How to know non blocking Recursive job is complete in nodejs

I have written this non-blocking nodejs sample recursive file search code, the problem is I am unable to figure out when the task is complete. Like to calculate the time taken for the task.

fs = require('fs');

searchApp = function() {

    var dirToScan = 'D:/';

    var stringToSearch = 'test';

    var scan = function(dir, done) {
        fs.readdir(dir, function(err, files) {
            files.forEach(function (file) {
                var abPath = dir + '/' + file;
                try {
                    fs.lstat(abPath, function(err, stat) {
                        if(!err && stat.isDirectory()) {
                            scan(abPath, done);;
                        }
                    });
                }
                catch (e) {
                    console.log(abPath);
                    console.log(e);
                }

                matchString(file,abPath);
            });
        });
    }

    var matchString = function (fileName, fullPath) {
        if(fileName.indexOf(stringToSearch) != -1) {
            console.log(fullPath);
        }
    }

    var onComplte = function () {
        console.log('Task is completed');
    }

    scan(dirToScan,onComplte);
}


searchApp();

Above code do the search perfectly, but I am unable to figure out when the recursion will end.