Databases
StackOverflow2013
Postsdbo
POMaking a while loop asynchronous in nodeJS
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POMaking a while loop asynchronous in nodeJS
    primarykey
    data
    text
    <p>There are several similar questions here on Stack but I can't get any answers working for me, I'm completely new to Node and the idea of asynchronous programming so please bear with me.</p> <p>I'm building a scraper that currently has a 4 step process:</p> <ol> <li>I give it a collection of links</li> <li>It goes to each of these links, finds all relevant <code>img src</code> on the page </li> <li>It finds the "next page" link, gets its <code>href</code>, retrieves the dom from said <code>href</code> and repeats step #2.</li> <li>All of these <code>img src</code> are put into an array and returned</li> </ol> <p>Here's the code. <code>getLinks</code> can be called asynchronously but the <code>while</code> loop in it currently cannot:</p> <pre><code>function scrape(url, oncomplete) { console.log("Scrape Function: " + url); request(url, function(err, resp, body) { if (err) { console.log(UHOH); throw err; } var html = cheerio.load(body); oncomplete(html); } ); } function getLinks(url, prodURL, baseURL, next_select) { var urls = []; while(url) { console.log("GetLinks Indexing: " + url); var html = scrape(url, function(data) { $ = data; $(prodURL).each(function() { var theHref = $(this).attr('href'); urls.push(baseURL + theHref); } ); next = $(next_select).first().attr('href'); url = next ? baseurl + next : null; } ); } console.log(urls); return urls; } </code></pre> <p>At present this goes into an infinite loop without scraping anything. If I put the <code>url = next ? baseurl + next : null;</code> outside of the callback I get a <code>"next" is not defined</code> error. </p> <p>Any ideas on how I can re-work this to make it node-friendly? It seems like, by this problem's very nature, it needs to be blocking, no?</p>
    singulars
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. USJascination
    1. USJascination
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. CORewrite the while-loop as a recursive function. You then will easily recognize how to make that async by passing callbacks.
      singulars
      1. POMaking a while loop asynchronous in nodeJS
      1. USBergi
    2. CO@Bergi Ah, turns out I just needed to know the name and I could work it out myself. For anyone else who stumbles upon this: http://tech.richardrodger.com/2011/04/21/node-js-%E2%80%93-how-to-write-a-for-loop-with-callbacks/ is a good resource
      singulars
      1. POMaking a while loop asynchronous in nodeJS
      1. USJascination
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload