json => console.log(json));

Handling exceptions

NOTE: 3xx-5xx responses are NOT exceptions and should be handled in then(); see the next section for more information.

Adding a catch to the fetch promise chain will catch all exceptions, such as errors originating from node core libraries, network errors and operational errors, which are instances of FetchError. See the error handling document for more details.

fetch('https://domain.invalid/')
    .catch(err => console.error(err));

Handling client and server errors

It is common to create a helper function to check that the response contains no client (4xx) or server (5xx) error responses:

function checkStatus(res) {
    if (res.ok) { // res.status >= 200 && res.status < 300
        return res;
    } else {
        throw MyCustomError(res.statusText);
    }
}

fetch('https://httpbin.org/status/400')
    .then(checkStatus)
    .then(res => console.log('will not get here...'))

Advanced Usage

Streams

The “Node.js way” is to use streams when possible:

fetch('https://assets-cdn.github.com/images/modules/logos_page/Octocat.png')
    .then(res => {
        const dest = fs.createWriteStream('./octocat.png');
        res.body.pipe(dest);
    });

In Node.js 14 you can also use async iterators to read body; however, be careful to catch errors -- the longer a response runs, the more likely it is to encounter an error.

const fetch = require('node-fetch');
const response = await fetch('https://httpbin.org/stream/3');
try {
	for await (const chunk of response.body) {
		console.dir(JSON.parse(chunk.toString()));
	}
} catch (err) {
	console.error(err.stack);
}

In Node.js 12 you can also use async iterators to read body; however, async iterators with streams did not mature until Node.js 14, so you need to do some extra work to ensure you handle errors directly from the stream and wait on it response to fully close.

const fetch = require('node-fetch');
const read = async body => {
    let error;
    body.on('error', err => {
        error = err;
    });
    for await (const chunk of body) {
        console.dir(JSON.parse(chunk.toString()));
    }
    return new Promise((resolve, reject) => {
        body.on('close', () => {
            error ? reject(error) : resolve();
        });
    });
};
try {
    const response = await fetch('https://httpbin.org/stream/3');
    await read(response.body);
} catch (err) {
    console.error(err.stack);
}

Buffer

If you prefer to cache binary data in full, use buffer(). (NOTE: buffer() is a node-fetch-only API)

const fileType = require('file-type');

fetch('https://assets-cdn.github.com/images/modules/logos_page/Octocat.png')
    .then(res => res.buffer())
    .then(buffer => fileType(buffer))
    .then(type => { /* ... */ });

Accessing Headers and other Meta data

fetch('https://github.com/')
    .then(res => {
        console.log(res.ok);
        console.log(res.status);
        console.log(res.statusText);
        console.log(res.headers.raw());
        console.log(res.headers.get('content-type'));
    });

Extract Set-Cookie Header

Unlike browsers, you can access raw Set-Cookie headers manually using Headers.raw(). This is a node-fetch only API.

fetch(url).then(res => {
    // returns an array of values, instead of a string of comma-separated values
    console.log(res.headers.raw()['set-cookie']);
});

Post data using a file stream

const { createReadStream } = require('fs');

const stream = createReadStream('input.txt');

fetch('https://httpbin.org/post', { method: 'POST', body: stream })
    .then(res => res.json())
    .then(json => console.log(json));

Post with form-data (detect multipart)

const FormData = require('form-data');

const form = new FormData();
form.append('a', 1);

fetch('https://httpbin.org/post', { method: 'POST', body: form })
    .then(res => res.json())
    .then(json => console.log(json));

// OR, using custom headers
// NOTE: getHeaders() is non-standard API

const form = new FormData();
form.append('a', 1);

const options = {
    method: 'POST',
    body: form,
    headers: form.getHeaders()
}

fetch('https://httpbin.org/post', options)
    .then(res => res.json())
    .then(json => console.log(json));

Request cancellation with AbortSignal

NOTE: You may cancel streamed requests only on Node >= v8.0.0

You may cancel requests with AbortController. A suggested implementation is abort-controller.

An example of timing out a request after 150ms could be achieved as the following:

import AbortController from 'abort-controller';

const controller = new AbortController();
const timeout = setTimeout(
  () => { controller.abort(); },
  150,
);

fetch(url, { signal: controller.signal })
  .then(res => res.json())
  .then(
    data => {
      useData(data)
    },
    err => {
      if (err.name === 'AbortError') {
        // request was aborted
      }
    },
  )
  .finally(() => {
    clearTimeout(timeout);
  });

See test cases for more examples.

API

fetch(url[, options])

• url A string representing the URL for fetching
• options Options for the HTTP(S) request
• Returns: Promise<Response>

Perform an HTTP(S) fetch.

url should be an absolute url, such as https://example.com/. A path-relative URL (/file/under/root) or protocol-relative URL (//can-be-http-or-https.com/) will result in a rejected Promise.

Options

The default values are shown after each option key.

{
    // These properties are part of the Fetch Standard
    method: 'GET',
    headers: {},        // request headers. format is the identical to that accepted by the Headers constructor (see below)
    body: null,         // request body. can be null, a string, a Buffer, a Blob, or a Node.js Readable stream
    redirect: 'follow', // set to `manual` to extract redirect headers, `error` to reject redirect
    signal: null,       // pass an instance of AbortSignal to optionally abort requests

    // The following properties are node-fetch extensions
    follow: 20,         // maximum redirect count. 0 to not follow redirect
    timeout: 0,         // req/res timeout in ms, it resets on redirect. 0 to disable (OS limit applies). Signal is recommended instead.
    compress: true,     // support gzip/deflate content encoding. false to disable
    size: 0,            // maximum response body size in bytes. 0 to disable
    agent: null         // http(s).Agent instance or function that returns an instance (see below)
}

Default Headers

If no values are set, the following request headers will be sent automatically:

Header	Value
Accept-Encoding	gzip,deflate (when options.compress === true)
Accept	*/*
Content-Length	(automatically calculated, if possible)
Transfer-Encoding	chunked (when req.body is a stream)
User-Agent	node-fetch/1.0 (+https://github.com/bitinn/node-fetch)

Note: when body is a Stream, Content-Length is not set automatically.

Custom Agent

The agent option allows you to specify networking related options which are out of the scope of Fetch, including and not limited to the following:

• Support self-signed certificate
• Use only IPv4 or IPv6
• Custom DNS Lookup

See http.Agent for more information.

If no agent is specified, the default agent provided by Node.js is used. Note that this changed in Node.js 19 to have keepalive true by default. If you wish to enable keepalive in an earlier version of Node.js, you can override the agent as per the following code sample.

In addition, the agent option accepts a function that returns http(s).Agent instance given current URL, this is useful during a redirection chain across HTTP and HTTPS protocol.

const httpAgent = new http.Agent({
    keepAlive: true
});
const httpsAgent = new https.Agent({
    keepAlive: true
});

const options = {
    agent: function (_parsedURL) {
        if (_parsedURL.protocol == 'http:') {
            return httpAgent;
        } else {
            return httpsAgent;
        }
    }
}

Class: Request

An HTTP(S) request containing information about URL, method, headers, and the body. This class implements the Body interface.

Due to the nature of Node.js, the following properties are not implemented at this moment:

• type
• destination
• referrer
• referrerPolicy
• mode
• credentials
• cache
• integrity
• keepalive

The following node-fetch extension properties are provided:

• follow
• compress
• counter
• agent

See options for exact meaning of these extensions.

new Request(input[, options])

(spec-compliant)

• input A string representing a URL, or another Request (which will be cloned)
• options [Options][#fetch-options] for the HTTP(S) request

Constructs a new Request object. The constructor is identical to that in the browser.

In most cases, directly fetch(url, options) is simpler than creating a Request object.

Class: Response

An HTTP(S) response. This class implements the Body interface.

The following properties are not implemented in node-fetch at this moment:

• Response.error()
• Response.redirect()
• type
• trailer

new Response([body[, options]])

(spec-compliant)

• body A String or Readable stream
• options A ResponseInit options dictionary

Constructs a new Response object. The constructor is identical to that in the browser.

Because Node.js does not implement service workers (for which this class was designed), one rarely has to construct a Response directly.

response.ok

(spec-compliant)

Convenience property representing if the request ended normally. Will evaluate to true if the response status was greater than or equal to 200 but smaller than 300.

response.redirected

(spec-compliant)

Convenience property representing if the request has been redirected at least once. Will evaluate to true if the internal redirect counter is greater than 0.

Class: Headers

This class allows manipulating and iterating over a set of HTTP headers. All methods specified in the Fetch Standard are implemented.

new Headers([init])

(spec-compliant)

• init Optional argument to pre-fill the Headers object

Construct a new Headers object. init can be either null, a Headers object, an key-value map object or any iterable object.

// Example adapted from https://fetch.spec.whatwg.org/#example-headers-class

const meta = {
  'Content-Type': 'text/xml',
  'Breaking-Bad': '<3'
};
const headers = new Headers(meta);

// The above is equivalent to
const meta = [
  [ 'Content-Type', 'text/xml' ],
  [ 'Breaking-Bad', '<3' ]
];
const headers = new Headers(meta);

// You can in fact use any iterable objects, like a Map or even another Headers
const meta = new Map();
meta.set('Content-Type', 'text/xml');
meta.set('Breaking-Bad', '<3');
const headers = new Headers(meta);
const copyOfHeaders = new Headers(headers);

Interface: Body

Body is an abstract interface with methods that are applicable to both Request and Response classes.

The following methods are not yet implemented in node-fetch at this moment:

• formData()

body.body

(deviation from spec)

• Node.js Readable stream

Data are encapsulated in the Body object. Note that while the Fetch Standard requires the property to always be a WHATWG ReadableStream, in node-fetch it is a Node.js Readable stream.

body.bodyUsed

(spec-compliant)

• Boolean

A boolean property for if this body has been consumed. Per the specs, a consumed body cannot be used again.

body.arrayBuffer()

body.blob()

body.json()

body.text()

(spec-compliant)

• Returns: Promise

Consume the body and return a promise that will resolve to one of these formats.

body.buffer()

(node-fetch extension)

• Returns: Promise<Buffer>

Consume the body and return a promise that will resolve to a Buffer.

body.textConverted()

(node-fetch extension)

• Returns: Promise<String>

Identical to body.text(), except instead of always converting to UTF-8, encoding sniffing will be performed and text converted to UTF-8 if possible.

(This API requires an optional dependency of the npm package encoding, which you need to install manually. webpack users may see a warning message due to this optional dependency.)

Class: FetchError

(node-fetch extension)

An operational error in the fetching process. See ERROR-HANDLING.md for more info.

Class: AbortError

(node-fetch extension)

An Error thrown when the request is aborted in response to an AbortSignal’s abort event. It has a name property of AbortError. See ERROR-HANDLING.MD for more info.

Acknowledgement

Thanks to github/fetch for providing a solid implementation reference.

node-fetch v1 was maintained by @bitinn; v2 was maintained by @TimothyGu, @bitinn and @jimmywarting; v2 readme is written by @jkantr.

License

MIT