client-side form validation

When a user hits the submit button I'd like to do some client-side validation before sending the form data to the server. What is the proper way to do this in Javascript? I.e. should I use the form's Sumbit button's "onClick()" event to trigger the client-side Javascript validation function and the call "document.location.href" from the function? (Will this pass the POST data?) Thanks! Jc