1

Hope you're having a good day. I recently discovered that it is not as easy to handle callbacks inside a for loop. I have tried a few things but couldn't find a solution. Here is the code:

var book = new Array;
var chapters = Object.keys(epub.spine.contents).length;

for (let i = 0; i < chapters; i++) {
    let cacheArray = [];
    epub.getChapter(epub.spine.contents[i].id, function (err, data) {
        if (err) {
            return console.log(err);
        }
        //remove html tags
        let str = data.replace(/<\/?[\w\s]*>|<.+[\W]>/g, '');
        book.push(str)
    })
}
console.log(book)//returns empty array ofc

After this code is executed, I need to loop over the array to search its contents. If that was not the case, my approach would be to just send it to a db.

My approach was the following:

var recursiveChapter = function (n) {
    var book = new Array;
    if (n < chapters) {
        // chapter function
        epub.getChapter(epub.spine.contents[n].id, function (err, data) {
            if (err) {
                throw err
            }
            //remove HTML tags
            let str = data.replace(/<\/?[\w\s]*>|<.+[\W]>/g, '');
            book.push(str)
            recursiveChapter(n + 1)
        });
    }
}
//start the recursive function
recursiveChapter(0)
console.log(book)//returns an empty array

I am stuck and can't think of a way of using this data without storing it in a db. Any help would be appreciated .

Improve this question
6
  • can u tell us what did you expect this code do and what are you currently facing? Commented Feb 26, 2021 at 14:52
  • Why not promisify your getChapter function. It would be easier to await this method Commented Feb 26, 2021 at 15:01
  • @AliHussam getChapter comes from an external library, so yeah i can modify it but it's going to take me some time i think. (github.com/julien-c/epub) this library btw. Commented Feb 26, 2021 at 18:23
  • @arminyahya this fuctions basically parse an epub to a string, once it does that i can interact with it via an event emmiter. Once im there i need to load every chapter and remove html tags, push each chapter into an Array, and then i want to do a search with a forEach loop. The problem im having is that getChapter returns a callback, so the for loop just keep going and if i try to use the Array it's allways empty, so i can't continue with the process i just mentioned. Commented Feb 26, 2021 at 18:26
  • Your function has an error first callback, you can simply do util.promisify to promisify it or write a promisifed wrapper for this call. Commented Feb 26, 2021 at 18:45

1 Answer 1

Reset to default
2

There are a few ways you can tackle this. One way is to use the async library, this allows to to run async. calls in parallel, or in series.

For example, we can use async.mapSeries() to get the result of a series of asynchronous calls as an array.

You input your array of ids, then the callback will return an array of the data returned, for example, I've mocked out the getChapter() function to give you an idea of how it would work:

// Mock out epub object
const epub = { 
    getChapter(id, callback) {
        setTimeout(() => callback(null, "Data for id " + id), 250);
    }
}

let ids = [1,2,3,4];
console.log("Calling async.mapSeries for ids:", ids);
async.mapSeries(ids, epub.getChapter, (err, result) => {
    if (err) {
        console.error(err);
    } else {
        console.log("Result:", result)
    }
})
<script src="https://cdnjs.cloudflare.com/ajax/libs/async/3.2.0/async.min.js" integrity="sha512-6K6+H87tLdCWvY5ml9ZQXLRlPlDEt8uXmtELhuJRgFyEDv6JvndWHg3jadJuBVGPEhhA2AAt+ROMC2V7EvTIWw==" crossorigin="anonymous"></script>

You could also promisify the epub call and use Promise.all to get the result, like so:

epub = { 
    getChapter(id, callback) {
        setTimeout(() => callback(null, "Data for id " + id), 250);
    }
}
 
let ids = [1,2,3,4];

function getChapterPromisified(id) {
    return new Promise((resolve, reject) => { 
        epub.getChapter(id, (err, data) => { 
            if (err) {
                reject(err);
            } else {
                resolve(data);
            }
        })
    })
}

// Create a list of promises, one for each call
const promises = ids.map(id => getChapterPromisified(id))
Promise.all(promises)
    .then(result => console.log("Result:", result))
    .catch(error => console.error("An error occurred:", err));

Improve this answer
Sign up to request clarification or add additional context in comments.

6 Comments

Thx for the response Terry, the function im using comes from an external library (this one to be exact github.com/julien-c/epub), so i don't think i can modify it. I mean, sure i can, but it would probably take me some time since i really don't know how it was made(this one github.com/julien-c/epub). Your first answer tho, that might work, since i have a way of obtaining the id's. I'm gonna give it a try, thx a lot.
Thanks for testing my answer! You shouldn't have to modify the function, maybe just add a wrapper if you want to go the promise route. If you use the async approach, you shouldn't have to modify the function at all!
Oh right, i could wrap it and return a promise, i didn't think of that. You see, i'm what people refers as a "noob" so i've never actually tryed that, but it looks interesting, i'm gonna check it out. Thx a lot.
Oh great, give those approaches a go. Just test them out and see if either suits you!
second one worked just fine, i had some issues just because i forgot to actually return the promise, wich gave me insanity for a few hours. Thx you!
|

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.