Sequential Promises using Map Reduce

example.js

[1, 2, 3, 4, 5]
    .reduce(
        sequentialPromise(element => new Promise(resolve =>
            setTimeout(
                () => {
                    console.log(`Handled ${element}`);
                    resolve(element * Math.floor(Math.random() * 100));
                },
                Math.random() * 1000
            )
        )),
        Promise.resolve()
    )
    .then(result => console.log(`Result: ${result}`));

sequentialPromise.js

function sequentialPromise(callback) {
    return (prevPromise, currentElement, index) => {
        return prevPromise.then(results =>
            callback(currentElement, index)
                .then(result => [...(results || []), result])
        );
    };
}