Form submits when input is null

I want to prevent form submission when there is no input in the text boxes but the form submits even when the input is empty. How do I fix this?

<input type="text" size="30" id="zip" name="zip"><br />
<input type="text" size="30" id="phone" name="phone">

var input = $("#zip").val();
var input1 = $("#phone").val();

if ( $.trim(input).length == 0 || $.trim(input1).length == 0 ){
    event.preventDefault();
    }

Code (JavaScript):

<button type="submit" class="btn btn-success">Continue</button>

 

It's because you haven't wrapped it in a function-call, and "event" is not defined. This works:

$('button[type=submit]').click(function(event) {
var input = $("#zip").val();
var input1 = $("#phone").val();
console.log(input.length);
if ( $.trim(input).length == 0 || $.trim(input1).length == 0 ){
event.preventDefault();
}
})

Code (JavaScript):

 

Thank you PoPSiCle