Getting started with Question JavaScript

I have questions throughout my surveys that require validation of numeric entries. In many questions I can use the validation options built into Qualtrics, but several have more advanced validation: I either need to compare the value of one question against the sum of two or more other questions (or the total of column sums of another question) elsewhere in the survey, or compare a date entered to ensure it is within a valid range (before or after another date entry, or a static date). I was able to use JavaScript for all of this in the previous online survey software I used with IF/ELSE conditional statements and alerts or HTML5 messages so that the survey stops progress and displays an error message if an invalid entry is made.

Presumably I can do something similar in Qualtrics with Question JavaScript, but I'm having trouble getting started. I set up a simple JavaScript validation for a question (though I know it could be done instead using built-in options) to make sure I've got the basics right; see below. It catches the error when the entry is less than 3 and displays an alert, but the survey still progresses to the next page after the respondent accepts the alert.

My JavaScript knowledge and experience is limited to the context of the online survey software I used prior to Qualtrics, and I can't tell what I'm overlooking even after searching the community and the wider internet. Any suggestions?

Qualtrics.SurveyEngine.addOnPageSubmit(function()
{
var q1 = parseInt(document.getElementById('QR~QID3').value, 10),
msg = 'Please enter a number greater than 2.';
if (q1 < 3)
{
alert(msg);
document.getElementById('QR~QID3').focus();
return false;
} else {
return true;
}
});