Web Scraping Js

Web Scraping Js
  1. Web Scraping Js Python
  2. Web Scraping Json
  3. Web Scraping Js

If you really want to do the web-scraping on the client-side, then use cheerio.You also need to find a way to do a CORS request from the client side. If you want to do the web-scraping on the server-side, use cheerio or pupeteer, cheerio is enough for most use case, but if you are doing some advance scraping, sometimes cheerio isn't enough, so you need to use pupeteer since pupeteer is a. Learn how to build a web scraper ⛏️ with NodeJS using two distinct strategies, including (1) a metatag link preview generator and (2) a fully-interactive bot. Jsdom: jsdom is a pure-JavaScript implementation of many web standards, notably the WHATWG DOM and HTML Standards, for use with Node.js. In general, the goal of the project is to emulate enough of a subset of a web browser to be useful for testing and scraping real-world web applications. Web scrapers can be developed using any programming language that is Turing complete. Java, PHP, Python, JavaScript, C/C, and C#, among others, have been used for writing web scrapers. Being that as it may, some languages are much more popular than others as far as developing web scrapers are concerned. JavaScript is not a popular choice. Web scrapers can be developed using any programming language that is Turing complete. Java, PHP, Python, JavaScript, C/C, and C#, among others, have been used for writing web scrapers. Being that as it may, some languages are much more popular than others as far as developing web scrapers are concerned. JavaScript is not a popular choice.

Is it possible to scrape an HTML page with JavaScript from inside of a web browser?

To be perfectly honest I wasn’t sure so I decided to try it out.

Full disclaimer here, I didn’t actually succeed. However, it was a great learning experience for me and I think you guys could benefit from seeing what I did and where I went wrong. Who knows, maybe you can take what I’ve done and figure it out for yourself!

You can jump to any of these methods if you like…

CORSNo Referer header requestWordPress pages’ load

Let’s say you’re at mysite.com (in your browser) and want to run a script to load some data from example.com. The simplest solution for requesting content via JavaScript would be to request it through an XML Http Request (XHR):

However, this pertains to the cross-origin requests (as opposed to same-origin request), which turns out to be the unbreakable wall for requesting page content from pure JavaScript.

Same Origin Policy

Same-origin policy restricts how a document or script loaded from one origin can interact with a resource from another origin. It is a critical security mechanism for isolating potentially malicious documents. Source.

Same-origin policy requires that in web requests made from the client side (mysite.com) both domain name and protocol (http, https) should be equal. So security limitations do not allow you to request another domain website (example.com) or any of its resources.

Can you bypass it? No.

Scraping

You might think you can just bypass this. In order to request foreign domain resources we imitate the same domain origin. How? By obfuscating Referer header in XML Http Request, see the following:

Web Scraping Js Python

This is explicitly disallowed by the XHR specification. So in JavaScript XHR you can’t explicitly set up an arbitrary header.

Resources allowed to be requested from a foreign domain

Web Scraping Js

Now, there are services that do allow cross-origin resource sharing. This is applicable for distributed resource sharing to diminish a server resource load. Eg. CSS stylesheets, images, and scripts might be served from foreign domain servers. Here are some examples of resources which may be embedded cross-origin:

  • JavaScript with <script src='...'></script>. Error messages for syntax errors are only available for same-origin scripts.
  • CSS with <link href='...'>.
  • Images with <img>. Supported image formats include PNG, JPEG, GIF, BMP, SVG.
  • Media files with <video> and <audio>.
  • Plug-ins with <object>, <embed> and <applet>.
  • Fonts with @font-face. Some browsers allow cross-origin fonts, others require same-origin fonts.
  • Anything with <frame> and <iframe>. A site can use the X-Frame-Options header to prevent this form of cross-origin interaction.

One example is the jQuery code which is often served from ajax.googleapis.com domain:

But, to the developers’ joy the Cross-Origin Resource Sharing policy has been recently (January 2014) introduced. This allows to resolve the Same-Origin Policy restriction.

Web scraping js console

Cross-Origin Resource Sharing (CORS)

The main concept is that a target server may allow some other origins (or all of them) to request its resources. Server configured for allowing cross-origin requests is useful for the cross-domain API access of its resources.

If a server allows CORS it’ll respond with Access-Control-Allow-Origin:* header.

If a resource owner’s restricts the sharing with only a certain domain, the server will respond with:

You might do a preflight request to make clear if a server allows foreign domain access.

Cross-Origin Resource Sharing (CORS) is a W3C spec that allows cross-domain communication from web browser.

Read more of CORS here.
How to set an access control in Apach server (enabling CORS) see here.
CORS tester.

Wrap up

Eventually, site owners will allow CORS only for the API access since it’s unlikely they will make their private web data cross-origin accessable. The attempt to scrape other sites’ content with JavaScript provides a very limited scope.

No Referer form submission

We’ve mentioned before that <iframe> loading foreing data in it works by neglecting same-domain policy.

Let’s try to use the form submission with no Referer header. Most of the sites approve the request if Referer header is empty (omitted). Websites do this because they don’t want to lose sort of 1% of their traffic. So we make a simple procedure that is called for a chosen domain with requesting thru virtual form submission:

This code, when called client-side, adds new <iframe> into a web page and loads needed resource into a browser page. The whole code is here. Kind of loading.

See the following web sniffer’s shot showing the Origin header being null and no Referer header present.

So basically you might load a foreign page into your browser page by JavaScript. But still the Same Origin Policy, applied in all major browsers, forbids access to the fetched HTML. Cross-origin contents can not be read by JavaScript. No major browser will allow that to secure against XSS attacks. Surprisingly, you can watch the cross-site request response HTML code thru browser’s web developer tools (F12, an example of using them) and manually copy/paste it:
The loaded site will seamlessly work in an iframe, yet, you can’t have an access to its HTML. You can get the page’s screenshot as an image, but it’s not sufficient for full-scale web scraping.

How does WordPress load foreign page shots into its admin panel

WordPress CMS can load of foreign resources with server-side call (if having access to wp-admin – just visit: <site>/wp-admin/edit-comments.php). If the user hovers a comment website link, the CMS’ JavaScript automatically invokes a request to the WordPress home server:

Now the CMS makes HTTP request to its own server, embedding the link to the foreign resource. Obviously the WordPress server makes request to the resource by provided link of interest and returns the content:

The only the thing is that the content returned by WordPress being an image: content-type: image/jpeg. You can program server to return HTML code, but that’s server-side data extraction.

Conclusion

The client-side (from your browser) scraping with JavaScript is not practical today. (1) The browser capabilities are far less compared to web servers (speed, memory, etc.). (2) The Same-Origin Policy safeguards sites from cross-origin requests, avoiding XSS attacks threat. CORS is limited scope applicable. (3) I’ve also tried to emulate the cross-domain HTTP request by a virtual form submission to load a result into an iframe, but this failed since the browser restrictions forbid scripts to handle raw response HTML cause of XSS attacks threat. (4) The last option is the indirect requesting thru a domain server (mysite.com, who actually extracts). Eg. WordPress loading foreign pages’ previews.

Feel free to add more to this topic (using comments).


This post will walk through how to use the requests_html package to scrape options data from a JavaScript-rendered webpage. requests_html serves as an alternative to Selenium and PhantomJS, and provides a clear syntax similar to the awesome requests package. The code we’ll walk through is packaged into functions in the options module in the yahoo_fin package, but this article will show how to write the code from scratch using requests_html so that you can use the same idea to scrape other JavaScript-rendered webpages.

Note:

requests_html requires Python 3.6+. If you don’t have requests_html installed, you can download it using pip:

Motivation

Let’s say we want to scrape options data for a particular stock. As an example, let’s look at Netflix (since it’s well known). If we go to the below site, we can see the option chain information for the earliest upcoming options expiration date for Netflix:

On this webpage there’s a drop-down box allowing us to view data by other expiration dates. What if we want to get all the possible choices – i.e. all the possible expiration dates?

We can try using requests with BeautifulSoup, but that won’t work quite the way we want. To demonstrate, let’s try doing that to see what happens.

Running the above code shows us that option_tags is an empty list. This is because there are no option tags found in the HTML we scrapped from the webpage above. However, if we look at the source via a web browser, we can see that there are, indeed, option tags:

Why the disconnect? The reason why we see option tags when looking at the source code in a browser is that the browser is executing JavaScript code that renders that HTML i.e. it modifies the HTML of the page dynamically to allow a user to select one of the possible expiration dates. This means if we try just scraping the HTML, the JavaScript won’t be executed, and thus, we won’t see the tags containing the expiration dates. This brings us to requests_html.

Using requests_html to render JavaScript

Now, let’s use requests_html to run the JavaScript code in order to render the HTML we’re looking for.

Similar to the requests package, we can use a session object to get the webpage we need. This gets stored in a response variable, resp. If you print out resp you should see the message Response 200, which means the connection to the webpage was successful (otherwise you’ll get a different message).

Web Scraping Json

Running resp.html will give us an object that allows us to print out, search through, and perform several functions on the webpage’s HTML. To simulate running the JavaScript code, we use the render method on the resp.html object. Note how we don’t need to set a variable equal to this rendered result i.e. running the below code:

stores the updated HTML as in attribute in resp.html. Specifically, we can access the rendered HTML like this:

Web Scraping Js

So now resp.html.html contains the HTML we need containing the option tags. From here, we can parse out the expiration dates from these tags using the find method.

Similarly, if we wanted to search for other HTML tags we could just input whatever those are into the find method e.g. anchor (a), paragraph (p), header tags (h1, h2, h3, etc.) and so on.

Alternatively, we could also use BeautifulSoup on the rendered HTML (see below). However, the awesome point here is that we can create the connection to this webpage, render its JavaScript, and parse out the resultant HTML all in one package!

Web Scraping Js

Lastly, we could scrape this particular webpage directly with yahoo_fin, which provides functions that wrap around requests_html specifically for Yahoo Finance’s website.

Scraping options data for each expiration date

Once we have the expiration dates, we could proceed with scraping the data associated with each date. In this particular case, the pattern of the URL for each expiration date’s data requires the date be converted to Unix timestamp format. This can be done using the pandas package.

Similarly, we could scrape this data using yahoo_fin. In this case, we just input the ticker symbol, NFLX and associated expiration date into either get_calls or get_puts to obtain the calls and puts data, respectively.

Note: here we don’t need to convert each date to a Unix timestamp as these functions will figure that out automatically from the input dates.

That’s it for this post! To learn more about requests-html, check out my web scraping course on Udemy here!

To see the official documentation for requests_html, click here.

Related