Sequential execution of Promises using reduce()

promises_reduce.js

function createPromiseForFunction(e, functionThatTakesOneItem)
{
    return new Promise((resolve, reject) => {
        
        resolve(functionThatTakesOneItem(e));
    });
}

function performSequentially(arr, functionThatTakesOneItem)
{
    var finalResult = [];
    return new Promise(function(resolve, reject){

        resolve(arr.reduce((promise, item) => {
            return promise
            .then((result) => {
                console.log('item '+item);
                return createPromiseForFunction(item, functionThatTakesOneItem)
                .then(result => finalResult.push(result));
            })
            .catch(function(reason)
            {
                console.error('reason: '+reason);
                return Promise.reject(reason);
            });
        }, Promise.resolve()));

    })
    .then(function(){
        return Promise.resolve(finalResult);
    });
}

function tryOut()
{

    var values = [];
    for(var i = 0; i < 4; i++)
    {
        values.push(i);
    }

    var processFile = function(filename)
    {
        console.log('processing '+filename);
        return new Promise(function(resolve, reject){
            var product = filename + '/differented.png';
            setTimeout(() => resolve(product), 200);
        });
    }

    performSequentially(values, processFile)
    .then(function(result){
        console.log('final result is: '+result);
        return Promise.resolve('hey '+result.join('\n'));
    })
    .catch(function(reason){
        console.error('blah '+reason.stack);
    });
}

if(require.main === module)
{
    tryOut();
}


exports.performSequentially = performSequentially;