fix: Recognize custom built ES5 era errors

error/is.js

@@ -1,28 +1,45 @@
 "use strict";
 
-var isPrototype = require("../prototype/is");
+var isPrototype   = require("../prototype/is")
+  , isPlainObject = require("../plain-object/is");
 
 var objectToString = Object.prototype.toString;
 
 // Recognize host specific errors (e.g. DOMException)
-var errorTaggedStringRe = /^\[object .*(?:Error|Exception)\]$/;
+var errorTaggedStringRe = /^\[object .*(?:Error|Exception)\]$/
+  , errorNameRe = /^[^\s]*(?:Error|Exception)$/;
 
 module.exports = function (value) {
 	if (!value) return false;
 
+	var name;
 	// Sanity check (reject objects which do not expose common Error interface)
 	try {
+		name = value.name;
+		if (typeof name !== "string") return false;
 		if (typeof value.message !== "string") return false;
 	} catch (error) {
 		return false;
 	}
 
-	// Ensure its a native Error object (has [[ErrorData]] slot)
-	// Note: There seems no 100% bulletproof way of confirming that as:
+	// Ensure its a native-like Error object
+	// (has [[ErrorData]] slot, or was created to resemble one)
+	// Note: It's not a 100% bulletproof check of confirming that as:
 	// - In ES2015+ string tag can be overriden via Symbol.toStringTag property
 	// - Host errors do not share native error tag. Still we rely on assumption that
 	//   tag for each error will end either with `Error` or `Exception` string
-	if (!errorTaggedStringRe.test(objectToString.call(value))) return false;
+	// - In pre ES2015 era, no custom errors will share the error tag.
+	if (!errorTaggedStringRe.test(objectToString.call(value))) {
+		// Definitely not an ES2015 error instance, but could still be an error
+		// (created via e.g. CustomError.prototype = Object.create(Error.prototype))
+		try {
+			if (name !== value.constructor.name) return false;
+		} catch (error) {
+			return false;
+		}
+		if (!errorNameRe.test(name)) return false;
+		if (isPlainObject(value)) return false;
+	}
 
 	return !isPrototype(value);
 };

test/error/is.js

@@ -15,17 +15,25 @@ describe("error/is", function () {
 		assert.equal(isError(Error.prototype), false);
 	});
 
+	if (typeof Object.create === "function") {
+		it("Should return true on custom built ES5 era error", function () {
+			var CustomEs5Error = function () { Error.call(this); };
+			CustomEs5Error.prototype = Object.create(Error.prototype);
+			assert.equal(isError(new CustomEs5Error()), true);
+		});
+
+		it("Should return false on object with no prototype", function () {
+			assert.equal(isError(Object.create(null)), false);
+		});
+	}
+
 	it("Should return false on plain object", function () { assert.equal(isError({}), false); });
 	it("Should return false on function", function () {
 		assert.equal(isError(function () { return true; }), false);
 	});
 
 	it("Should return false on array", function () { assert.equal(isError([]), false); });
-	if (typeof Object.create === "function") {
-		it("Should return false on object with no prototype", function () {
-			assert.equal(isError(Object.create(null)), false);
-		});
-	}
+
 	it("Should return false on string", function () { assert.equal(isError("foo"), false); });
 	it("Should return false on empty string", function () { assert.equal(isError(""), false); });
 	it("Should return false on number", function () { assert.equal(isError(123), false); });