触发级联下拉菜单

Question

我在修改表单中有级联下拉菜单，目的是在级联下拉菜单保持功能正常的情况下，以编程方式填充下拉列表（来自mysql查询的值）而不是仅单击，选择了第一个下拉菜单，但其他下拉菜单是没有填写/选择。如果有人可以帮助我，那将非常好。

Here is the Demo

var stateObject = {
	"California": {
		"Monterey": ["Salinas", "Gonzales"],
		"Alameda": ["Oakland", "Berkeley"]
	},
	"Oregon": {
		"Douglas": ["Roseburg", "Winston"],
		"Jackson": ["Medford", "Jacksonville"]
	}
}
var stateSel = document.getElementById("stateSel"),
	countySel = document.getElementById("countySel"),
	citySel = document.getElementById("citySel");
	
$(document).ready(function() {
	for (var state in stateObject) {
		stateSel.options[stateSel.options.length] = new Option(state, state);
	}
	stateSel.onchange = function () {
		countySel.length = 1; // remove all options bar first
		citySel.length = 1; // remove all options bar first
		if (this.selectedIndex < 1) return; // done   
		for (var county in stateObject[this.value]) {
			countySel.options[countySel.options.length] = new Option(county, county);
		}
	}
	stateSel.onchange(); // reset in case page is reloaded
	countySel.onchange = function () {
		citySel.length = 1; // remove all options bar first
		if (this.selectedIndex < 1) return; // done   
		var cities = stateObject[stateSel.value][this.value];
		for (var i = 0; i < cities.length; i++) {
			citySel.options[citySel.options.length] = new Option(cities[i], cities[i]);
		}
	}
});
$( "#testing" ).click(function() {
	var mystate = "Oregon",
		mycounty = "Jackson",
		mycity = "Jacksonville";
	$("#stateSel option").filter(function() {return $(this).text() == mystate;}).prop("selected", true);
	$("#countySel option").filter(function() {return $(this).text() == mycounty;}).prop("selected", true);
	$("#citySel option").filter(function() {return $(this).text() == mycity;}).prop("selected", true);

});

<div class="container">
	<div class="row mb-4">
		<div class="col-xl-4 col-lg-4 col-md-4">
			<select name="stateSel" id="stateSel" class="form-control input-md">
				<option value="">Select</option>
			</select>
			<br />
			<select name="countySel" id="countySel" class="form-control input-md">
				<option value="">Select</option>
			</select>
			<br />
			<select name="citySel" id="citySel" class="form-control input-md">
				<option value="">Select</option>
			</select>
		</div>
	</div>
	<button type="button" class="btn btn-sm btn-success" id="testing">Trigger Button</button>
</div>

Here is the demo