fix: support some virtual contexts in toThrow

This adds support for VM situations where we pass a `RegExp` from
another process.

Note that we don't have a full fix for this stuff until `check-error`
also supports `Error` being from another origin.

lib/chai/core/assertions.js

@@ -2676,7 +2676,7 @@ function assertThrows (errorLike, errMsgMatcher, msg) {
     , negate = flag(this, 'negate') || false;
   new Assertion(obj, flagMsg, ssfi, true).is.a('function');
 
-  if (errorLike instanceof RegExp || typeof errorLike === 'string') {
+  if (_.isRegExp(errorLike) || typeof errorLike === 'string') {
     errMsgMatcher = errorLike;
     errorLike = null;
   }
@@ -2709,7 +2709,7 @@ function assertThrows (errorLike, errMsgMatcher, msg) {
     }
 
     this.assert(
-        caughtErr
+        caughtErr !== undefined
       , 'expected #{this} to throw ' + errorLikeString
       , 'expected #{this} to not throw an error but #{act} was thrown'
       , errorLike && errorLike.toString()
@@ -2760,7 +2760,7 @@ function assertThrows (errorLike, errMsgMatcher, msg) {
   if (caughtErr && errMsgMatcher !== undefined && errMsgMatcher !== null) {
     // Here we check compatible messages
     var placeholder = 'including';
-    if (errMsgMatcher instanceof RegExp) {
+    if (_.isRegExp(errMsgMatcher)) {
       placeholder = 'matching'
     }
 

lib/chai/utils/index.js

@@ -94,3 +94,7 @@ export {isNaN} from './isNaN.js';
 
 // getOperator method
 export {getOperator} from './getOperator.js';
+
+export function isRegExp(obj) {
+  return Object.prototype.toString.call(obj) === '[object RegExp]';
+}

test/assert.js

@@ -1644,6 +1644,9 @@ describe('assert', function () {
       assert[throws](function() { throw new Error('bar'); }, Error, 'bar');
       assert[throws](function() { throw new Error(''); }, Error, '');
       assert[throws](function() { throw new Error('foo') }, '');
+      assert[throws](function() { throw ''; }, '');
+      assert[throws](function() { throw ''; }, /^$/);
+      assert[throws](function() { throw new Error(''); }, /^$/);
 
       var thrownErr = assert[throws](function() { throw new Error('foo'); });
       assert(thrownErr instanceof Error, 'assert.' + throws + ' returns error');

web-test-runner.config.js

@@ -5,7 +5,10 @@ const commonjs = fromRollup(rollupCommonjs);
 
 export default {
   nodeResolve: true,
-  files: ["test/*.js"],
+  files: [
+    "test/*.js",
+    "!test/virtual-machines.js"
+  ],
   plugins: [
     commonjs({
       include: [