fix: getConstructor name working with PoorlyConstructedErrors

index.js

@@ -129,8 +129,10 @@ function getConstructorName(errorLike) {
   if (errorLike instanceof Error) {
     constructorName = getFunctionName(errorLike.constructor);
   } else if (typeof errorLike === 'function') {
-    // if `err` is not an instance of Error it is an error constructor itself
-    constructorName = getFunctionName(errorLike);
+    // If `err` is not an instance of Error it is an error constructor itself or another function.
+    // If we've got a common function we get its name, otherwise we may need to create a new instance
+    // of the error just in case it's a poorly-constructed error. Please see chaijs/chai/issues/45 to know more.
+    constructorName = getFunctionName(errorLike) || getFunctionName(new errorLike()); // eslint-disable-line new-cap
   }
 
   return constructorName;

test/index.js

@@ -85,8 +85,22 @@ describe('checkError', function () {
       return 1;
     }
 
+    var anonymousFunc = function () { // eslint-disable-line func-style
+      return 2;
+    };
+
+    // See chaijs/chai/issues/45: some poorly-constructed custom errors don't have useful names
+    // on either their constructor or their constructor prototype, but instead
+    // only set the name inside the constructor itself.
+    var PoorlyConstructedError = function () { // eslint-disable-line func-style
+      this.name = 'PoorlyConstructedError'; // eslint-disable-line no-invalid-this
+    };
+    PoorlyConstructedError.prototype = Object.create(Error.prototype);
+
     assert(checkError.getConstructorName(correctName) === 'correctName');
     assert(checkError.getConstructorName(withoutComments) === 'withoutComments');
+    assert(checkError.getConstructorName(anonymousFunc) === '');
+    assert(checkError.getConstructorName(PoorlyConstructedError) === 'PoorlyConstructedError');
   });
 
   it('getMessage', function () {