check-zip-code.js

(function($){

	var hide_fields = function() {
		$( '#wpsc-checkout-field-billingcity, #wpsc-checkout-field-billingstate, #wpsc-checkout-field-billingcountry' ).addClass( 'ui-helper-hidden-accessible' );
	},

	show_fields = function() {
		$( '#wpsc-checkout-field-billingcity, #wpsc-checkout-field-billingstate, #wpsc-checkout-field-billingcountry' ).removeClass( 'ui-helper-hidden-accessible' );
	},

	ping_api = function() {

		var $this = $( this ), zip = $this.val();

		if ( zip.length < 5 ) {
			return;
		}

		// Note: 'us' here could be set via wp_localize_script(), allowing to work for any of the whitelisted countries available to the API.
		// Ensuring that we're always (even on API failure or an invalid response) showing the fields
		$.getJSON( 'http://api.zippopotam.us/us/' + zip, function( response ) {
			$( '#wpsc-checkout-field-billingcity-control' ).val( response.places[0]['place name'] );

			var state = response.places[0]['state'];

			$( 'input.wpsc-form-select-region' ).val( state );
			$( '#wpsc-checkout-field-billingstate-control option' ).filter(function () { return $(this).html() == state; } ).attr( 'selected', true );

		} ).always( show_fields );
	};

	$(function() {
		$( '#wpsc-checkout-field-billingpostcode-control' ).keyup( ping_api );
		hide_fields();
	});

})(jQuery);