@jrc03c/web-crawler
v0.0.13
Published
This is a simple web crawler I plan to use for personal projects.
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Readme
Intro
This is a simple web crawler I plan to use for personal projects.
Installation
npm install --save @jrc03c/web-crawler
Usage
const WebCrawler = require("@jrc03c/web-crawler")
const crawler = new WebCrawler({
delay: 3000,
filter: url => url.includes("example.com"),
requestTimeout: 3000,
})
crawler.on("after-crawl", payload => {
console.log("Just crawled: " + payload.url)
})
crawler.on("error", payload => {
console.error("ERROR: " + payload.message)
})
crawler.start("https://example.com")
API
WebCrawler(options)
(constructor)
The WebCrawler
constructor can optionally accept a configuration object with these properties (all of which are optional):
delay
= see thedelay
propertyfilter
= see thefilter
propertyrequestTimeout
= see therequestTimeout
propertyshouldHonorBotRules
= see theshouldHonorBotRules
propertyshouldOnlyFollowSitemap
= see theshouldOnlyFollowSitemap
property
Class (static) properties
DisallowReasons
An object whose keys and values are strings representing reasons for which the crawling of a particular URL might be disallowed. It is defined as:
{
DISALLOWED_BY_BOT_RULES: "DISALLOWED_BY_BOT_RULES",
DISALLOWED_BY_META_TAG: "DISALLOWED_BY_META_TAG",
DISALLOWED_BY_RESPONSE_HEADER: "DISALLOWED_BY_RESPONSE_HEADER",
}
Where:
- "DISALLOWED_BY_BOT_RULES" = the URL was disallowed by a "robots.txt" rule
- "DISALLOWED_BY_META_TAG" = the URL was disallowed by a "noindex"
<meta>
tag in a response's HTML - "DISALLOWED_BY_RESPONSE_HEADER" = the URL was disallowed by an "x-robots-tag" header in a response
Instance properties
delay
A positive integer representing the time in milliseconds between fetch requests. The default is 3000.
domainConfigs
An object whose keys and values, respectively, are domain names and bot configuration objects. The default is an empty object. Configuration objects are instances of the RobotsConfig
class.
filter
A function that receives a URL and returns a boolean indicating whether or not the URL should be crawled (true
= the URL should be crawled; false
= the URL should not be crawled). The function can be either synchronous or asynchronous. The default value is an asynchronous function that always returns true
.
frontier
An array of strings representing URLs to be crawled. The default is an empty array.
isCrawling
A boolean indicating whether or not the crawler is currently in the process of crawling URLs in its frontier
. The default is false
.
isPaused
A boolean indicating whether or not the crawler is currently paused in the process of crawling. The default is false
.
requestTimeout
A positive integer representing the time in milliseconds the crawler should wait for a response to a request before aborting the request.
shouldHonorBotRules
A boolean indicating whether or not the crawler should abide by rules listed in "robots.txt" files, "x-robots-tag" headers in responses, and/or "noindex" <meta>
tags in responses HTMLs. The default is true
.
shouldOnlyFollowSitemap
A boolean indicating whether or not the crawler should only crawl pages listed in a site's sitemap. The default is true
.
subscriptions
An object whose keys and values, respectively, are strings representing event names and arrays of callback functions to be called in response to emissions of events with those names. The default is an empty object.
visited
An array of strings representing URLs that have already been crawled. The default is an empty array.
Instance methods
addUrlToFrontier(url : string, shouldSkipFilter : boolean)
Attempts to add a URL to the frontier
. Returns a boolean indicating whether or not the URL was added. If the given URL is already in the frontier
, then the URL is not added, and false
is returned. If the given URL is already in visited
, then the URL is not added, and false
is returned. The shouldSkipFilter
parameter is a boolean indicating whether or not the filter
function should be skipped. By default, it's false
, meaning that the filter
function will be applied. If the filter
function is applied, and if it returns false
, then the URL is not added, and false
is returned. If the URL passes all of the above hurdles, then it is added to the frontier
, the "add-url" event is emitted, and true
is returned.
createDomainConfiguration(domain : string)
Creates a new RobotsConfig
instance for the given domain (based on the domain's "robots.txt" file, if it exists), and then fetches the list of URLs from the domain's sitemap(s) (if they exist) and adds them to the frontier
. The configuration is stored in the instance's domainConfigs
object, and then the configuration is returned.
emit(eventName : string, payload : any)
Emits an event with a given name, passing payload
to each callback function in the array of callback functions corresponding to the event name in the instance's subscriptions
object. Returns the WebCrawler
instance.
getDomainConfiguration(domain : string)
Returns a RobotsConfig
instance for a given domain either by pulling it from the instance's domainConfigs
object (if it exists there) or by creating a new configuration via the createDomainConfiguration
instance method.
off(eventName : string, callback : function)
Removes the given callback from the array of callback functions in the instance's subscriptions
object for the given event name. Returns the WebCrawler
instance.
on(eventName : string, callback : function)
Adds the given callback to the array of callback functions in the instance's subscriptions
object for the given event name. Returns the WebCrawler
instance.
pause()
Pauses the crawling process and emits the "pause" event. Returns the WebCrawler
instance.
removeUrlFromFrontier(url)
Removes all instances of the URL from the frontier
. Returns the WebCrawler
instance.
start(url : string)
Starts the crawling process, using the given URL as the first (and probably only) URL in the frontier
, and emits the "start" event. Returns the WebCrawler
instance.
Once the crawling process begins, other events are fired in sequence for each URL to be crawled. They are:
- "skip-url"
- "filter-url"
- "disallow-url"
- "before-crawl"
- "fetch"
- "after-crawl"
- "error"
See the Events documentation below for more information about these events.
stop()
Stops the crawling process completely and emits the "stop" event. Returns the WebCrawler
instance.
Events
"add-url"
Is emitted when a new URL is added to the frontier
. The payload passed to callback functions is an object with these properties:
{
url: string,
}
"after-crawl"
Is emitted after a URL has been successfully crawled. The payload passed to callback functions is an object with these properties:
{
dom: object, // a JSDOM instance
raw: string, // the raw response text
response: object, // the response object
url: string, // the URL that was crawled
}
See the JSDOM docs for more info.
"before-crawl"
Is emitted before a URL is crawled. The payload passed to callback functions is an object with these properties:
{
url: string,
}
"disallow-url"
Is emitted when a URL is prevented from being crawled by a "robots.txt" rule, an "x-robots-tag" header in a response, or a "noindex" <meta>
tag in a response's HTML. The payload passed to callback functions is an object with these properties:
{
reason: string, // one of WebCrawler.DisallowReasons
url: string,
}
"error"
Is emitted when an error is thrown (but then caught) during the crawling process. The payload passed to callback functions is an object with these properties:
{
message: string, // the error message
response: object, // the response object
url: string, // the URL being crawled when the error occurred
}
Note that the "response" property may not always be present. It is only attached to the payload when the error occurred in response to a fetch
request.
"fetch"
Is emitted after a fetch
quest succeeds and a response is returned. The payload passed to callback functions is an object with these properties:
{
raw: string, // the raw response text
response: object, // the response object
url: string,
}
"filter-url"
Is emitted when an object fails to pass the filter
function. The payload passed to callback functions is an object with these properties:
{
url: string,
}
"finish"
Is emitted once all URLs in the frontier
have been crawled and there is nothing left to do. No payload is passed to callback functions.
"pause"
Is emitted in response to an invocation of the pause
instance method. No payload is passed to callback functions.
"skip-url"
Is emitted when a URL is skipped because it already exists in visited
. The payload passed to callback functions is an object with these properties:
{
url: string,
}
"start"
Is emitted in response to an invocation of the start
instance method. No payload is passed to callback functions.
"stop"
Is emitted in response to an invocation of the stop
instance method. No payload is passed to callback functions.
"warn"
Is emitted when the crawler wants to warn the user about something even though no error was thrown.
Currently, it is only emitted during the createDomainConfiguration
instance method if no "robots.txt" file was found for a given domain. The payload passed to callback functions is an object with these properties:
{
domain: string, // the domain for which no "robots.txt" file was found
message: string, // the warning message
}