all-fetchers.js

function fruitFetcher(prefix, callback) {
    callback([
        'apples',
        'bananas',
        'oranges',
    ]);
}

function animalFetcher(prefix, callback) {
    callback([
        'cats',
        'dogs',
        'sheep',
    ]);
}

function combineFetchers(fetchers) {
    return function(prefix, callback) {
        let allResults = [];
        let numFinished = 0;

        fetchers.forEach(fetcher => {
            fetcher(prefix, (results) => {
                allResults = allResults.concat(results);
                numFinished++;

                if (numFinished === fetchers.length) {
                    callback(allResults);
                }
            });
        });
    }
}

const allFetcher = combineFetchers([fruitFetcher, animalFetcher]);

allFetcher('aaa', (results) => {
    console.log(results);
});