s3.js

const aws = require("aws-sdk");
const fs = require("fs");

let secrets;
if (process.env.NODE_ENV == "production") {
    secrets = process.env; // in prod the secrets are environment variables
} else {
    secrets = require("./secrets"); // in dev they are in secrets.json which is listed in .gitignore
}

const s3 = new aws.S3({
    accessKeyId: secrets.AWS_KEY,
    secretAccessKey: secrets.AWS_SECRET
});

// ---------------------
exports.upload = (req, res, next) => {
    if (!req.file) {
        console.log(`req.file ain't there`);
        return res.sendStatus(500);
    }
    const { filename, mimetype, size, path } = req.file;
    s3.putObject({
        Bucket: "ninas-imageboard",
        ACL: "public-read",
        Key: filename,
        Body: fs.createReadStream(path),
        ContentType: mimetype,
        ContentLength: size
    }).promise().then(
        () => {
            // console.log('it worked!');
            next();
            // will delete file from upload folder(in our server), as soon as it is uploaded on AWS:
            fs.unlink(path, () => {});
        }
    ).catch(
        err => {
            console.log(err, 'failure :-(');
            res.sendStatus(500);
        }
    );
};