Understanding js prototyping

proto-test.js

res = function(){
    mr = function(obj){
        this.testing = obj
        // or _.extend(this, obj) or Object.assign(this, obj) to extend this
    };
    mr.prototype.tester = function(){
        return this.testing.test
    };
    return mr
}
m = res() // mr
r = new m({test: 123}) // Object { testing: Object { test: 123 } }
r.tester() // 123