Delays between promises in promise chain

Question

Let\'s say I am using the following code to run a couple of promises in series:

let paramerterArr = [\'a\',\'b\',\'c\',\'d\',\'e\',\'f\']
parameterArr.reduce         


        

Answer 1

A really handy utility function to have around is something I call delay():

function delay(t, val) {
    return new Promise(function(resolve) {
        if (t <= 0) {
            resolve(val);
        } else {
            setTimeout(resolve.bind(null, val), t);
        }
    });
}

Then, you can use it in a promise chain like this:

let paramerterArr = ['a','b','c','d','e','f']
parameterArr.reduce(function(promise, item, index) {
  return promise.then(function(result) {
    // no delay on first iteration
    var delayT = index ? 50 : 0;
    return delay(delayT, item).then(mySpecialFunction);
  })
}, Promise.resolve());

---

You could also make a little utility function for doing the sequential iteration with optional delay:

// delayT is optional (defaults to 0)
function iterateSerialAsync(array, delayT, fn) {
    if (!fn) {
        fn = delayT;
        delayT = 0;
    }
    array.reduce(function(p, item, index) {
        return p.then(function() {
            // no delay on first iteration
            if (index === 0) delayT = 0;
            return delay(delayT, item).then(fn)
        });
    }, Promise.resolve());
}

And, then you would use it like this:

iterateSerialAsync(paramerterArr, 50, mySpecialFunction).then(function(finalVal) {
    // all done here
});