为什么这是未处理的承诺拒绝

我正在尝试重写我的代码以正确利用承诺。

完整的程序应该从T恤站点抓取数据。第一个代码块应该进入网站的首页，获取立即可用的产品页面，然后将URL存储在数组中。剩余的 URL 将存储在"剩余"中，以便稍后执行第二次抓取。

目前手动对每个部分进行单元测试：

//TASK: Create a command line application that goes to an ecommerce site to get the latest prices.
    //Save the scraped data in a spreadsheet (CSV format).

//Modules being used:
var cheerio = require('cheerio');
var request = require('request');
//harcoded url
var url = 'http://shirts4mike.com/';
//url for tshirt pages
var urlSet = new Set();
var remainder;
   const requestPromise = function(url) {
    return new Promise(function(resolve, reject) {
        request(url, function(error, response, html) {
            if(error) return reject(error);
            if(!error && response.statusCode == 200){
                return resolve(html);   
            }       
        });
    });
}

function firstScrape (url) {
    return requestPromise(url)
        .then(function(html) {
            var $ = cheerio.load(html);
            var links = [];
            //get all the links
            $('a[href*=shirt]').each(function(){
                var a = $(this).attr('href');
                //add into link array
                links.push(url + a);
            });
            return links;
            // return this array you've made
        });
}

function nextStep (arrayOfLinks) { 
    var promiseArray = [];
    for(var link in arrayOfLinks){
        promiseArray.push(requestPromise(link));
        return Promise.all(promiseArray);
    }                   
}

function lastStep (arrayOfHTMLresults){ 
    for(var html in arrayOfHTMLresults){
        var $ = cheerio.load(html);
        //if page has a submit it must be a product page
        if($('[type=submit]').length !== 0){
            //add page to set
            urlSet.add(scrapeLink);
        } else if(remainder == undefined) {
            //if not a product page, add it to remainder so it another scrape can be performed.
            remainder = scrapeLink;                         
        }
    }
    console.log(urlSet);
    console.log(remainder);
}

firstScrape(url)
    .then(nextStep)
    .then(lastStep);

我目前收到以下错误：

(node:71094) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 3): Error: Invalid URI "0"

这是我试图承诺的代码：

// Load front page of shirts4mike
function firstScrape(){
    request(url, function(error, response, html) {
        if(!error && response.statusCode == 200){
            var $ = cheerio.load(html);
        //iterate over links with 'shirt'
            $('a[href*=shirt]').each(function(){
                var a = $(this).attr('href');
                //create new link
                var scrapeLink = url + a;
                //for each new link, go in and find out if there is a submit button. 
                //If there, add it to the set
                request(scrapeLink, function(error,response, html){
                    if(!error && response.statusCode == 200) {
                        var $ = cheerio.load(html);
                        //if page has a submit it must be a product page
                        if($('[type=submit]').length !== 0){
                            //add page to set
                            urlSet.add(scrapeLink);
                        } else if(remainder == undefined) {
                            //if not a product page, add it to remainder so it another scrape can be performed.
                            remainder = scrapeLink;                         
                        }
                    }
                });
            });     
        }
    });
}

我无法解决的是，当lastStep()不知道scrapeLink是什么时，如何在中使用urlSet.add(scrapeLink);？

知道为什么吗？谢谢