Get the array value of a promise

Question

I'm trying to send multiple attachments with Sendgrid with a new promise as below. I want to return an array of objects but I don't understand how to do so. emailAttachments contains my 3 objects, but how can I return the array? Can you tell me what's wrong in my code?

const emailAttachments = []

return new Promise(function(resolve, reject){

    pdfList.map(item => {

        fs.readFile((item.frDoc), (err, data) => {
            if (err) {
              return { handled: false }
            }

            if (data) {
                
                emailAttachments.push({
                    content: data.toString('base64'),
                    filename: `document.pdf`,
                    type: 'application/pdf',
                    disposition: 'attachment'
                })

            }
        });
    })

    resolve(emailAttachments)
})

`map` returns a new array so there's no need to `push` elements into an empty array. [This may also give you some clues](https://stackoverflow.com/questions/34628305/using-promises-with-fs-readfile-in-a-loop) as to using `readFile` with promises. — Andy, Oct 10 '21 at 14:26

score 3 · Answer 1 · answered Oct 10 '21 at 14:35

You are mixing callback functions and promises. That won't work. I'd suggest to use the promise API of the fs module supported since node 10.

Furthermore, you need to await the filereading before resolving promise, which you currently don't do.

import { promises as fsp} from "fs"; //use the Promise API from fs

async function getAttachments(pdffiles) {
  return Promise.all(pdffiles.map(pdf => fsp.readFile(pdf.frDoc)))
    .then(files => files.map(f => ({
       content: f.toString("base64"),
       filename: "document.pdf",
       type: "application/pdf",
       disposition: "attachment"
     })));

}

cj-2307 · Accepted Answer · 2021-10-10T14:43:41.143

I think you need something like this:

async function getEmailAttachments(pdfList) {
    const emailAttachments = await Promise.all(pdfList.map((item) => {
        return new Promise(function (resolve, reject) {
            fs.readFile((item.frDoc), (err, data) => {
                if (err) {
                    reject({ handled: false });
                }
                if (data) {
                    resolve({
                        content: data.toString('base64'),
                        filename: `document.pdf`,
                        type: 'application/pdf',
                        disposition: 'attachment'
                    });
                }
            });
        });
    }));
    return emailAttachments;
}

[EDIT] Or using the promise version of readFile (sugestion of @derpirscher):

async function getEmailAttachments(pdfList) {
    const emailAttachments = await Promise.all(pdfList.map(async (item) => {
        const data = await fs.promises.readFile(item.frDoc);
        return {
            content: data.toString('base64'),
            filename: `document.pdf`,
            type: 'application/pdf',
            disposition: 'attachment'
        };
    }));
    return emailAttachments;
}

Why wrapping the callback based `readFile` if there is a promise based version already included in the node api? — derpirscher, Oct 10 '21 at 14:29
Now you will get a syntax error, because `await` can only be used inside of an `async` function (and `item => { ...}` is not async) — derpirscher, Oct 10 '21 at 14:41
I implemented the second solution and it's working, thank you both ! — taurOan, Oct 10 '21 at 15:15

Get the array value of a promise

2 Answers2