I have been reading many documents here and there but I don't quite understand how the error object in javascript can be used as boolean in one place and as an object at another place.

var request = require('request'); //http wrapped module
function requestWrapper(url, callback) {
    request.get(url, function (err, response) {
        if (err) {  //as boolean
            console.log("Im here as an object "+err);
        } else {
            callback(null, response);
        }
    })
}