Bluebird. Then method

There is sample code in the Bluebird docs for the .call method labeled "Lo-dash chaining or underscore methods".

What is the purpose of chained .then(_)

in the code snippet below?

var Promise = require("bluebird");

...

var _ = require("lodash");
var fs = Promise.promisifyAll(require("fs"));

fs.readdirAsync(".").then(_) <-- ???
    .call("groupBy", function(fileName) {
        return fileName.charAt(0);
    })

...

      

        
        
        
      

    

Thank!