Do not close the browser untill fill the form

Hi Everyone,
                   Once the page is executed then must fill all the fields. Once all the fields are filled then only we closed the browser. How to restrict the browser close button. I used this javascript below, but if i click the ok button then it closed. Please tell me. How to do.

window.onbeforeunload = function() { var loggedIn = true; if (loggedIn) { alert("Fill the form then close"); return false; } };