add warning

Author: timdeschryver

src/__tests__/matches.js

@@ -16,11 +16,19 @@ test('matchers accept regex', () => {
 })
 
 // https://stackoverflow.com/questions/1520800/why-does-a-regexp-with-global-flag-give-wrong-results
-test('matchers recreate regex to prevent global mistakes', () => {
+test('a regex with the global flag consistently (re-)finds a match', () => {
   const regex = /ABC/g
+  const spy = jest.spyOn(console, 'warn').mockImplementation()
+
   expect(matches('ABC', node, regex, normalizer)).toBe(true)
-  // without recreation of the regexp this would be false
   expect(fuzzyMatches('ABC', node, regex, normalizer)).toBe(true)
+
+  expect(spy).toBeCalledTimes(2)
+  expect(spy).toHaveBeenCalledWith(
+    `To match all elements we had to reset the lastIndex of the RegExp because the global flag is enabled. We encourage to remove the global flag from the RegExp.`,
+  )
+
+  console.warn.mockClear()
 })
 
 test('matchers accept functions', () => {

src/matches.ts

@@ -36,7 +36,7 @@ function fuzzyMatches(
   } else if (typeof matcher === 'function') {
     return matcher(normalizedText, node)
   } else {
-    return matcher.test(normalizedText)
+    return matchRegExp(matcher, normalizedText)
   }
 }
 
@@ -56,11 +56,7 @@ function matches(
   if (matcher instanceof Function) {
     return matcher(normalizedText, node)
   } else if (matcher instanceof RegExp) {
-    const match = matcher.test(normalizedText)
-    if (matcher.global) {
-      matcher.lastIndex = 0
-    }
-    return match
+    return matchRegExp(matcher, normalizedText)
   } else {
     return normalizedText === String(matcher)
   }
@@ -116,4 +112,15 @@ function makeNormalizer({
   }
 }
 
+function matchRegExp(matcher: RegExp, text: string) {
+  const match = matcher.test(text)
+  if (matcher.global && matcher.lastIndex !== 0) {
+    console.warn(
+      `To match all elements we had to reset the lastIndex of the RegExp because the global flag is enabled. We encourage to remove the global flag from the RegExp.`,
+    )
+    matcher.lastIndex = 0
+  }
+  return match
+}
+
 export {fuzzyMatches, matches, getDefaultNormalizer, makeNormalizer}