Improve code (thanks Alex for help!)

main.js

@@ -1,4 +1,4 @@
-var request = new XMLHttpRequest()
+let request = new XMLHttpRequest()
 request.open(
     "GET",
     "https://cors-proxy.blaseball-reference.com/database/allTeams",
@@ -6,34 +6,23 @@ request.open(
 request.send();
 
 request.onload = function() {
-    var teams = JSON.parse(this.response)
-    var totals = teams.map(team => team.lineup.length + team.rotation.length)
+    let teams = JSON.parse(this.response)
+    let totals = teams.map(team => team.lineup.length + team.rotation.length)
     // Get all player counts that there are from greatest to least
-    var playercounts = totals.slice().sort(function (x, y) {
-        return y - x
-    }).filter(function (item, pos, array) {
-        return !pos || item != array[pos - 1]
-    })
-
-    // Now make headers & lists for all the player counts
-    playercounts.forEach(function(item) {
+    // & make headers & lists for them
+    totals.slice().sort((x, y) => y - x)
+        .filter((item, pos, array) => !pos || item != array[pos - 1])
+        .forEach(function(item) {
             document.body.insertAdjacentHTML("beforeend",
-            "<h1>"
-            + item 
-            + " players</h1><ul id='tlist"
-            + item
-            + "'></ul>")
+                `<h1>${item} player${(item > 1 ? "s" : "")}</h1>
+                <ul id="tlist${item}"></ul>`)
     })
     // Put the teams in the lists
     teams.forEach(function(item, pos) {
         document.getElementById("tlist" + totals[pos]).insertAdjacentHTML("beforeend", 
-            "<li>" 
-            + item.fullName
-            + " ("
-            + item.lineup.length
-            + " hitters, "
-            + item.rotation.length
-            + " pitchers)</li>")
+            `<li>${item.fullName} 
+            (${item.lineup.length} hitter${(item.lineup.length > 1 ? "s" : "")},
+             ${item.rotation.length} pitcher${(item.rotation.length > 1 ? "s" : "")})</li>`)
     })
 
     document.getElementById("progress").remove()