[Promise waterfall] Async function queue #interview

index.js

var func1 = function() {
    return new Promise(function(resolve) {
        setTimeout(function() {
            console.log('func1');
            resolve();
        }, 500);
    });
};
var func2 = function() {
    return new Promise(function(resolve) {
        setTimeout(function() {
            console.log('func2');
            resolve();
        }, 300);
    });
};
var func3 = function() {
    return new Promise(function(resolve) {
        setTimeout(function() {
            console.log('func3');
            resolve();
        }, 100);
    });
};

var asyn = function(arr, cb) {
    arr.reduce((p, func) => p.then(func), Promise.resolve())
        .then(cb);
};

asyn([func1, func2, func3], () => { console.log('Finished!') })