javascript form validation. Check for input

We get the user to enter zipcode first and then auto-fill city and state. Here is the code from zipcodeapi.com/examples:

<script type="text/javascript">//<![CDATA[
$(function() {
    // IMPORTANT: Fill in your client key
    var clientKey = "js-9qZHzu2Flc59Eq5rx10JdKERovBlJp3TQ3ApyC4TOa3tA8U7aVRnFwf41RpLgtE7";

    var cache = {};
    var container = $("#example1");
    var errorDiv = container.find("div.text-error");

    /** Handle successful response */
    function handleResp(data)
    {
        // Check for error
        if (data.error_msg)
            errorDiv.text(data.error_msg);
        else if ("city" in data)
        {
            // Set city and state
            container.find("input[name='city']").val(data.city);
            container.find("input[name='state']").val(data.state);
        }
    }

    // Set up event handlers
    container.find("input[name='zipcode']").on("keyup change", function() {
        // Get zip code
        var zipcode = $(this).val().substring(0, 5);
        if (zipcode.length == 5 && /^[0-9]+$/.test(zipcode))
        {
            // Clear error
            errorDiv.empty();

            // Check cache
            if (zipcode in cache)
            {
                handleResp(cache[zipcode]);
            }
            else
            {
                // Build url
                var url = "https://www.zipcodeapi.com/rest/"+clientKey+"/info.json/" + zipcode + "/radians";

                // Make AJAX request
                $.ajax({
                    "url": url,
                    "dataType": "json"
                }).done(function(data) {
                    handleResp(data);

                    // Store in cache
                    cache[zipcode] = data;
                }).fail(function(data) {
                    if (data.responseText && (json = $.parseJSON(data.responseText)))
                    {
                        // Store in cache
                        cache[zipcode] = json;

                        // Check for error
                        if (json.error_msg)
                            errorDiv.text(json.error_msg);
                    }
                    else
                        errorDiv.text('Request failed.');
                });
            }
        }
    }).trigger("change");
});

//]]> Zip Code Distance

It will be better to use HTML5 required form field attribute. I create the jsfiddle for your.

Note that, as it stands, they will have always selected a state, since Alabama is selected by default. You can add an option with no value and a label of "Pick a state" to change this. With that advice taken and an id "city" on your city input

var city = document.getElementById('city'),
    state = document.getElementById('state'),
    cityFilled = city.value !== ''
    stateBlank = state.value ==='';

if(cityFilled && stateBlank){
  alert("Enter a zipcode, chump");
}

Using pure js you can write the code as

var val = document.getElementById("name").value; // get value
if(val == "") { // check its value, if null
  alert("please write something here"); // alert the user..
}

And do that similarly for each and every input.

You need to add the id to each element to make the JS capture the element.