fix: faithfully match an empty set of browsers (#1583)

If the query matches no browsers, generate `/[]/` which matches nothing
rather than `/(?:)/` which matches everything.

Signed-off-by: Anders Kaseorg <[email protected]>

Author: andersk

src/regex/nodes.ts

@@ -109,10 +109,15 @@ export function RepetitionNode(expression: Expression, quantifier: Quantifier):
   }
 }
 
-export function DisjunctionNode(...expressions: (Expression | Expression[])[]): Disjunction | Expression {
+export function DisjunctionNode(...expressions: (Expression | Expression[])[]): Disjunction | Expression | CharacterClass {
   const exprs = concat(expressions).filter(Boolean)
 
-  if (exprs.length === 1) {
+  if (exprs.length === 0) {
+    return {
+      type: 'CharacterClass',
+      expressions: []
+    }
+  } else if (exprs.length === 1) {
     return exprs[0]
   }
 

test/useragents.ts

@@ -111,7 +111,8 @@ export const useragents = [
    */
   {
     ua: 'Mozilla/5.0 (Windows NT 5.2; rv:42.0) Gecko/20100101 Firefox/42.0',
-    yes: ['firefox >= 40']
+    yes: ['firefox >= 40'],
+    no: ['> 9000%']
   },
   /**
    * Firefox Desktop bug #1530