0.5.5 • Public • Published

Simple web-crawler for Node.js

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Simplecrawler is designed to provide the most basic possible API for crawling websites, while being as flexible and robust as possible. I wrote simplecrawler to archive, analyse, and search some very large websites. It has happily chewed through 50,000 pages and written tens of gigabytes to disk without issue.

Example (simple mode)

var Crawler = require("simplecrawler");
    .on("fetchcomplete", function(queueItem) {
        console.log("Completed fetching resource:", queueItem.url);

What does simplecrawler do?

  • Provides a very simple event driven API using EventEmitter
  • Extremely configurable base for writing your own crawler
  • Provides some simple logic for auto-detecting linked resources - which you can replace or augment
  • Has a flexible queue system which can be frozen to disk and defrosted
  • Provides basic statistics on network performance
  • Uses buffers for fetching and managing data, preserving binary data (except when discovering links)


npm install simplecrawler

Getting Started

There are two ways of instantiating a new crawler - a simple but less flexible method inspired by anemone, and the traditional method which provides a little more room to configure crawl parameters.

Regardless of whether you use the simple or traditional methods of instantiation, you'll need to require simplecrawler:

var Crawler = require("simplecrawler");

Simple Mode

Simple mode generates a new crawler for you, preconfigures it based on a URL you provide, and returns the crawler to you for further configuration and so you can attach event handlers.

Simply call Crawler.crawl, with a URL first parameter, and two optional functions that will be added as event listeners for fetchcomplete and fetcherror respectively.

Crawler.crawl("", function(queueItem) {
    console.log("Completed fetching resource:", queueItem.url);

Alternately, if you decide to omit these functions, you can use the returned crawler object to add the event listeners yourself, and tweak configuration options:

var crawler = Crawler.crawl("");
crawler.interval = 500;
crawler.on("fetchcomplete", function(queueItem) {
    console.log("Completed fetching resource:", queueItem.url);

Advanced Mode

The alternative method of creating a crawler is to call the simplecrawler constructor yourself, and to initiate the crawl manually.

var myCrawler = new Crawler("");

Nonstandard port? HTTPS? Want to start archiving a specific path? No problem:

myCrawler.initialPath = "/archive";
myCrawler.initialPort = 8080;
myCrawler.initialProtocol = "https";
// Or:
var myCrawler = new Crawler("", "/archive", 8080);

And of course, you're probably wanting to ensure you don't take down your web server. Decrease the concurrency from five simultaneous requests - and increase the request interval from the default 250 ms like this:

myCrawler.interval = 10000; // Ten seconds
myCrawler.maxConcurrency = 1;

You can also define a max depth for links to fetch:

myCrawler.maxDepth = 1; // Only first page is fetched (with linked CSS & images)
// Or:
myCrawler.maxDepth = 2; // First page and discovered links from it are fetched
// Or:
myCrawler.maxDepth = 3; // Etc.

For brevity, you may also specify the initial path and request interval when creating the crawler:

var myCrawler = new Crawler("", "/", 8080, 300);

Running the crawler

First, you'll need to set up an event listener to get the fetched data:

myCrawler.on("fetchcomplete", function(queueItem, responseBuffer, response) {
    console.log("I just received %s (%d bytes)", queueItem.url, responseBuffer.length);
    console.log("It was a resource of type %s", response.headers['content-type']);
    // Do something with the data in responseBuffer

Then, when you're satisfied you're ready to go, start the crawler! It'll run through its queue finding linked resources on the domain to download, until it can't find any more.


Of course, once you've got that down pat, there's a fair bit more you can listen for...


  • crawlstart Fired when the crawl begins or is restarted.
  • queueadd (queueItem) Fired when a new item is automatically added to the queue (not when you manually queue an item yourself.)
  • queueduplicate (URLData) Fired when an item cannot be added to the queue because it is already present in the queue. Frequent firing of this event is normal and expected.
  • queueerror (errorData, URLData) Fired when an item cannot be added to the queue due to error.
  • fetchstart (queueItem, requestOptions) Fired when an item is spooled for fetching. If your event handler is synchronous, you can modify the crawler request options (including headers and request method.)
  • fetchheaders (queueItem, responseObject) Fired when the headers for a resource are received from the server. The node http response object is returned for your perusal.
  • fetchcomplete (queueItem, responseBody, responseObject) Fired when the resource is completely downloaded. The response body is provided as a Buffer per default, unless decodeResponses is truthy, in which case it's a decoded string representation of the body.
  • fetchdataerror (queueItem, response) Fired when a resource can't be downloaded, because it exceeds the maximum size we're prepared to receive (16MB by default.)
  • fetchredirect (queueItem, parsedURL, response) Fired when a redirect header is encountered. The new URL is validated and returned as a complete canonical link to the new resource.
  • fetch404 (queueItem, response) Fired when a 404 or 410 HTTP status code is returned for a request.
  • fetcherror (queueItem, response) Fired when an alternate 400 or 500 series HTTP status code is returned for a request.
  • gziperror (queueItem, error, resourceData) Fired when a gzipped resource cannot be unzipped.
  • fetchtimeout (queueItem, crawlerTimeoutValue) Fired when a request time exceeds the internal crawler threshold.
  • fetchclienterror (queueItem, errorData) Fired when a request dies locally for some reason. The error data is returned as the second parameter.
  • discoverycomplete (queueItem, resources) Fired when linked resources have been discovered. Passes an array of resources (as URLs) as the second parameter.
  • complete Fired when the crawler completes processing all the items in its queue, and does not find any more to add. This event returns no arguments.

A note about HTTP error conditions

By default, simplecrawler does not download the response body when it encounters an HTTP error status in the response. If you need this information, you can listen to simplecrawler's error events, and through node's native data event (response.on("data",function(chunk) {...})) you can save the information yourself.

If this is annoying, and you'd really like to retain error pages by default, let me know. I didn't include it because I didn't need it - but if it's important to people I might put it back in. :)

Waiting for Asynchronous Event Listeners

Sometimes, you might want to wait for simplecrawler to wait for you while you perform some asynchronous tasks in an event listener, instead of having it racing off and firing the complete event, halting your crawl. For example, if you're doing your own link discovery using an asynchronous library method.

Simplecrawler provides a wait method you can call at any time. It is available via this from inside listeners, and on the crawler object itself. It returns a callback function.

Once you've called this method, simplecrawler will not fire the complete event until either you execute the callback it returns, or a timeout is reached (configured in crawler.listenerTTL, by default 10000 ms.)

Example Asynchronous Event Listener
crawler.on("fetchcomplete", function(queueItem, data, res) {
    var continue = this.wait();
    doSomeDiscovery(data, function(foundURLs) {

Configuring the crawler

Here's a complete list of what you can stuff with at this stage:

  • - The domain to scan. By default, simplecrawler will restrict all requests to this domain.
  • crawler.initialPath - The initial path with which the crawler will formulate its first request. Does not restrict subsequent requests.
  • crawler.initialPort - The initial port with which the crawler will formulate its first request. Does not restrict subsequent requests.
  • crawler.initialProtocol - The initial protocol with which the crawler will formulate its first request. Does not restrict subsequent requests.
  • crawler.interval - The interval with which the crawler will spool up new requests (one per tick.) Defaults to 250 ms.
  • crawler.maxConcurrency - The maximum number of requests the crawler will run simultaneously. Defaults to 5 - the default number of http agents node will run.
  • crawler.timeout - The maximum time in milliseconds the crawler will wait for headers before aborting the request.
  • crawler.listenerTTL - The maximum time in milliseconds the crawler will wait for async listeners.
  • crawler.userAgent - The user agent the crawler will report. Defaults to Node/SimpleCrawler <version> (
  • crawler.decodeResponses - The response bodies will be intelligently character converted to standard JavaScript strings using the iconv-lite module. The character encoding is interpreted from the Content-Type header firstly, and secondly from any <meta charset="xxx" /> tags.
  • crawler.queue - The queue in use by the crawler (Must implement the FetchQueue interface)
  • crawler.filterByDomain - Specifies whether the crawler will restrict queued requests to a given domain/domains.
  • crawler.scanSubdomains - Enables scanning subdomains (other than www) as well as the specified domain. Defaults to false.
  • crawler.ignoreWWWDomain - Treats the www domain the same as the originally specified domain. Defaults to true.
  • crawler.stripWWWDomain - Or go even further and strip WWW subdomain from requests altogether!
  • crawler.stripQuerystring - Specify to strip querystring parameters from URLs. Defaults to false.
  • crawler.discoverResources - Use simplecrawler's internal resource discovery function. You can replace it with your own function, which must accept a buffer and a queueItem, and add the discovered resources to the crawler queue:
crawler.discoverResources = function(buf, queueItem) {
    // scan buffer for URLs, and then:
    crawler.queueURL(aDiscoveredURL, queueItem);
  • crawler.discoverRegex - Array of regex objects that simplecrawler uses to discover resources.
  • crawler.cache - Specify a cache architecture to use when crawling. Must implement SimpleCache interface. You can save the site to disk using the built in file system cache like this: crawler.cache = new Crawler.cache('pathToCacheDirectory');
  • crawler.useProxy - The crawler should use an HTTP proxy to make its requests.
  • crawler.proxyHostname - The hostname of the proxy to use for requests.
  • crawler.proxyPort - The port of the proxy to use for requests.
  • crawler.proxyUser - The username for HTTP/Basic proxy authentication (leave unset for unauthenticated proxies.)
  • crawler.proxyPass - The password for HTTP/Basic proxy authentication (leave unset for unauthenticated proxies.)
  • crawler.domainWhitelist - An array of domains the crawler is permitted to crawl from. If other settings are more permissive, they will override this setting.
  • crawler.supportedMimeTypes - An array of RegEx objects used to determine supported MIME types (types of data simplecrawler will scan for links.) If you're not using simplecrawler's resource discovery function, this won't have any effect.
  • crawler.allowedProtocols - An array of RegEx objects used to determine whether a URL protocol is supported. This is to deal with nonstandard protocol handlers that regular HTTP is sometimes given, like feed:. It does not provide support for non-http protocols (and why would it!?)
  • crawler.maxResourceSize - The maximum resource size, in bytes, which will be downloaded. Defaults to 16MB.
  • crawler.downloadUnsupported - Simplecrawler will download files it can't parse. Defaults to true, but if you'd rather save the RAM and GC lag, switch it off. When false, it closes sockets for unsupported resources.
  • crawler.needsAuth - Flag to specify if the domain you are hitting requires basic authentication
  • crawler.authUser - Username provided for needsAuth flag
  • crawler.authPass - Password provided for needsAuth flag
  • crawler.customHeaders - An object specifying a number of custom headers simplecrawler will add to every request. These override the default headers simplecrawler sets, so be careful with them. If you want to tamper with headers on a per-request basis, see the fetchqueue event.
  • crawler.acceptCookies - Flag to indicate if the crawler should hold on to cookies
  • crawler.urlEncoding - Set this to iso8859 to trigger URIjs' re-encoding of iso8859 URLs to unicode. Defaults to unicode.
  • crawler.parseHTMLComments - Whether to scan for URLs inside HTML comments. Defaults to true.
  • crawler.parseScriptTags - Whether to scan for URLs inside script tags. Defaults to true.
  • crawler.maxDepth - Defines a maximum distance from the original request at which resources will be downloaded. Asset files are excluded from this distance condition if crawler.fetchWhitelistedMimeTypesBelowMaxDepth is true. Defaults to 0 — no max depth.
  • crawler.fetchWhitelistedMimeTypesBelowMaxDepth — Defaults to false. If true, then resources (fonts, images, CSS) will be excluded from maxDepth checks. (And therefore downloaded regardless of their depth.)
  • crawler.ignoreInvalidSSL - Treat self-signed SSL certificates as valid. SSL certificates will not be validated against known CAs. Only applies to https requests. You may also have to set the environment variable NODE_TLS_REJECT_UNAUTHORIZED to '0'. For example: process.env.NODE_TLS_REJECT_UNAUTHORIZED = '0'; Defaults to false.

Excluding certain resources from downloading

Simplecrawler has a mechanism you can use to prevent certain resources from being fetched, based on the URL, called Fetch Conditions. A fetch condition is just a function, which, when given a parsed URL object, will return a true or a false value, indicating whether a given resource should be downloaded.

You may add as many fetch conditions as you like, and remove them at runtime. Simplecrawler will evaluate every single condition against every queued URL, and should just one of them return a falsy value (this includes null and undefined, so remember to always return a value!) then the resource in question will not be fetched.

Adding a fetch condition

This example fetch condition prevents URLs ending in .pdf from downloading. Adding a fetch condition assigns it an ID, which the addFetchCondition function returns. You can use this ID to remove the condition later.

var conditionID = myCrawler.addFetchCondition(function(parsedURL) {
    return !parsedURL.path.match(/\.pdf$/i);

NOTE: simplecrawler uses slightly different terminology to URIjs. parsedURL.path includes the query string too. If you want the path without the query string, use parsedURL.uriPath.

Removing a fetch condition

If you stored the ID of the fetch condition you added earlier, you can remove it from the crawler:

Excluding resources based on robots.txt

Simplecrawler purposely doesn't come with any built in support for parsing robots.txt rules. Adding support manually is very straightforward using fetch conditions however, and in examples/robots-txt-example.js you'll find an example that makes use of the robots-parser module to do just that.

The Simplecrawler Queue

Simplecrawler has a queue like any other web crawler. It can be directly accessed at crawler.queue (assuming you called your Crawler() object crawler.) It provides array access, so you can get to queue items just with array notation and an index.


For compatibility with different backing stores, it now provides an alternate interface which the crawler core makes use of:


It's not just an array though.

Adding to the queue

The simplest way to add to the queue is to use the crawler's own method, crawler.queueURL. This method takes a complete URL, validates and deconstructs it, and adds it to the queue.

If you instead want to add a resource by its components, you may call the queue.add method directly:

crawler.queue.add(protocol, hostname, port, path);

That's it! It's basically just a URL, but comma separated (that's how you can remember the order.)

Queue items

Because when working with simplecrawler, you'll constantly be handed queue items, it helps to know what's inside them. These are the properties every queue item is expected to have:

  • url - The complete, canonical URL of the resource.
  • protocol - The protocol of the resource (http, https)
  • host - The full domain/hostname of the resource
  • port - The port of the resource
  • path - The bit of the URL after the domain - includes the query string.
  • fetched - Has the request for this item been completed? You can monitor this as requests are processed.
  • status - The internal status of the item, always a string. This can be one of:
    • queued - The resource is in the queue to be fetched, but nothing's happened to it yet.
    • spooled - A request has been made to the remote server, but we're still waiting for a response.
    • headers - The headers for the resource have been received.
    • downloaded - The item has been entirely downloaded.
    • redirected - The resource request returned a 300 series response, with a Location header and a new URL.
    • notfound - The resource could not be found. (404)
    • failed - An error occurred when attempting to fetch the resource.
  • stateData - An object containing state data and other information about the request:
    • requestLatency - The time taken for headers to be received after the request was made.
    • requestTime - The total time taken for the request (including download time.)
    • downloadTime - The total time taken for the resource to be downloaded.
    • contentLength - The length (in bytes) of the returned content. Calculated based on the content-length header.
    • contentType - The MIME type of the content.
    • code - The HTTP status code returned for the request.
    • headers - An object containing the header information returned by the server. This is the object node returns as part of the response object.
    • actualDataSize - The length (in bytes) of the returned content. Calculated based on what is actually received, not the content-length header.
    • sentIncorrectSize - True if the data length returned by the server did not match what we were told to expect by the content-length header.

You can address these properties like you would any other object:

queueItem.status === "queued";

As you can see, you can get a lot of meta-information out about each request. The upside is, the queue actually has some convenient functions for getting simple aggregate data about the queue...

Queue Statistics and Reporting

First of all, the queue can provide some basic statistics about the network performance of your crawl (so far.) This is done live, so don't check it thirty times a second. You can test the following properties:

  • requestTime
  • requestLatency
  • downloadTime
  • contentLength
  • actualDataSize

And you can get the maximum, minimum, and average values for each with the crawler.queue.max, crawler.queue.min, and crawler.queue.avg functions respectively. Like so:

console.log("The maximum request latency was %dms.", crawler.queue.max("requestLatency"));
console.log("The minimum download time was %dms.", crawler.queue.min("downloadTime"));
console.log("The average resource size received is %d bytes.", crawler.queue.avg("actualDataSize"));

You'll probably often need to determine how many items in the queue have a given status at any one time, and/or retrieve them. That's easy with crawler.queue.countWithStatus and crawler.queue.getWithStatus.

crawler.queue.countWithStatus returns the number of queued items with a given status, while crawler.queue.getWithStatus returns an array of the queue items themselves.

var redirectCount = crawler.queue.countWithStatus("redirected");
crawler.queue.getWithStatus("failed").forEach(function(queueItem) {
    console.log("Whoah, the request for %s failed!", queueItem.url);
    // do something...

Then there's some even simpler convenience functions:

  • crawler.queue.complete - returns the number of queue items which have been completed (marked as fetched)
  • crawler.queue.errors - returns the number of requests which have failed (404s and other 400/500 errors, as well as client errors)

Saving and reloading the queue (freeze/defrost)

You'll probably want to be able to save your progress and reload it later, if your application fails or you need to abort the crawl for some reason. (Perhaps you just want to finish off for the night and pick it up tomorrow!) The crawler.queue.freeze and crawler.queue.defrost functions perform this task.

A word of warning though - they are not CPU friendly as they rely on JSON.parse and JSON.stringify. Use them only when you need to save the queue - don't call them every request or your application's performance will be incredibly poor - they block like crazy. That said, using them when your crawler commences and stops is perfectly reasonable.

Note that the methods themselves are asynchronous, so if you are going to exit the process after you do the freezing, make sure you wait for callback - otherwise you'll get an empty file.

// Freeze queue
crawler.queue.freeze("mysavedqueue.json", function() {
// Defrost queue


Simplecrawler now has an internal cookie jar, which collects and resends cookies automatically, and by default.

If you want to turn this off, set the crawler.acceptCookies option to false.

The cookie jar is accessible via crawler.cookies, and is an event emitter itself:

Cookie Events

  • addcookie (cookie) Fired when a new cookie is added to the jar.
  • removecookie (cookie array) Fired when one or more cookies are removed from the jar.


I'd like to extend sincere thanks to:

And everybody else who has helped out in some way! :)


Copyright (c) 2013, Christopher Giffard.

All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.


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