How to learn Nodejs library?

I have difficulties when I study nodejs libraries for using its functions. They don't write document for every function so I don't know how I can learn about these functions. For example,

var db = redis.createClient();
User.prototype.save = function(fn) {
if (this.id) {
    this.update(fn);
} else {
    var user = this;
    db.incr('user:ids', function(err, id) {
        if (err) {
            return fn(err);
        }
...

In this code, they are using db.incr('user:ids', function(err, id){}) function for communication with Redis server, when I research homepage of this library(http://ift.tt/1Ccwn3d), I cannot find any line to explain how to use 'incr(..)' function.

Therefore, I would like to ask if there is a way to learn how to use nodejs libraries properly.

Thanks