Promise sequence in pure javascript (browser and server)

promiseSequence

//similar to: https://www.npmjs.com/package/promise-sequential
//input: array of functions that returns a promise
//rules:
//- if one promise fails, the sequence is cancelled and the error is returned (catch)
//returns:
//array of results

function promiseSequence(arr) {
    return new Promise(function(resolve, reject) {
        var result = [];
        var interval = setInterval(function() {
            if (result.length === 0) {
                var p = arr[0]()
                result[0] = p;
                p.then(function(res) {
                    result[0] = res
                }).catch(function(err) {
                    result[0] = err
                });
            }
            var last = result[result.length - 1];
            if (!(last instanceof Promise)) {
                if (last instanceof Error) {
                    clearInterval(interval);
                    return reject(last);
                }
                if (result.length - 1 == arr.length - 1) {
                    clearInterval(interval);
                    resolve(result);
                } else {
                    if (typeof arr[result.length] === 'function') {
                        var p = arr[result.length]();
                        result.push(p);
                        p.then(function(res) {
                            result[result.length - 1] = res
                        }).catch(function(err) {
                            result[result.length - 1] = err
                        });
                    }
                }
            } else {
                //wait
            }
        }, 100);
    });
}



/*
//test
var msgs = ['foo', 'bar']
promiseSequence(msgs.map(function(n) {
    return function() {
        return new Promise(function(resolve, reject) {
            setTimeout(function() {
                if (n == 'bar') {
                    return reject(new Error('ups'));
                }
                resolve(n)

            }, 2000)
        });
    }
})).then(console.info).catch(console.error)
 */

If you are reading this today, I recommend this simple snippet instead:

iterable.reduce((p, fn) => p.then(fn), Promise.resolve())