You can wait until the current expression is cleared before applying the new one. Here's an example:
<!DOCTYPE html>
<html>
<head>
<title>QueryMap</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="viewport" content="initial-scale=1, maximum-scale=1,user-scalable=no">
<link rel="stylesheet" href="http://js.arcgis.com/3.13/dijit/themes/claro/claro.css">
<link rel="stylesheet" href="http://js.arcgis.com/3.13/esri/css/esri.css">
<style>
html, body, #mapDiv{
padding: 0;
margin: 0;
height: 100%;
}
#clearButton{
z-index: 40;
position: absolute;
top:5px;
right:10px;
}
</style>
<script src="http://js.arcgis.com/3.13/"></script>
<script>
require(["esri/map", "esri/layers/FeatureLayer", "esri/tasks/query", "esri/InfoTemplate", "dojo/on","dojo/dom", "dojo/domReady!"], function(Map, FeatureLayer, Query, InfoTemplate, on, dom) {
var map = new Map("mapDiv", {
center: [-118.29, 34.07],
zoom: 13,
basemap: "gray"
});
//Clear the existing expression and once the layer is updated apply
//the new expression
on(dom.byId("clearButton"),"click",function(){
on.once(flayer, "update-start", function(){
updateQuery(flayer, "Ligustrum lucidum");
});
flayer.setDefinitionExpression(null);
});
var flayer = new FeatureLayer("http://services.arcgis.com/V6ZHFr6zdgNZuVG0/arcgis/rest/services/ParkTrees/FeatureServer/0",{
outFields: ["*"],
infoTemplate: new InfoTemplate("${COMMON}","${BOTANICAL}")
});
map.addLayer(flayer);
flayer.on("load",function(){
updateQuery(flayer, "Fraxinus uhdei");
});
function updateQuery(layer, expression){
var query = new Query();
query.where = "BOTANICAL = '" + expression + "'";
layer.queryIds(query, function(objectIds) {
var exp = layer.objectIdField + " IN(" + objectIds.join(",") + ")";
layer.setDefinitionExpression(exp);
});
}
});
</script>
</head>
<body class="claro">
<div id="mapDiv"></div>
<input id="clearButton" type="button" value="Clear"/>
</body>
</html>
