executeBatched() – Run a list of async tasks as promises and wait after each batch

executedBatched.js

/**
 * Run a list of async tasks as promises and wait after each batch
 * @param {Number} batchSize - Number of tasks to run in parallel
 * @param {Number} waitTime - Time to wait after each batch, in milliseconds
 * @param {Array<Function>} tasks - Promise executor functions (not promise objects!)
 * @return {Promise<Array<*>>}
 */
function executeBatched(batchSize, waitTime, tasks) {

    tasks = tasks.slice();
    const allResults = [];

    return new Promise((resolve, reject) => {

        const exec = function() {

            const executors = tasks.slice(0, batchSize);
            tasks = tasks.slice(batchSize);

            Promise.all(executors.map(fn => new Promise(fn)))
                .then(results => {
                    allResults.push.apply(allResults, results);
                    if (tasks.length) {
                        setTimeout(exec, waitTime);
                    } else {
                        resolve(allResults);
                    }
                })
                .catch(reject);

        };

        exec();

    });

}