Promise based serial task executor (version #1 - Only creates promises if any task returns one)

gistfile1.js

"use strict";

function isThenable(something) {

    return (
        typeof something === "object"
        && something.then instanceof Function
    );
}

function serial(tasks, ...args) {

    let results = [];
    let error;
    let prevPromise;

    for (let curTask of tasks) {
        if (prevPromise) {
            prevPromise = prevPromise.then(function(prevResult) {

                results.push(prevResult);

                return curTask(...args);
            });
        }
        else {
            let curResult;

            try {
                curResult = curTask(...args);
            }
            catch(e) {
                error = e;
                break;
            }

            if (isThenable(curResult)) {
                prevPromise = curResult;
            }
            else {
                results.push(curResult);
            }
        }
    }

    if (error) {
        return Promise.reject(error);
    }

    // none of the tasks returned a promise
    if (!prevPromise) {
        return Promise.resolve(results);
    }

    return prevPromise.then(function(prevResult) {

        results.push(prevResult);

        return results;
    });
}