fix: stable sort for undefined keys, fixes #191

src/util/underscore.ts

@@ -1,6 +1,7 @@
 import { Drop } from '../drop/drop'
 
 const toStr = Object.prototype.toString
+const toLowerCase = String.prototype.toLowerCase
 
 /*
  * Checks if value is classified as a String primitive or object.
@@ -127,3 +128,15 @@ export function changeCase (str: string): string {
 export function ellipsis (str: string, N: number): string {
   return str.length > N ? str.substr(0, N - 3) + '...' : str
 }
+
+// compare string in case-insensitive way, undefined values to the tail
+export function caseInsensitiveCompare (a: any, b: any) {
+  if (a == null && b == null) return 0
+  if (a == null) return 1
+  if (b == null) return -1
+  a = toLowerCase.call(a)
+  b = toLowerCase.call(b)
+  if (a < b) return -1
+  if (a > b) return 1
+  return 0
+}