How to run this package to download app?

[trungducng]

The guide of this package is "too clear" for me to use.
npm install gpapi --save
create index.js:

var api = require('gpapi').GooglePlayAPI({
username: "xxxxx@gmail.com",
password: "xxxxxxxxx",
androidId: "334340280EA00B10"
// apiUserAgent: optional API agent override (see below)
// downloadUserAgent: optional download agent override (see below)
});

// usage via Promise
api.details("com.viber.voip").then(console.log);

// usage via node callback convention
api.details("com.viber.voip", function (err, res) {
console.log(err ? err : res);
});

Then how???