fix: simplify error class checking (#53)

Previously, we were unnecessarily trying to determine if the passed
object was in fact a class which extends `Error`.

We don't actually care about this, really. We just want to know if the
`thrown` is an instance of the thing you passed.

Due to this, we can simplify by simply checking that the `errorLike` is
something with a `prototype` and assert that the `thrown` is an instance
of it.

index.js

@@ -3,10 +3,6 @@ function isErrorInstance(obj) {
   return obj instanceof Error || Object.prototype.toString.call(obj) === '[object Error]';
 }
 
-function isErrorClass(obj) {
-  return obj === Error || (typeof obj === 'function' && obj.name === 'Error');
-}
-
 function isRegExp(obj) {
   // eslint-disable-next-line prefer-reflect
   return Object.prototype.toString.call(obj) === '[object RegExp]';
@@ -50,7 +46,7 @@ function compatibleConstructor(thrown, errorLike) {
   if (isErrorInstance(errorLike)) {
     // If `errorLike` is an instance of any error we compare their constructors
     return thrown.constructor === errorLike.constructor || thrown instanceof errorLike.constructor;
-  } else if (isErrorClass(Object.getPrototypeOf(errorLike)) || isErrorClass(errorLike)) {
+  } else if ((typeof errorLike === 'object' || typeof errorLike === 'function') && errorLike.prototype) {
     // If `errorLike` is a constructor that inherits from Error, we compare `thrown` to `errorLike` directly
     return thrown.constructor === errorLike || thrown instanceof errorLike;
   }

test/index.js

@@ -31,6 +31,17 @@ describe('checkError', function () {
 
     assert(checkError.compatibleConstructor(errorInstance, anObject) === false);
     assert(checkError.compatibleConstructor(errorInstance, aNumber) === false);
+
+    function PrototypeError() {}
+    PrototypeError.prototype = Object.create(Error.prototype);
+    assert(checkError.compatibleConstructor(new PrototypeError(), PrototypeError) === true);
+    assert(checkError.compatibleConstructor(new PrototypeError(), Error) === true);
+
+    // eslint-disable-next-line func-style
+    const WeirdNamelessError = function () {};
+    WeirdNamelessError.prototype = Object.create(Error.prototype);
+    assert(checkError.compatibleConstructor(new WeirdNamelessError(), WeirdNamelessError) === true);
+    assert(checkError.compatibleConstructor(new WeirdNamelessError(), Error) === true);
   });
 
   it('compatibleMessage', function () {