Count elements in ajax/json response - ajax

Question

This is properly pretty simple and has been asked many times before, but it just eludes me, who to do it.
I have 2 dropdowns, where the last one gets populated from the select of the first one. That works, but now I want to preselect for the second dropdown, when there is only one element in the response.
Function:
$( "select[name='r53b']" ).change(function () {
var r53bID = $(this).val();
if(r53bID) {
$.ajax({
url: "/overfladeajax.php",
dataType: 'Json',
data: {'id':r53bID},
success: function(data) {
$('select[name="r53c"]').empty();
$('select[name="r53c"]').append('<option value="">Vælg Lagtykkelse</option>');
$.each(data, function(key, value) {
$('select[name="r53c"]').append('<option value="'+ key +'">'+ value +'</option>');
});
}
});
}
});
2 kind of reponses:
{"7":"2 x 100 \u03bcm HS 150 (totalt 200 \u03bcm t\u00f8rfilm)","8":"1 x 40 \u03bcm zink primer + 1 x 120 \u03bcm HS 150 (totalt 160 \u03bcm t\u00f8rfilm)"}
or
{"2":"1 x 80 \u03bcm HS 150"}
How to I make the append, so it will also select the option where only one is available?

Answer 1

You can use Object.keys(yourjson).length to get the length of keys in your json object and then if the count is 1 add selected to that options.
Demo Code :
var data = {
"2":"1 x 80 \u03bcm HS 150"
}
var count = Object.keys(data).length//get key length
console.log("length is "+count)
$('select[name="r53c"]').append('<option value="">Vælg Lagtykkelse</option>');
$.each(data, function(key, value) {
//if length is 1
if (count == 1) {
$('select[name="r53c"]').append('<option selected value="' + key + '">' + value + '</option>');
} else {
$('select[name="r53c"]').append('<option value="' + key + '">' + value + '</option>');
}
})
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select name="r53c">
</select>