Preventing nested callbacks in JavaScript that uses iteration

Question

Currently I'm using promises to try to prevent the need for nested callbacks in my code, but I've hit a setback. In this case, I'm using node's request-promise and cheerio to emulate jQuery on the server. However, at some point I need to call jQuery.each(), to create a request for each <a> element. Is there any way I can use promises to prevent this nested callback?

request("http://url.com").then(function (html) { 
    var $ = cheerio.load(html);
    var rows = $("tr.class a");
    rows.each(function (index, el) {

        //Iterate over all <a> elements, and send a request for each one.
        //Can this code be modified to return a promise?
        //Is there another way to prevent this from being nested?

        request($(el).attr("href")).then(function (html) {
            var $ = cheerio.load(html);
            var url = $("td>img").attr("src");
            return request(url);
        })
        .then(function (img) {
            //Save the image to the database
        });
    });
});

Answer 1

This is the best solution I got in the end. Some incidental changes I made include using url.resolve to allow relative URLs to work.

var $ = require('cheerio');
var request = require('request-promise');
var url = require('url');

var baseURL = "http://url.com";

request(baseURL).then(function (html) {
    $("tr.class a", html).toArray(); 
}).map(function (el) {
    return request(url.resolve(baseURL, jq.attr("href")));
}).map(function (html) {
    var src = $("td>img", html).attr("src");
    return request(url.resolve(baseURL, src));
}).map(function (img) {
    //Save the image to the database
});

Thanks to Benjamin Gruenbaum for alterting me to the .map() method in bluebird.

Answer 2

Assuming Bluebird promises (code in other libraries is similar):

Promise.resolve(request("http://url.com").then(function (html) { 
    var $ = cheerio.load(html)("tr.class a");
})).map(function(el){ // map is `then` over an array
    return el.href;
}).map(request).map(function(html){
    return cheerio.load(html)("td>img").src;
}).map(request).map(function(img){
    // save to database.
});

Alternatively, you can define actions for a single link and then process those. It would look similar.