{"id":5395,"date":"2026-05-06T10:01:00","date_gmt":"2026-05-06T10:01:00","guid":{"rendered":"https:\/\/scraping-bot.io\/blogs\/how-to-build-a-web-crawler\/"},"modified":"2026-05-07T10:47:41","modified_gmt":"2026-05-07T10:47:41","slug":"how-to-build-a-web-crawler","status":"publish","type":"post","link":"https:\/\/scraping-bot.io\/blogs\/how-to-build-a-web-crawler\/","title":{"rendered":"How to build a web crawler ?"},"content":{"rendered":"\t\t<div data-elementor-type=\"wp-post\" data-elementor-id=\"5395\" class=\"elementor elementor-5395\" data-elementor-post-type=\"post\">\n\t\t\t\t<div class=\"elementor-element elementor-element-c7988ea e-flex e-con-boxed e-con e-parent\" data-id=\"c7988ea\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-b3e5a25 elementor-widget elementor-widget-html\" data-id=\"b3e5a25\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"html.default\">\n\t\t\t\t\t<article class=\"sb-article\">\r\n\r\n  <div class=\"sb-meta\">\r\n    <span class=\"sb-tag\">Web scraping<\/span>\r\n    <span class=\"sb-read-time\">10 min read &nbsp;\u00b7&nbsp; Published: 06\/05\/2026<\/span>\r\n  <\/div>\r\n\r\n  <h1>How to Build a Web Crawler: A Step-by-Step Guide<\/h1>\r\n\r\n  <p class=\"sb-intro\">Building a <strong>web crawler<\/strong> is one of the most practical skills you can develop as a developer working with data. Rather than manually visiting pages one by one, a web crawler automates the entire process \u2014 following links, discovering URLs, and feeding them to a scraper. In this guide, you will learn exactly how a web crawler works, how to build one from scratch in Node.js, and how to combine it with ScrapingBot to extract structured data at scale.<\/p>\r\n\r\n  <div class=\"sb-toc\">\r\n    <p class=\"sb-toc-title\">Table of contents<\/p>\r\n    <ol>\r\n      <li><a href=\"#what-is\">What is a web crawler?<\/a><\/li>\r\n      <li><a href=\"#crawler-vs-scraper\">Web crawler vs web scraper<\/a><\/li>\r\n      <li><a href=\"#how-it-works\">How does a web crawler work?<\/a><\/li>\r\n      <li><a href=\"#why-need\">Why do you need a web crawler?<\/a><\/li>\r\n      <li><a href=\"#how-to-build\">How to build a web crawler<\/a><\/li>\r\n      <li><a href=\"#best-practices\">Best practices and rules to follow<\/a><\/li>\r\n      <li><a href=\"#with-scrapingbot\">Combining your crawler with ScrapingBot<\/a><\/li>\r\n    <\/ol>\r\n  <\/div>\r\n\r\n  <h2 id=\"what-is\">1. What is a web crawler?<\/h2>\r\n  <p>A web crawler \u2014 also called a spider or bot \u2014 is a program that systematically browses the internet by following hyperlinks from page to page. Starting from one or more entry URLs, it fetches each page, extracts all the links it finds, and adds them to a queue of pages to visit next. This process repeats until the queue is empty or a stopping condition is met.<\/p>\r\n  <p>The most well-known web crawlers are search engine bots such as Google's Googlebot or Bing's Bingbot. When you publish a new website, these crawlers will eventually find it, read its content, and index it so it appears in search results. Beyond search engines, however, developers use web crawlers daily for data collection, competitive intelligence, price monitoring, and more.<\/p>\r\n\r\n  <div class=\"sb-note\">\r\n    <strong>\ud83d\udca1 Key concept:<\/strong> A web crawler <em>discovers<\/em> URLs. A web scraper <em>extracts data<\/em> from those URLs. The two tools work best together.\r\n  <\/div>\r\n\r\n  <h2 id=\"crawler-vs-scraper\">2. Web crawler vs web scraper \u2014 what's the difference?<\/h2>\r\n  <p>These two terms are often confused, but they serve different purposes:<\/p>\r\n\r\n  <table class=\"sb-table\">\r\n    <thead>\r\n      <tr><th>Web Crawler<\/th><th>Web Scraper<\/th><\/tr>\r\n    <\/thead>\r\n    <tbody>\r\n      <tr><td>Follows links to discover pages<\/td><td>Extracts data from specific pages<\/td><\/tr>\r\n      <tr><td>Always works on the web<\/td><td>Can work on the web or any data source<\/td><\/tr>\r\n      <tr><td>Builds a list of URLs<\/td><td>Parses page content into structured data<\/td><\/tr>\r\n      <tr><td>Output: a list of URLs<\/td><td>Output: JSON, CSV, database records<\/td><\/tr>\r\n    <\/tbody>\r\n  <\/table>\r\n\r\n  <p>In practice, a crawler and a scraper are typically used together: the crawler discovers all the product pages on an e-commerce site, and the scraper then extracts the price, title, and description from each one.<\/p>\r\n\r\n  <h2 id=\"how-it-works\">3. How does a web crawler work?<\/h2>\r\n  <p>Understanding the internal mechanics of a crawler will help you build a reliable one. At its core, a crawler manages two lists:<\/p>\r\n\r\n  <ul>\r\n    <li><strong>The queue<\/strong> (also called the horizon) \u2014 URLs waiting to be visited<\/li>\r\n    <li><strong>The visited set<\/strong> \u2014 URLs that have already been crawled<\/li>\r\n  <\/ul>\r\n\r\n  <h3>The crawling loop<\/h3>\r\n  <p>Here is the basic flow, step by step:<\/p>\r\n\r\n  <table class=\"sb-table\">\r\n    <thead>\r\n      <tr><th>Step<\/th><th>Action<\/th><\/tr>\r\n    <\/thead>\r\n    <tbody>\r\n      <tr><td>1<\/td><td>Add the root URL(s) to the queue<\/td><\/tr>\r\n      <tr><td>2<\/td><td>Pop the first URL from the queue<\/td><\/tr>\r\n      <tr><td>3<\/td><td>Add it to the visited set<\/td><\/tr>\r\n      <tr><td>4<\/td><td>Fetch the page content<\/td><\/tr>\r\n      <tr><td>5<\/td><td>Extract all links from the page<\/td><\/tr>\r\n      <tr><td>6<\/td><td>For each link: if not already visited and matches your rules \u2192 add to queue<\/td><\/tr>\r\n      <tr><td>7<\/td><td>Repeat from step 2 until the queue is empty<\/td><\/tr>\r\n    <\/tbody>\r\n  <\/table>\r\n\r\n  <h3>URL prioritization<\/h3>\r\n  <p>To prioritize which URLs to visit first, more advanced crawlers take into account signals such as the number of inbound links pointing to a URL or the frequency at which regular users visit the page. Consequently, the most important pages are crawled first, even when the queue contains thousands of URLs.<\/p>\r\n\r\n  <h2 id=\"why-need\">4. Why do you need a web crawler?<\/h2>\r\n  <p>Web scraping alone requires you to know every URL you want to scrape in advance. For small, well-defined datasets, this works fine. However, when dealing with large websites \u2014 e-commerce catalogues, news archives, job boards \u2014 manually listing every page is impossible.<\/p>\r\n  <p>A web crawler solves this by automating URL discovery. For instance, you can point your crawler at a product category page on Amazon, and it will automatically find and queue every product page linked from there.<\/p>\r\n  <p>Additionally, you can set rules to exclude irrelevant pages \u2014 login pages, cart pages, pagination \u2014 so only the pages you care about end up in your scraping queue. As a result, you save hours of manual work and collect far more complete datasets.<\/p>\r\n\r\n  <h2 id=\"how-to-build\">5. How to build a web crawler<\/h2>\r\n\r\n  <h3>Data structures you need<\/h3>\r\n  <p>Before writing any code, set up two core data structures:<\/p>\r\n  <ul>\r\n    <li><strong>A queue<\/strong> \u2014 use an array or a proper queue structure to store URLs to visit. A FIFO (first in, first out) queue gives you breadth-first crawling, which is usually what you want.<\/li>\r\n    <li><strong>A visited set<\/strong> \u2014 use a Set or hash map so URL lookups are O(1). This is critical for performance at scale.<\/li>\r\n  <\/ul>\r\n\r\n  <h3>Handling duplicate URLs with canonical tags<\/h3>\r\n  <p>On many websites \u2014 especially e-commerce ones \u2014 a single page can be accessible via multiple URLs. For example:<\/p>\r\n\r\n  <pre><code>https:\/\/example.com\/product?id=123&ref=homepage\r\nhttps:\/\/example.com\/product?id=123&ref=search\r\nhttps:\/\/example.com\/product\/blue-sneakers<\/code><\/pre>\r\n\r\n  <p>All three might display the exact same content. To avoid scraping the same page multiple times, look for the <strong>canonical tag<\/strong> in the HTML head of each page:<\/p>\r\n\r\n  <pre><code>&lt;link rel=\"canonical\" href=\"https:\/\/example.com\/product\/blue-sneakers\" \/&gt;<\/code><\/pre>\r\n\r\n  <p>By using the canonical URL as the key in your visited set, you ensure that each unique page is crawled only once \u2014 regardless of how many different URLs point to it.<\/p>\r\n\r\n  <h3>Setting URL filtering rules<\/h3>\r\n  <p>Not every link on a page is worth crawling. Therefore, define filtering rules before you start. Common rules include:<\/p>\r\n  <ul>\r\n    <li>Only follow links within the same domain (avoid leaving the target site)<\/li>\r\n    <li>Exclude URLs matching patterns like <code>\/login<\/code>, <code>\/cart<\/code>, <code>\/account<\/code><\/li>\r\n    <li>Exclude file extensions like <code>.pdf<\/code>, <code>.jpg<\/code>, <code>.zip<\/code><\/li>\r\n    <li>Only include URLs matching a specific path prefix, e.g. <code>\/products\/<\/code><\/li>\r\n  <\/ul>\r\n\r\n  <h3>Complete Node.js crawler example<\/h3>\r\n  <p>Here is a working web crawler in Node.js using only two dependencies: <a href=\"https:\/\/axios-http.com\/docs\/intro\" target=\"_blank\" rel=\"noopener\">axios<\/a> for HTTP requests and <a href=\"https:\/\/cheerio.js.org\/\" target=\"_blank\" rel=\"noopener\">cheerio<\/a> for HTML parsing. This requires Node.js 8 or above for <code>async\/await<\/code> support.<\/p>\r\n\r\n  <pre><code>const axios = require('axios');\r\nconst cheerio = require('cheerio');\r\n\r\nconst ROOT_URL = 'https:\/\/example.com\/products';\r\nconst DOMAIN   = 'https:\/\/example.com';\r\n\r\nconst queue   = [ROOT_URL];\r\nconst visited = new Set();\r\n\r\nasync function crawl(url) {\r\n  if (visited.has(url)) return;\r\n  visited.add(url);\r\n\r\n  console.log(`Crawling: ${url}`);\r\n\r\n  try {\r\n    const { data } = await axios.get(url, { timeout: 10000 });\r\n    const $ = cheerio.load(data);\r\n\r\n    \/\/ Extract canonical URL to avoid duplicates\r\n    const canonical = $('link[rel=\"canonical\"]').attr('href');\r\n    const pageUrl = canonical || url;\r\n\r\n    \/\/ TODO: pass pageUrl to your ScrapingBot scraper here\r\n\r\n    \/\/ Find and queue all links on the page\r\n    $('a[href]').each((_, el) => {\r\n      const href = $(el).attr('href');\r\n      const absolute = toAbsolute(href, DOMAIN);\r\n\r\n      if (\r\n        absolute &&\r\n        absolute.startsWith(DOMAIN) &&\r\n        !visited.has(absolute) &&\r\n        !isExcluded(absolute)\r\n      ) {\r\n        queue.push(absolute);\r\n      }\r\n    });\r\n\r\n  } catch (err) {\r\n    console.error(`Failed to crawl ${url}: ${err.message}`);\r\n  }\r\n}\r\n\r\nfunction toAbsolute(href, base) {\r\n  if (!href) return null;\r\n  if (href.startsWith('http')) return href;\r\n  if (href.startsWith('\/')) return base + href;\r\n  return null;\r\n}\r\n\r\nfunction isExcluded(url) {\r\n  const excluded = ['\/login', '\/cart', '\/account', '\/checkout'];\r\n  return excluded.some(pattern => url.includes(pattern));\r\n}\r\n\r\n\/\/ Main loop \u2014 process queue sequentially\r\nasync function run() {\r\n  while (queue.length > 0) {\r\n    const url = queue.shift(); \/\/ FIFO\r\n    await crawl(url);\r\n    await sleep(500); \/\/ Polite delay between requests\r\n  }\r\n  console.log(`Done. Visited ${visited.size} pages.`);\r\n}\r\n\r\nfunction sleep(ms) {\r\n  return new Promise(resolve => setTimeout(resolve, ms));\r\n}\r\n\r\nrun();<\/code><\/pre>\r\n\r\n  <div class=\"sb-note\">\r\n    <strong>\ud83d\udca1 Note:<\/strong> The <code>sleep(500)<\/code> call adds a 500ms delay between requests. This is important \u2014 without it, your crawler may overload the target server and get your IP banned. See the best practices section below.\r\n  <\/div>\r\n\r\n  <h2 id=\"best-practices\">6. Best practices and rules to follow<\/h2>\r\n  <p>Before deploying any crawler, it is essential to follow a set of rules \u2014 both technical and ethical:<\/p>\r\n\r\n  <table class=\"sb-table\">\r\n    <thead>\r\n      <tr><th>Rule<\/th><th>Why it matters<\/th><\/tr>\r\n    <\/thead>\r\n    <tbody>\r\n      <tr><td>Check <code>robots.txt<\/code><\/td><td>Specifies which paths crawlers are not allowed to visit. Always respect it.<\/td><\/tr>\r\n      <tr><td>Set a crawl delay<\/td><td>Avoid overloading the server. A 500ms\u20131s delay between requests is a good baseline.<\/td><\/tr>\r\n      <tr><td>Set a User-Agent header<\/td><td>Identify your crawler honestly in the request headers.<\/td><\/tr>\r\n      <tr><td>Handle errors gracefully<\/td><td>Use try\/catch and retry logic for failed requests \u2014 don't let one bad URL crash your crawler.<\/td><\/tr>\r\n      <tr><td>Deduplicate aggressively<\/td><td>Use canonical tags and a visited Set to avoid crawling the same content twice.<\/td><\/tr>\r\n      <tr><td>Limit crawl depth<\/td><td>Set a maximum depth to prevent your crawler from going too deep into a site.<\/td><\/tr>\r\n    <\/tbody>\r\n  <\/table>\r\n\r\n  <p>You can find the <code>robots.txt<\/code> file at the root of any website, e.g. <code>https:\/\/example.com\/robots.txt<\/code>. Furthermore, some websites include <code>Crawl-delay<\/code> directives directly in their robots.txt \u2014 check for these and respect them.<\/p>\r\n\r\n  <h2 id=\"with-scrapingbot\">7. Combining your crawler with ScrapingBot<\/h2>\r\n  <p>Building a crawler to discover URLs is only half the work. Once you have a queue of pages to scrape, you still need to extract structured data from each one \u2014 and that's where anti-bot protections, JavaScript rendering, and IP bans become a problem.<\/p>\r\n  <p>ScrapingBot handles all of this for you. Rather than fetching pages directly in your crawler, pass each URL to the ScrapingBot API instead. As a result, you gain automatic IP rotation, JavaScript rendering, and CAPTCHA handling \u2014 without changing your crawler logic.<\/p>\r\n\r\n  <pre><code>const axios = require('axios');\r\n\r\nconst USERNAME = 'your_username';\r\nconst API_KEY  = 'your_api_key';\r\n\r\nasync function scrapeWithBot(url) {\r\n  const response = await axios.post(\r\n    'https:\/\/api.scraping-bot.io\/scrape\/raw-html',\r\n    { url },\r\n    { auth: { username: USERNAME, password: API_KEY } }\r\n  );\r\n  return response.data; \/\/ Returns the rendered HTML\r\n}\r\n\r\n\/\/ In your crawler loop, replace direct axios.get() with:\r\nconst html = await scrapeWithBot(url);\r\nconst $ = cheerio.load(html);\r\n\/\/ ... parse the content as usual<\/code><\/pre>\r\n\r\n  <p>This approach gives you the best of both worlds: your crawler handles URL discovery and queue management, while ScrapingBot handles the hard part of actually fetching the pages reliably.<\/p>\r\n\r\n  <div class=\"sb-cta\">\r\n    <p><strong>Ready to combine your web crawler with ScrapingBot?<\/strong> Get 1,000 free API calls when you sign up \u2014 no credit card required.<\/p>\r\n    <a href=\"https:\/\/scraping-bot.io\/pricing\" class=\"sb-cta-btn\">Try ScrapingBot for free \u2192<\/a>\r\n  <\/div>\r\n\r\n<\/article>\r\n<style>\r\n.sb-article { max-width: 800px; margin: 0 auto; font-family: inherit; color: inherit; line-height: 1.7; }\r\n.sb-article h1 { font-size: 28px; font-weight: 700; margin: 0 0 1.25rem; line-height: 1.3; }\r\n.sb-meta { display: flex; align-items: center; gap: 12px; margin-bottom: 1.5rem; flex-wrap: wrap; }\r\n.sb-tag { background: #e6f1fb; color: #185fa5; font-size: 12px; padding: 4px 12px; border-radius: 6px; font-weight: 500; }\r\n.sb-read-time { font-size: 13px; color: #888; }\r\n.sb-intro { font-size: 16px; border-left: 3px solid #378add; padding-left: 1rem; color: #444; margin-bottom: 2rem; }\r\n.sb-toc { background: #f8f8f8; border: 1px solid #e8e8e8; border-radius: 8px; padding: 1rem 1.5rem; margin-bottom: 2rem; }\r\n.sb-toc-title { font-size: 13px; font-weight: 600; color: #666; margin: 0 0 8px; text-transform: uppercase; letter-spacing: 0.05em; }\r\n.sb-toc ol { margin: 0; padding-left: 1.25rem; }\r\n.sb-toc li { font-size: 14px; padding: 3px 0; }\r\n.sb-toc a { color: #185fa5; text-decoration: none; }\r\n.sb-toc a:hover { text-decoration: underline; }\r\n.sb-article h2 { font-size: 22px; font-weight: 600; margin: 2.5rem 0 0.75rem; border-bottom: 1px solid #eee; padding-bottom: 0.5rem; }\r\n.sb-article h3 { font-size: 17px; font-weight: 600; margin: 1.5rem 0 0.5rem; }\r\n.sb-article p { margin: 0 0 1rem; }\r\n.sb-article ul, .sb-article ol { margin: 0 0 1rem; padding-left: 1.5rem; }\r\n.sb-article li { margin-bottom: 6px; }\r\n.sb-article pre { background: #1e1e1e; color: #d4d4d4; border-radius: 8px; padding: 1.25rem; overflow-x: auto; margin: 1rem 0 1.5rem; }\r\n.sb-article code { font-family: 'Courier New', monospace; font-size: 13px; line-height: 1.6; }\r\n.sb-article p code { background: #f4f4f4; padding: 2px 6px; border-radius: 4px; font-size: 13px; color: #c7254e; }\r\n.sb-table { width: 100%; border-collapse: collapse; margin: 1rem 0 1.5rem; font-size: 14px; }\r\n.sb-table th { text-align: left; padding: 10px 14px; background: #f4f4f4; font-weight: 600; border-bottom: 2px solid #ddd; }\r\n.sb-table td { padding: 10px 14px; border-bottom: 1px solid #eee; }\r\n.sb-table tr:last-child td { border-bottom: none; }\r\n.sb-img-block { margin: 1.5rem 0 2rem; }\r\n.sb-screenshot { width: 100%; border-radius: 8px; border: 1px solid #ddd; box-shadow: 0 2px 12px rgba(0,0,0,0.08); display: block; }\r\n.sb-img-caption { font-size: 13px; color: #888; margin-top: 0.5rem; text-align: center; font-style: italic; }\r\n.sb-note { background: #fffbea; border: 1px solid #f0e28a; border-radius: 8px; padding: 1rem 1.25rem; margin: 1rem 0 1.5rem; font-size: 14px; color: #5a4a00; }\r\n.sb-cta { background: #e6f1fb; border: 1px solid #b5d4f4; border-radius: 10px; padding: 1.5rem; margin: 2.5rem 0 0; text-align: center; }\r\n.sb-cta p { margin: 0 0 1rem; font-size: 15px; }\r\n.sb-cta-btn { display: inline-block; background: #185fa5; color: white; padding: 10px 24px; border-radius: 6px; text-decoration: none; font-size: 14px; font-weight: 500; }\r\n.sb-cta-btn:hover { background: #0c447c; }\r\n<\/style>\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-8963957 e-flex e-con-boxed e-con e-parent\" data-id=\"8963957\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t","protected":false},"excerpt":{"rendered":"<p>Web scraping 10 min read \u00a0\u00b7\u00a0 Published: 06\/05\/2026 How to Build a Web Crawler: A Step-by-Step Guide Building a web crawler is one of the most practical skills you can develop as a developer working with data. Rather than manually visiting pages one by one, a web crawler automates the entire process \u2014 following links, [&hellip;]<\/p>\n","protected":false},"author":3,"featured_media":5971,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[6],"tags":[],"class_list":["post-5395","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-web-scraping-in-general"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v27.5 (Yoast SEO v27.5) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>Web Crawler \u2014 How to Build One Step by Step with ScrapingBot<\/title>\n<meta name=\"description\" content=\"Learn how to build a web crawler from scratch. Understand how crawling works and combine it with ScrapingBot to extract data at scale.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/scraping-bot.io\/blogs\/how-to-build-a-web-crawler\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"How to build a web crawler ?\" \/>\n<meta property=\"og:description\" content=\"Learn how to build a web crawler from scratch. 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