Iterate over an array and process each item in a promise with a given delay. Ideal if you need to run items at a set or variable pace. As used in https://github.com/cballenar/propertygrubber to delay items due to timeouts possibly put in place to stop crawlers.

pacer.js

/**
 * Pacer
 * Iterate over an array and process each item in a promise with a given delay.
 * Ideal if you need to run items at a set or variable pace. As used in
 * https://github.com/cballenar/propertygrubber to delay items due to timeouts
 * possibly put in place to stop crawlers.
 * 
 * @param {Array} list List of items to process
 * @param {Number} delay Time in milliseconds to delay each item
 * @returns {Array} New list of processed items
 * 
 * Credits
 * https://medium.com/developer-rants/running-promises-in-a-loop-sequentially-one-by-one-bd803181b283
 */
function pacer(list, delay=1000) {
    const startTime = Date.now();
    const newList = [];

    function wait(data, ms) {
        var newData = 'new '+data;
        console.log(`Waiting: ${ms / 1000} seconds.`);
        return new Promise((resolve) => {
            setTimeout(resolve, ms, newData);
        });
    }

    function paceList(list, i=0) {
        var item = list[i];
        wait(item, delay)
            .then(newData => {
                newList.push(newData)
                console.log(`Received: ${newData}\n`);
                i++;
                if (i < list.length) {
                    paceList(list, i);
                } else {
                    console.log(`Total: ${(Date.now() - startTime) / 1000} seconds.`);
                    return newList;
                }
            });
    }
    return paceList(list);
}

const myData = ['a','b','c','d','e'],
      myNewData = pacer(myData,1500);