Flattening Promises

translation.js

const translateSongLyrics = (songLines, callback) => {
    
    let config = {
        headers: {
            'Content-Type': 'application/x-www-form-urlencoded'
        }
    };
    let endpoint = "https://translate.yandex.net/api/v1.5/tr.json/translate?key=" + yandexApiKey;

    songLines.forEach((line) => {
        let requestBody = {
            text: removeHTMLTags(line),
            lang: 'fr-en'
        let request = axios.post(endpoint, querystring.stringify(requestBody), config);
          
        /* Storing each promise returned from the requests into a promise chain */
        promiseChain.push(request);
    })

    let combinedChain = [];
    
    /* Evaluating all the promises together with a single .then() */
    Promise.all(promiseChain)
        .then((response) => {
            let translationIndex = 0;
            songLines.forEach((songLine) => {
                combinedChain.push({
                    original: removeHTMLTags(songLine),
                    translation: response[translationIndex].data.text[0]
                });
            })
            callback(combinedChain);
        },
        (error) => {
            console.log("There was an error in translation", error);
            callback([]);
        })
}