Number( "" ) == 0
or, If you want to ensure that it's an integer, parseInt() can be used instead.
var postcode = parseInt(form.elements.postcode.value, 10);
That ensures that it starts with an integer.
parseInt( "1oo", 10) == 1
The most satisfactory solution I've seen is to use isFinite combined with parseFloat to eliminate null strings.
return !isNaN( parseFloat( n ) ) && isFinite( n );