Gracefully handle Error instances without a stack property

index.js

@@ -3,6 +3,7 @@ const indentString = require('indent-string');
 const cleanStack = require('clean-stack');
 
 const cleanInternalStack = stack => stack.replace(/\s+at .*aggregate-error\/index.js:\d+:\d+\)?/g, '');
+const isString = value => typeof value === 'string' || value instanceof String;
 
 class AggregateError extends Error {
 	constructor(errors) {
@@ -23,7 +24,14 @@ class AggregateError extends Error {
 			return new Error(error);
 		});
 
-		let message = errors.map(error => cleanInternalStack(cleanStack(error.stack))).join('\n');
+		let message = errors
+			.map(error => {
+				// Unfortunately stack is not standardized as a property of Error instances
+				// which makes it necessary to explicitly check for it and its type.
+				// In case the stack property is missing the stringified error should be used instead.
+				return isString(error.stack) ? cleanInternalStack(cleanStack(error.stack)) : String(error);
+			})
+			.join('\n');
 		message = '\n' + indentString(message, 4);
 		super(message);
 

test.js

@@ -26,3 +26,28 @@ test('main', t => {
 		Object.assign(new Error(), {code: 'EQUX'})
 	]);
 });
+
+test('gracefully handle Error instances without a stack', t => {
+	class StacklessError extends Error {
+		constructor(...args) {
+			super(...args);
+			this.name = this.constructor.name;
+			delete this.stack;
+		}
+	}
+
+	const error = new AggregateError([
+		new Error('foo'),
+		new StacklessError('stackless')
+	]);
+
+	console.log(error);
+
+	t.regex(error.message, /Error: foo\n {8}at /);
+	t.regex(error.message, /StacklessError: stackless/);
+
+	t.deepEqual([...error], [
+		new Error('foo'),
+		new StacklessError('stackless')
+	]);
+});