Use jQuery.slim

Underscore.js _.uniq(_.compact( to native Array.filter((value, index, self) => !!value && self.indexOf(value) == index)
Underscore.js _.compact to native Array.filter(value => !!value)
Underscore.js _.uniq to native Array.filter((value, index, self) => self.indexOf(value) == index)
Underscore.js _.values to native Object.values
Underscore.js _.flatten to native Array.flat
Underscore.js _.union to native Array.concat + unique filter
Underscore.js _.reduce to native Array.reduce
Underscore.js _.escape replaced with advanced htmlspecialchars()
Underscore.js _.memoize replaced
Now Underscore.js is a slim custom version (only _.debounce, _.defer & _.throttle)

dev/polyfill.js

@@ -0,0 +1,16 @@
+
+Array.prototype.flat || Object.defineProperty(Array.prototype, 'flat', {
+	configurable: true,
+	value: function flat(depth) {
+		depth = isNaN(depth) ? 1 : Number(depth);
+		return depth ? Array.prototype.reduce.call(this, (acc, cur) => {
+			if (Array.isArray(cur)) {
+				acc.push.apply(acc, flat.call(cur, depth - 1));
+			} else {
+				acc.push(cur);
+			}
+			return acc;
+		}, []) : this.slice();
+	},
+	writable: true
+});